Second Week of Notes Flashcards
Characteristics of Autosomal Recessive Disorders
o Phenotype is only expressed in homozygotes rr
Need to have both mutant alleles
o Males and females are usually equally affected
Does not target based on gender
o Horizontal Inheritance Pattern
-Normally see it across in siblings (they are rare unless there are carrier parents
Parents of an affected child are obligate carriers
o Recurrent risk for each unborn child is ¼ (both parents are carriers)
RR, Rr, Rr, rr (rr is 25%)
o The chance of unaffected siblings being a carrier is 2/3
Carrier =Rr
Only 3 unaffected made RR, Rr, Rr….. 2 of the three are carriers
Calculating Frequencies of AR Disorders (PRACTICE)
o Amount of an autosomal recessive disease 1/10,000
♣ This is the q^2
o Find the square root to get q
♣ 1/100 is the amount of q alleles there are
o Find the amount of carriers (the Rr)
♣ Find 2pq
♣ Rare diseases, just do 2q
• 2* 1/100= 2/100
o REMEMBER, when finding carriers for rare diseases
♣ 2pq~ 2q
• Carrier Frequency vs. Disease Frequency (Practice!!)
o Scotland 1/5,300 1/36 (2pq)
o Finland 1/200,000 1/250
Parental Consanguinity
• Parental Consanguinity The more affected children of an autosomal recessive disease, the more likely there is Parental Consanguinity (common ancestor or inbred
o If the two parents inherited the same mutant allele from a common ancestor, then the disease risk for their child
Much higher risk to have the carrier traits passed down in consanguinity
Compound Heterozygosit
Compound Heterozygosity: presence of multiple common mutant alleles of the same gene in a population. There are different mutant alleles
o Example: is an individual who carries two different mutant alleles of the same gene
♣ Bc// Bs (both of these are mutant alleles in hemoglobin)
• One is sickle cell, other is the hemoglobin C
Simple Heterozygotes
Simple Heterozygotes Have two different alleles, only one of them could be mutant
♣ Ba//Bs (one is normal, the other is mutant sickle-cell)
o Homozygots= Ba//Ba Bs//Bs (one healthy homozygous, other is mutant homozygous)
Phenylketonuria and it’s genetics
Phenylketonuria High phenylalanine in blood due to lacking a critical enzyme, phenylalanine hydroxylase. Will lead to mental retardation, hyperactivity and epilepsy. Caused by mutations for phenylalanine hydroxylase at chromosome 12. Treated by a low phenylalanine diet and possible neutral amino acid supplementation
Cause of Phenylketonuria
♣ High amount of Phenylalanine levels in the blood, due to the defective nature or lack of enzyme phenylalanine hydroxylase
• Phenylalanine Hydroxylase (PAH): Normally found in the liver, is used to turn phenylalanine Tyrosine
• Defects cause 98% of the diseases
Defect in Cofactor BH4 ( a cofactor for PAH) Need the cofactor B4 to bind to the phenylalanine hydroxylase enzyme (and for dopamine/serotonin)
• This causes 1-2% of the cases of Phenylketonuria
• Lacking BH4 Also leads to a loss of neurotransmitters such as dopamine and serotonin
Possible phenotypes from Phenylketonuria
High phenylalanine metabolites in urine, hyperactivity, mental retardation, epilepsy, microcephaly
Molecular defects in PAH The PAH gene is at chromosome 12q22-24. Most mutations in PAH are partial or complete loss-of-function alleles. PAH gene exhibits high allelic heterogeneity; over 400 alleles have been identified. Most PKU patients are compound heterozygotes (i.e. having two different mutant alleles of the PAH gene). Severity of phenotype varies and probably reflects compound heterozygosity
Phenylketonuria (PKU) Defects in BH4 synthesis and Recycle
♣ Defects in different enzymes that give rise to cofactor BH4 (BH4 is for the phenylalanine hydroxylase)
• Enzymes defected to build BH4: GTP-CH, 6-PTS
o Phenylketonuria (PKU) Screening
Detecting Phenylketonuria (PKU) and treating it
Phenylketonuria (PKU) Screening
-Mass Spectrometry(MS/MS) is the current method of choice for many newborn screenings including PKU. A mass spectrometer simultaneously sorts many molecules in a blood specimen by weight (mass) and size, and also measures the quantity of each molecule.
• Sorts molecules in a blood specimen by size, weight, and quantity.
• Fast detection and high sensitivity.
• Measures multiple molecules simultaneously.
Timing of Test: The sensitivity of PKU screening is influenced by the age of the newborn when the blood sample is obtained. Phenylalanine level is typically normal in PKU babies at birth because of normal PAH in maternal supply and increases progressively with the initiation of protein feedings during the first days of life. Early detection and treatment is crucial to prevent irreversible damage to the developing brain. However, if tested too early (within 1-2 days of birth), some affected children can be missed.
• Newborns are tested first after birth and then again at their first pediatrician’s visit days later.
Guthrie test (aka Guthrie bacterial inhibition assay) for PKU was developed in the mid- 1960s. This test is based on the findings that phenylalanine inhibits the growth of the bacterium Bacillus subtilis and that such inhibition can be overcome by a high level of phenylalanine in the blood sample of a PKU baby.
o Phenylketonuria Treatment When treated early with low-phenylalanine diet, the mental retardation can be prevented. Phenylalanine is an essential amino acid and thus cannot be eliminated from the diet. The low-phenylalanine diet should be maintained throughout childhood and school years, and preferably the patient’s whole life. BH4-deficient PKU patients are treated with oral BH4, low-phenylalanine diet, and supplements (L-dopa and 5-hydroxytryptophan etc) to balance neurotransmitter levels.
Low-Phenylalanine Diet:
• From birth through childhood, and beyond
• For women, throughout child-bearing years
BH4 supplementation • Kuvan® : long-term safety and effectiveness?
Other treatments:
• Neutral amino acid supplementation
o Dietary supplementation with large neutral amino acids(LNAAs), with or without the traditional PKU diet is another treatment strategy. The LNAAs (e.g.leu,tyr,trp,met,his,ile,val,thr) compete with phe for specific carrier proteins that transport LNAAs across the intestinal mucosa into the blood and across theblood brain barrierinto the brain .
• Enzyme replacement therapy
• Gene therapy
o PKU: Maternal Effect
♣ Problem: Phenylketonuria women who are off low-Phe diet in pregnancy
• markedly increased risk of miscarriage.
• congenital malformations and mental retardation in babies.
Cause: elevated phenylalanine level in maternal circulation (due to being off diet)
• Prevention: maintain low-Phe diet for females with PKU throughout their child-bearing years.
1-antitrypsin deficiency (ATD)
1-antitrypsin deficiency (ATD) A disease where a protease inhibitor (Serpina1 gene) is inhibited, allowing the protease elastase run rampant, destroying connective tissue elastin in lungs, leading to lung emphysema and liver disease
Phenotype
ATD patients have a 20-fold increased risk of developing emphysema, with more severe symptoms among smokers. This disorder is late-onset, especially in non-smokers, but 80- 90% of deficient individuals will eventually develop disease symptoms. Many patients also develop liver cirrhosis and have increased risk of liver carcinoma due to the accumulation of a misfolded α1-AT mutant protein in the liver
Different conditions it leads to :
• Emphysema of lungs and lung disease
• Liver Cancer and Liver Cirhosis
Earlier and more severe symptoms in smoker (ecogenetics).
Mechanism of 1-antitrypsin deficiency (ATD)
α1-antitrypsin (ATT or SERPINA1) is made in the liver and secreted into plasma. SERPINA1 is a member of serpins (serine protease inhibitor), which are suicide substrates that bind and inhibit specific serine proteases. The main target of SERPINA1 is elastase, which is released by neutrophils in the lung. When left unchecked, elastase can destroy the connective tissue proteins (particularly elastin) of the lung, causing alveolar wall damage and emphysema.
♣ Deficiency in a1-antitrypsin protein (SERPINA1, AAT), a protease inhibitor
♣ SERPINA1 is a suicide substrate of the serine protease elastase.
• Elastase is released by activated neutrophils at the airway, destroying elastin in the connective tissues.
• It’s macrophages that release signals to summon neutrophils the neutrophils will release the Elastase
♣ ATD patients have an imbalance of elastase and SERPINA1 levels
ATD: Molecular Basis The SERPINA1 gene is on chromosome 14 (14q32.13). There are ~20 different mutant alleles, although the Z & S alleles account for most of the disease cases. The Z allele (Glu342Lys) encodes a misfolded protein that aggregates in the endoplasmic reticulum (ER) of liver cells, causing damage to the liver in addition to the lung. The S allele (Glu264Val) expresses an unstable protein that is less effective.
♣ ATD is caused by mutations in a1-AT (SERPINA1) gene.
♣ a1-antitrypsin (elastin inhibitor) is produced in the liver and transported to the lungs via the blood.
Three common M alleles encodes functional proteins.
• Z allele (Glu342Lys) is the most common mutant allele. (Glu at 342 is replaced by Lysine)
o Individuals with Z/Z genotype have ~15% of normal SERPINA1 level.
o The Z allele makes a protein that is not folded properly and tends to accumulate in the endoplasmic reticulum of liver cells, leading to liver damage.
S allele (Glu264Val) makes unstable SERPINA1 protein. o Individuals with S/S genotype has 50-60% of normal SERPINA1 level.
1-antitrypsin deficiency (ATD) Treatment and
effects of smoking on ATD
antitrypsin deficiency (ATD) treatment:
Two approaches of delivering human SERPINA1 to the pulmonary epithelium are being studied: intravenous infusion and aerosol inhalation.
♣ Inhaled bronchodilators and inhaled steroids
♣ O2 therapy and possibly lung replacement
♣ In the future: Enzyme Replacement Therapy and Gene therapy are possibilities
1-antitrypsin deficiency (ATD) and Smoking:
Ecogenetics (Ecogenetics) Smoking accelerates the onset of emphysema in ATD patients. Tobacco smoke damages the lung, prompting the body to send more neutrophils to the lung for protection. More neutrophils release more elastase, causing more severe lung damage. (Dont have α1-antitrypsin to inhibit elastase)
Tay-Sachs Disease (GM2 gangliosidosis type I)
Tay-Sachs is an inherited disorder that progressively destroys neurons in the brain and spinal cord. The most common form of T-S is an early-onset, fatal disorder apparent in infancy. T-S infants appear normal until the age of 3-6 months, when early symptoms such as muscle weakness, decreased attentiveness, and increased startle response appear. As the disease progresses, the T-S children experience symptoms of neurodegeneration including seizures, vision and hearing loss, diminishing mental function, and paralysis. An eye abnormality called “cherry-red spot” is a characteristic of T-S. Children of T-S usually live only till 3-4 years of age.
T-S is caused by a defective α subunit; only HexA activity is affected. Sandhoff disease is caused by a defective Hex A and defective Hex B, leading to defective Hexosaminidase A and B
Phenotype A fatal genetic disorder in children that causes progressive destruction of the central nervous system.
-T-S babies start to develop neurological deterioration around 3 - 6 months of age, and die by 2 - 4 years.
First signs - muscle weakness and a startle response at sudden sound.
Advanced Symptoms: Loss of voluntary movement, muscle seizure, mental retardation, vegetative state
Biochemical defects of Tay-Sachs disease T-S is a lysosomal storage disease. Inability to degrade GM2 ganglioside results in up to 300-fold accumulation of this sphingolipid inside swollen lysosomes in neurons of the central nervous system. A defective hexosaminidase A (HexA) needed for in metabolizing GM2 is responsible for T-S. HexA is a heterodimer of αβ, which are encoded by the HEXA and HEXB genes, respectively. Although HexA is a ubiquitous enzyme, the impact of T-S is primarily in the brain where most of GM2 ganglioside is synthesized.
T-S is a lysosomal storage disorder with (>300x) accumulation of GM2 ganglioside in the lysosome.
• GM2 ganglioside is primarily synthesized in neurons of the brain and a componentof neuron cell membrane.
♣ T-S patients are unable to degrade GM2 ganglioside because of a defective hexosaminidase A.
• Hexosaminidase A consists of two subunits: a and b, which are encoded by the HEXA and HEXB genes, respectively.
o T-S patients usually have mutations in the HEXA gene.
• Mutations in HEX B causes Sandhoff disease (GM2 gangliosidosis II),a similar lysosomal storage disorder.
Sandhoff disease
Sandhoff disease (GM2 gangliosidosis type II) presents the same neurological symptoms as T-S. Sandhoff disease patients have defects in both genes Hexosaminidase A and Hexosaminidase B (HexB) • T-S is caused by a defective α subunit; only HexA activity is affected. Sandhoff disease is caused by a defective Hex A and defective Hex B, leading to defective Alpha and Beta units in Hexosaminidase A • The α subunit gene HEXA and the β subunit gene HEXB reside on chromosomes #15 and #5, respectively.
GM2 ganglioside degradation requires three proteins
The first two proteins are the hexosaminidase A and the hexosaminidase B (the two proteins that enzymatic degrade GM2 ganglioside
GM2A Need this protein to recognize the ganglioside and bring it the hexosaminidase A and B complex for degradation
• If GM2A is defective, can lead to lysosome storage disease
GM2A
GM2A Need this protein to recognize the ganglioside and bring it the hexosaminidase A and B complex for degradation
• If GM2A is defective, can lead to lysosome storage disease
Recognize The effects of Tay-Sachs and Sandhoffs Disease
Tay-Sachs is caused by a defective α subunit; only HexA gene activity is affected.
Sandhoff disease is caused by a defective Hex A and defective Hex B, leading to defective Alpha and Beta units in Hexosaminidase A
If the GM2AP activator is defective, then you cant bring Ganglioside to the hexosaminidase
Screening for Tay-Sachs disease:
o Enzymatic activity assay: both HexA and HexB enzymes are present in the serum. Their activities can be distinguished in such assays because only HexA is inactivated by heat.
o Carrier Screening: primarily among Ashkenazi Jewish population, the enzyme test has 97% accuracy because carriers have lower HexA enzyme levels in the blood.
o Prenatal screening: the enzyme test can also be performed on cultured amniotic fluid cells to detect T-S fetus when both parents are known to be carriers. Notably, this screening has reduced the number of T-S cases by about 95% over the past 30 years.
o DNA testing: The tests currently available can detect about 95% of carriers in the Ashkenazi Jewish population and about 60% of carriers among non-Jewish individuals. Therefore, some carriers will be missed by DNA test alone.
Complex Traits
Complex Traits—> Multifactorial Inheritance
o For a large portion of diseases there are multiple genetic components and environmental components
o Complex traits aggregate in families
Need to distinguish between clustering in families due to genetic factors and those due to shared environmental factors
o Epidemiologic twin, adoption, and immigration studies used
-Each measure of genetic contribution needs to be interpreted carefully, but as a group can provide compelling evidence for genetic contribution to trait.
Concordance
Usually means the presence of the same trait in both members of a pair of twins. However, the strict definition is the probability that a pair of individuals will both have a certain characteristic, given that one of the pair has the characteristic.
o For example, twins are concordant when both have or both lack a given trait
o A twin study compares the concordance rate of identical twins to that of fraternal twins. This can help suggest whether a disease or a certain trait has a genetic cause
Twin Studies
o Compare monozygotic to dizygotic twins
If twins raised together and assume same degree of similar environment, then differences in concordance rate between mono- and dizygotic twins likely due to genetic factors
- Monozygotic Twins Share 100% of the same genetic code
- Dizygotic Twins Shares roughly 50% of the same genetic code
Twin studies provide a potential means of overcoming this problem. Monozygous (MZ) twins are identically matched for DNA sequence, age, and gender, and perhaps closely matched for environmental exposures. Dizygous (DZ) twins on average share 1/2 of their DNA sequences, but may be about as closely matched for other factors as are MZ twins. If it can be assumed that MZ and DZ twins are equally similar with respect to noninherited factors, then twins can be used to get an estimate of the relative contribution of genetic vs. environmental variation to the trait.
Compare Monozygotic twins raised apart
If twins raised apart and assume had different environments, then similarities in trait (high concordance rates) likely due to genetic factors.
• Share 100% same genetics, different environments with Environment 1 and Environment 2
o Adoption Studies
Compare similarity between biological siblings raised apart and adoptive siblings
• If biological sib 2 more concordant with biological sibling than adopted sibling, then have evidence for genetic component as opposed to environmental component.
o This means that despite being in a different environment, the biological twin 2 is more concordant with biological sibling 1 traits are genetic component
Risk of Disease in Relatives
o Can compare the frequency of disease in relatives of patients to see if higher than in general population
o Risk of disease in sibling/ risk of disease in general population= Risk of disease in relative
♣ Risk in general population ~ 0.4%
♣ Risk to sibs of type 1 diabetes patient ~ 6%
o S ~ 15 for type 1 diabetes.
Heritability
Heritability—> Proportion of variance in trait that is due to genetic variation (compared to non-genetic variation)
o The Heritability of a trait is the proportion of total variance in a trait that is due to variation in genes. A high heritability implies that differences among individuals with respect to a trait
♣ such as blood pressure in a population can be attributed to differences in the genetic makeup.
The key to interpreting heritability estimates is to remember that we’re talking about and describing variation in BOTH genetic factors AND non-genetic factors.
♣ If one (alleles or environment) doesn’t demonstrate much variability, then it doesn’t have much potential to explain variability in a trait.
o ***Implication: A high heritability does not imply that non-genetic factors are not important. A low heritability does not imply that environment is not important.
Allele and Locus Difference
Locus is that point on chromosome where the trait or gene is situated. Gene is found at a certain locus. Homologous chromosomes have same genes at the same locus but have different alleles. Gene mapping is used very commonly to determine locus in a certain biological trait. Hence, locus is a maker of DNA. Locus is usually referred as a chromosome marker. It could be called as a gene but it is used specifically to locate the position of chromosome on the gene. Single allele can be found on one single loci and that is what makes it distinct.
Characteristics of Complex Traits
Contains one or more of the following:
♣ Incomplete Penetrance= phenotype does not always show
♣ Variable Expressivity= different ranges/types of expression with the phenotype
♣ Heterogeneity – allele and locus
• The trait exists on different locuses or allels throughout the genome (on different chromosomes)
♣ Presence of phenocopies
Incomplete Penetrance
Penetrance: describes the relationship between trait and genotype
♣ The penetrance of a genotype is the probability that an individual will develop the trait if they have the genotype
Complete penetrance: everyone with pre-disposing genotype will get trait
Incomplete penetrance: some with genotype will not get trait
Susceptibility variants for complex diseases are generally thought to have low-penetrance
♣ Example: Type 1 diabetes: up to 20% of general population has one of two highest-risk haplotypes, but incidence of disease is only .4%
Incomplete penetrance
some with genotype will not get trait
♣ Susceptibility variants for complex diseases are generally thought to have low-penetrance
♣ Example: Type 1 diabetes: up to 20% of general population has one of two highest-risk haplotypes, but incidence of disease is only .4%
Variable expressivity
Variable Expressivity–>individuals with the same variant do not show precisely the same disease or quantitative phenotype characteristics.
o Light dimming: People will show variety of phenotype expressions with the mutated genotype
Allelic heterogeneity
Allelic Heterogeneity
o Different alleles in the same gene result in same trait
o Different alleles in the same gene result in different traits
Example: Cystic Fibrosis
• Many alleles appear to have very similar clinical progression of disease.
o Alleles can be grouped into classes; severity of lung and pancreatic involvement depends on allele class
CTRF Genotype: Example of Heterogeneity Based on the genes active, may or may not have pancreatic disease/ insufficiency. There is also a trait component for lungs
♣ Severe: two F508/F508 genes
• 85% more likely to have severe pancreatic disease
♣ Mild: R117H/F508
• 15% pancreatic insufficiency
Locus Heterogeneity
o Variants in different genes result in very similar clinical presentation
o Classic example: Early onset Alzheimer disease (AD)
♣ Mutations in 3 different genes all result in identical clinical presentation of early-onset AD
♣ Different Loci across chromosomes can bring on different types of Alzheimer’s disease
Phenocopy
Phenocopy—> environmentally caused phenotype that mimics the genetic version of the trait.
o Note that it can be argued that almost everything has some genetic component; intent here is that primary reason for the phenotype is not genetic.
Example: Thalidomide-induced limb malformation vs. genetically-induced
o Type 2 Diabetes as an example
Risk factors:
• increasing age
• obesity, physical inactivity
• family history
• prior gestational diabetes
• impaired glucose tolerance (IGT)
Associated with:
• LDL cholesterol, triglycerides, blood pressure
Control often attained with diet, exercise
o Type 2 Diabetes: Evidence for Genetic Component
♣ Studies in early 1900’s type 2 aggregates in families
• More recent studies
♣ Estimates of concordance rates for MZ twins:
• About 2 times those for dizygotic twins - 35%-100%
• Risk to 1st degree relatives of affected:
o ~ 3 to 4 times that of general population
♣ Intermediate traits of blood glucose and insulin levels have heritability estimates as high as 50%
Summary of Turner’s Syndrom
45,X, is a condition in which afemaleis partly or completely missing anX chromosome.[1]Signs and symptoms vary among those affected. Often, a short andwebbed neck,low-set ears, low hairline at the back of the neck,short stature, andswollenhands and feet are seen at birth. Typically they arewithout menstrual periods, do not developbreasts, and areunable to have children.Heart defects,diabetes, andlow thyroid hormoneoccur more frequently. Most people with TS have normal intelligence. Many, however, have troubles withspatial visualizationsuch as that needed formathematics.[2]Vision and hearing problems occur more often
No cure for Turner syndrome is known. Treatment, however, may help with symptoms.Human growth hormoneinjections during childhood may increase adult height.Estrogen replacement therapycan promote development of thebreastsand hips. Medical care is often required to manage other health problems with which TS is associated.
Inheritance
In the majority of cases where monosomy occurs, the X chromosome comes from the mother.[37]This may be due to anondisjunctionin the father.Meioticerrors that lead to the production of X with p arm deletions or abnormal Y chromosomes are also mostly found in the father.[38]IsochromosomeX orring chromosomeX on the other hand are formed equally often by both parents.
Autosomal Dominant
Manifests in homozygotes and heterozygotes
♣ Will see it in either RR or an Rr, the dominant allele will reign supreme
o Equal in males and females
-Is not gender linked
o Can be passed by either parent
Achondroplasia- Dwarfism
o Autosomal dominant o Most common skeletal dysplasia o 1 in 15,000-40,000 newborns o 80% new mutation rate o 100% penetrance ♣ They will show the disease for sure
o Clinical Meifestation
♣Small stature Males 4’ 3” Females 4’
Rhizomelic limb shortening
Short fingers
Genu varum
Trident hands
Large head/frontal bossing
Midfacial retrusion Their nose and cheeks look bound
Small Foramen Magnum/Craniocervical instability
• Small hole in the head
o Gene of Achondroplasia Mutation of the FGFR3 gene leads to an amino acid substitution (missense mutation) ultimately affecting bone growth by limiting bone formation from cartilage
FGFR3
Fibroblast Growth Factor Receptor 3
Regulates bone growth by limiting the formation of bone from cartilage
Chromosome 4p16.3 nucleotide 1138
♣ Amino acid substitution – missense mutation c.1138G>A;p.Gly380Arg
♣ Mutation increases the activity of the protein interfering with skeletal development
o Nucleotide 1138 of the FGFR3 gene has the highest new mutation rate known in man
Like Down’s Syndrome with women—>As men grow older, the paternal passing down of FGFR3 will mutate
• Aging= greater risk of mutated FGFR3
Retinoblastoma
Malignant tumor of the retina
o 1 in 15,000 live births
o RB1 gene on chromosome 13
RB protein is part of the cell cycle
o Retinoblastoma Associated protein regulates the cell cycle
o 90% penetrance
♣ Will have a 90% chance of showing the disease
Sometimes will not see it in the parents, still are carrying the gene and they will likely pass it down
oThis happen when you see the white reflection (bad) in youths
-Want a healthy red reflection for normal retina health
Neurofibromatosis Type 1
Neurofibromatosis Type 1: Is a tumor disorder that is caused by the mutation of a gene on chromosome 17 that is responsible for control of cell division. NF-1 causes tumors along the nervous system and can grow anywhere on the body. NF-1 is one of the most common genetic disorders and is not limited to any persons race or sex there are at least 100,000 people currently in the U.S. who have been diagnosed with NF. Common symptoms of NF-1 include brownish-red spots in the colored part of the eye called Lisch nodules, benign skin tumors called neurofibromas, and larger benign tumors of nerves called plexiform neurofibromas. o Autosomal dominant o 1 in 3000 births o 50% new mutation rate o Exhibits Variable Expressivity
NIH Diagnostic Criteria 2 or more of the following
6 or more café-au-lait spots
•Little blotches on the skin, will develop over time
2 or more neurofibromas
• Light brown raised skin (larger than moles)
• Will cluster
1 plexiform neurofibroma
♣ Freckling in the axillary or inguinal area
Optic glioma
♣ 2 or more Lisch Nodules
• These are freckling in the eyes
♣ Distinctive osseous lesions
♣ Affected first degree relative
o Mutation for Neurofibromatosis Type 1 NF-1 is a microdeletion syndrome caused by a mutation of a gene located on chromosomal segment 17q11.2 on the long arm of chromosome 17 which encodes a protein known as neurofibromin
♣ NF1
• Neurofibromin- tumor suppressor gene
♣ Chromosome 17q11.2
• Loss of function mutation
• Over 1000 mutations have been described on this c
♣ Although considered dominant must have a mutation in both genes to show the phenotype of NH1
• If you carry this mutation you are still at major risk
• If you have inherited one mutation, you are more likely to develop the second mutation De novo
o Possibly will develop the second mutation and have Neurofibromatosis Type 1
Locus Heterogeneity
A mutation in more than one locus causing the same clinical condition
Summary and Genetic Component of Tuberous Sclerosis
TS is a rare multi-system genetic disease that causes benign tumors to grow in the brain and on other vital organs such as the kidneys, heart, eyes, lungs, and skin. A combination of symptoms may include seizures, intellectual disability, developmental delay, behavioral problems, skin abnormalities, lung and kidney disease. TSC is caused by a mutation of either of two genes, TSC1 and TSC2, which code for the proteins hamartin and tuberin respectively. These proteins act as tumor growth suppressors, agents that regulate cell proliferation and differentiation.[1]
Mutation in Tuberous Sclerosis TSC1 encodes for the protein hamartin, is located on chromosome 9 q34. TSC2 encodes for the protein Tuberin, is located on chromosome 16 p13.3 The Hamartin and the Tuberin protein both regulate cell growth and proliferation TSC1, TSC2 -Encode hamartin and tuberin proteins • Regulate cell growth and proliferation Chromosome 9 and 16 -Loss of function mutations
Osteogenesis Imperfecta Type 1
•Osteogenesis Imperfecta Type 1 also known as brittle bone disease or Lobstein syndrome,[1] is a congenital bone disorder characterized by brittle bones that are prone to fracture. People with OI are born with defective connective tissue, or without the ability to make it, usually because of a deficiency of type I collagen.[2] Eight types of OI can be distinguished. Most cases are caused by mutations in the COL1A1 and COL1A2 genes.
♣ Autosomal dominant
♣ 1 in 30,000-50,000
♣ Exhibits Variable Expressivity
o Clinical Manifestations of Osteogenisis Imperfecta Type 1 ♣ Multiple fractures ♣ Mild short stature ♣ Adult onset hearing loss ♣ Blue sclera not always the case o Mutation of Osteogenisis Imperfecta 1 ♣ COL1A1
• Collagen type 1 alpha 1
♣ Chromosome 7q21.3
♣ Reduced production of pro-alpha 1 chains that reduces the type 1 collagen production by half
♣ Molecular and biochemical testing available
Marfan Syndrome
Marfan Syndrome Is a genetic disorder of connective tissue. It has a variable clinical presentation, ranging from mild to severe systemic disease. The most serious manifestations involve defects of the heart valves and aorta, which may lead to early death if not properly managed. The syndrome also may affect the lungs, eyes, dural sac surrounding the spinal cord, the skeleton, and the hard palate. People with Marfan syndrome tend to be unusually tall, with long limbs and long, thin fingers and toes.
♣ Autosomal dominant
♣ 1 in 5000 births
♣ 25% new mutation rate
♣ Variable Expressivity
o Clinical Manifestation of Marfan Syndrome
♣ Systemic disorder of connective tissue
Connective Tissues are the tissues in our body including muscle, ligaments and tendons. Connective tissues are also part of our vascular system, most importantly in the large arteries in our bodies
♣ Ocular
♣ Skeletal
♣ Cardiovascular
o Diagnosis:
o Scoliosis, Thumb and wrist sign, Pectus Excavatum
♣ Signs for the disease
o Ectopia Lentis
♣ Displaced lens
o Cardiovasuclar System
♣ Looking at the Aorta, seeing if it has a larger Diameter
♣ No family History • Aortic root enlargement • Plus one of the following: o Ectopia Lentis ♣ Displaced lens o FBN1 mutation o Systemic score >7 ♣ Positive Family History • Ectopia Lentis • Systemic score of >7 • Aortic root enlargement
Mutation in the Mafran syndrome rfan syndrome is caused by mutations in the FBN1 gene on chromosome 15,[14] which encodes fibrillin-1, a glycoprotein component of the extracellular matrix. Fibrillin-1 is essential for the proper formation of the extracellular matrix, including the biogenesis and maintenance of elastic fibers. The extracellular matrix is critical for both the structural integrity of connective tissue
FBN1
• Fibrillin- extracellular matrix protein
Chromosome 15q21.1
• Dominant negative activity mutation
• A mutation whose gene product adversely affects the normal, wild-type gene product within the same cell
♣ Severe reduction in the number of microfibrils
• Slipped Mispairing
o Mispairing of bases in regions of repetitive DNA replication coupled with inadequate DNA repair systems
o As the repeat grows longer the probability of subsequent mispairing increases
Trinucleotide Repeat Disorders
Trinucleotide Repeat Disorders are a set of genetic disorders caused by trinucleotide repeat expansion, a kind of mutation where trinucleotide repeats in certain genes exceed the normal, stable threshold, which differs per gene. The mutation is a subset of unstable microsatellite repeats that occur throughout all genomic sequences.
o Expansion of a segment of DNA consisting of three or more nucleotides
Just adding a bunch of repeating trio’s of nucleotides
♣ CAGCAGCAGCAGCAGCAG
o Slipped Mispairing
♣ Mispairing of bases in regions of repetitive DNA replication coupled with inadequate DNA repair systems
♣ As the repeat grows longer the probability of subsequent mispairing increases
o Anticipation In genetics, anticipation is a phenomenon whereby as a genetic disorder is passed on to the next generation, the symptoms of the genetic disorder become apparent at an earlier age with each generation. In most cases, an increase of severity of symptoms is also noted. Anticipation is common in trinucleotide repeat disorders
Severity and/or onset of disease increases in next generation
o
Parental Transmission Bias
♣ Trinucleotide expansion more prone to occur in gametogenesis of the male or the female
o AD, AR and X-linked transmission
Anticipation
Anticipation In genetics, anticipation is a phenomenon whereby as a genetic disorder is passed on to the next generation, the symptoms of the genetic disorder become apparent at an earlier age with each generation. In most cases, an increase of severity of symptoms is also noted.
Anticipation is common in trinucleotide repeat disorders
-Severity and/or onset of disease increases in next generation
EX: Huntington disease: More trinucleotides added with each generation, more likely to develop disease sooner with greater severity
o Parental Transmission Bias
♣ Trinucleotide expansion more prone to occur in gametogenesis of the male or the female
o AD, AR and X-linked transmission
Huntington Disease
Huntington Disease is a neurodegenerative genetic disorder that affects muscle coordination and leads to mental decline and behavioral symptoms ♣ Autosomal Dominant ♣ Trinucleotide repeat disorder (CAG) ♣ 1 in 10,000 ♣ Anticipation ♣ Parent of origin • Early onset – paternal • Later onset - maternal ♣ Clinical Manifestations:
Progressive neuronal degeneration causing motor, cognitive and psychiatric disturbances
• Age of onset 35-44
• Death approximately 15 years after onset
♣ Mutation: HD is one of several trinucleotide repeat disorders which are caused by the length of a repeated section of a gene exceeding a normal range. The HTT gene is located on the short arm of chromosome 4at 4p16.3. HTT contains a sequence of three DNA bases—cytosine-adenine-guanine (CAG)—repeated multiple times (i.e. … CAGCAGCAG …), known as a trinucleotide repeat. CAG is the 3-letter genetic code (codon) for the amino acid glutamine
•HTT—> Huntingtin
o Chromosome 4p16.3
• The expansion of glutamine may cause an altered structure or biochemical property of the protein
Myotonic Dystrophy Type 1
Myotonic Dystrophy Type 1 ♣ Autosomal Dominant ♣ Trinucleotide repeat disorder (CTG) ♣ 1 in 20,000 ♣ Anticipation ♣ Maternal transmission
o Clinical Manifestations ♣ Adult onset muscular dystrophy ♣ Progressive muscle wasting and weakness ♣ Myotonia ♣ Cataracts ♣ Cardiac conduction defects
o Mutation In DM1, the affected gene is called DMPK, which codes for myotonic dystrophy protein kinase,[2] a protein expressed predominantly in skeletal muscle.[3] The gene is located on the long arm of chromosome 19.
DMPK—>Myotonic dystrophy protein kinase
♣ Chromosome 19q13.3
♣ Plays important role in muscle, heart and brain cells
o CTG Repeat for Mystonic Dystrophy In DM1, there is an expansion of the cytosine-thymine-guanine (CTG) triplet repeat in the DMPK gene. Between 5 and 37 repeats is considered normal, while individuals with between 38 and 49 repeats are considered to have a pre-mutation and are at risk of having children with further expanded repeats and, therefore, symptomatic disease.[5] Individuals with greater than 50 repeats are almost invariably symptomatic, with some noted exceptions.[ref] Longer repeats are usually associated with earlier onset and more severe disease.
♣ 5-34 normal
♣ 34-49 premutation range
• Risk at having children with more repeats and the disease
♣ >50 full mutation with 100% penetrance
Hemoglobin Structure
Hemoglobin is the main oxygen carrier in the body
o Consists of four subunits called globin: 2 alpha and 2 beta (or beta like) chains
Each subunit has a polypeptide chain (alpha or beta) that is called a globin, a prosthetic group(heme) which carries an Iron
• The iron gives the oxygen transporting capabilities
o Hemoglobin states
♣ Tense state: Deoxygeneated, difficult to bind O2 with high binding affinity, need to have a high pO2 such as in the lungs (will allow the hemoglobin to get past tense state)
♣ Relaxed State: The hemoglobin changes shape, and will have it’s four heme’s carry oxygen, has a much lower binding affinity
• Drop off oxygens in the tissues (pH activasion)
o The Alpha and alpha like genes are located on chromosome 16 while the Beta and Beta like genes are located on chromosome 11
Chromosome 11 Will start with gamma chains in fetal, switch to beta after fertilization
Summary of hemoglobin chromosomes
The Alpha and alpha like genes are located on chromosome 16 while the Beta and Beta like genes are located on chromosome 11
Chromosome 11 Will start with gamma chains in fetal, switch to beta after fertilization
Globin switching
Globin switching—>Expression of various globin through development
o On the Beta chain of chromosome 11, the Genes are in the order that will be produced Start with Hemoglobin F (has 2 alpha and 2 gamma chains) during fetal development.
After birth Hemoglobin A (2 alpha and 2 beta) become predominant
Hb F and Hb A
During Fetal life, the Hemoglobin F is the predominant expressed hemoglobin
• Alpha chains are produced constantly through life on chromosome 16
• The genes of Globin gamma are closest to the locust control (start of the gene) on chromosome 11
o Hence gamma and alpha globin/chains will be made first
Come time of Birth, Beta-chain synthesis will begin to take over, and by 3 months of age, the Hemoglobin A will be dominant (2 alpha 2 beta chains)
•
The Beta globin genes are further down on the DNA expression area
♣ From Yolk Sac to Adult, Different hemoglobin take over
Alpha Hemoglobin is roughly always close to 50% dominant
o During Fetal period, gamma will take up around 40%
o After birth, and over the next several months, The Beta Globin/chains will take over and transition to the 40-50% amount
o Alpha Chains Always Exist around 45-50% throughout life
Need some way to control the Beta globin gene and when its going to be expressed Especially since it doesn’t start till around birth
Locus Control Region A regulatory element of DNA that is upstream of the Beta-Globin complex
o Critical proteins and activators will turn on Locus Control Region during birth
o If the Locus Control Region for Beta globin is deleted Then the patient CANNOT make Beta chains and has the more severe form of Beta thalassemia
♣ Will need to shoot up gamma-globin production if the Locus control region for Beta
o Locus Control Region (LCR), which is located at the most upstream region of each cluster. It is currently thought that the distance between the LCR and a particular globin gene affects its expression. The LCR presumably makes physical contact with the promoter and/or negative regulatory regions via specific transcriptional factors to influence gene expression.
♣ Deletions of the entire LCR of the beta cluster cause beta-thalassemias, a condition in which zero β-globin synthesis leads to precipitation of the α-globin chains.
Structural Variants Hemoglobin Disease
Altered components of the globin (the chain protein)
o Protein still made, just defective
o Example: Sickle Cell disease
o HbS or HbC trait Both sickle cell anemia and hemoglobin CC disease are of autosomal recessive inheritance. Sickle cell trait (HbS trait) or hemoglobin C trait (HbC trait) describes the conditions expressed in individuals who are heterozygous of HbS/HbA and HbC/HbA, respectively. They are clinically normal except when under severe low pO2 stress.
Sickle Cell Anemia
Sickle Cell Anemia
Single base mutation at codon #6 in the β-globin gene changes glutamate to valine. HbS is 80% less soluble than HbA when not bound to O2, and polymerizes into long fibers that distort the RBC into a characteristic sickle shape.
o These sickled cells become lodged in the micro-capillaries and further exacerbate the sickling crisis.
-Due to HbS being 80% less sobule than HbA
o The loss of red blood cell elasticity is central to the pathophysiology of sickle-cell disease. Normal red blood cells are quite elastic, which allows the cells to deform to pass through capillaries. In sickle-cell disease, low-oxygen tension promotes red blood cell sickling and repeated episodes of sickling damage the cell membrane and decrease the cell’s elasticity. These cells fail to return to normal shape when normal oxygen tension is restored
o Is typically considered an autosomal recessive disease
Hemoglobin C Disease: HbCC
HbCC
Caused by a single base mutation at codon#6 of the β-globin gene, changing glutamate to lysine.
o A milder form of hemolytic anemia than sickle cell anemia. Hemoglobin C disease (HbCC) A milder form of hemolytic anemia than sickle cell anemia. Caused by a single base mutation at codon#6 of the β-globin gene, changing glutamate to lysine.
♣ HbC is less soluble than HbA and tends to form crystals, reducing the deformability of RBC. (glutamate to lysine at codon six)
♣ Is also an autosomal recessive disease
DNA Diagnosis: Sickle Cell Anemia
DNA Diagnosis: Sickle Cell Anemia Use Southern Blot to find the different sizes based
o Both diseases will sepperate at different sizes on a southern blot test (after they are given southern probes)
o Hemoglobin electrophoresis
♣ is ablood testthat can detect different types ofhemoglobin. It uses the principles ofgel electrophoresisto separate out the various types of hemoglobin and is a type ofnative gel electrophoresis. The test can detect abnormal levels of HbS, the form associated withsickle-cell disease, as well as other abnormal hemoglobin-related blood disorders, such ashemoglobin C. Different hemoglobins have different charges, and according to those charges and the amount, hemoglobins move at different speeds in the gel whether in alkaline gel or acid gel. The hemoglobin electrophoresis is also known to be thalessemia screening, this also can be helpful for the patient who is frequently need of fresh blood transfusion
Symptoms of altered Hb-O2 binding
HbKempsey The O2 affinity of the hemoglobin is too high, not letting O2 go in the tissues
♣ Polycythemia Over production of Red blood cells
• Happens due to less O2 in the tissues, body is signaled to produce more Red blood cells
HbKansas too low of an O2 affinity to the hemoglobin, won’t carry enough O2 on RBC
♣ Have lower O2 in the red blood cells
♣ Cyanosis Bluish skin
Thalassemias
Thalassemias are caused by an imbalance in the relative levels of the α and β globin chains, which leads to the precipitation of the globin in excess and decreases the life span of the RBC.
Alpha Thalassemia
• low or zero alpha-globin
o beta- & gamma-globin in excess & precipitates.
• both fetal and postnatal defects.
• usually caused by deletion of the a-globin gene(s).
Beta Thalassemia
• low or zero beta-globin, alpha-globin in excess & precipitates.
• postnatal defects only.
• usually caused by point mutations in the b-globin gene.
• occasionally caused by deletions in the LCR or the b-gene cluster.
o Low or zero synthesis of one globin chain
♣ Due to the gene being altered
α-Thalassemias
o Mostly caused by deletions of one or both copies of the α-globin gene in the α-cluster. Thus, γ- and β-globin are in excess. Affects the formation of both fetal and adult hemoglobins.
α-thal-1 allele (- -) Common in Southeast Asia. Caused by deletion of both copies of α-globin genes in the α- cluster. Homozygous state (- -/- -) results in “hydrops fetalis” (stillborn). Most fetal hemoglobin is γ4 (Hb Bart’s) although there is enough ζ2γ2 (Hb Portland) to sustain fetal development. Heterozygotes (αα/–) have mild anemia, a.k.a. α-thalassemia-1 trait.
α-thal-2 allele (α -) Common in Africa, Mediterranean, and Asia. Deletion of one of the two α-globin genes in the α-cluster. 50% decrease in α-globin synthesis. No disease phenotype in heterozygote (αα/α-, silent carrier). Mild anemia in homozygotes (α-/α-), a.k.a. α-thalassemia-2 trait.
α-thal-1/α-thal-2 (α -/- -) Compound heterozygous individuals with only 25% of normal α-globin level. Severe anemia. a.k.a. HbH disease. About 5-30% of their hemoglobin is β4 (HbH), which precipitates.
HPFH
HPFH (hereditary persistent fetal hemoglobin) is a benign condition in which significant fetal hemoglobin (hemoglobin F) production continues well into adulthood, disregarding the normal shutoff point after which only adult-type hemoglobin should be produced.
o No δ or β synthesis because of deletions of both genes. Increased γ-globin expression caused by either of the two following mechanisms:
(1) extended deletion of additional downstream sequences, which likely brings a cis-acting enhancer element closer to the γ-globin gene, or
(2) mutations in the promoter region of one of the two γ-globin genes that destroy the binding site of a repressor, thereby relieving postnatal repression of γ.
HPFH individuals are disease free, since adequate levels of γ chains are still made due to the disruption of the perinatal globin switch from γ to β. 100% of hemoglobin is HbF (α2γ2), which is about 17-35% of normal level of total hemoglobin production. HPFH individuals have higher HbF (17-35%) level than δβ0 -thalassemia individuals (5-18%).
Significance of HPFH Understanding the mechanism of HPFH may make it possible to express HbF at high levels postnatally to treat patients of β-thalassemia and sickle cell anemia.
Four major Mutations
1) Loss of Function/Loss of protein function (greatest majority)
-Protein is abnormal (enzyme doesn’t work, it gets degraded
• Decreases necessary function
Typically flaw in coding region (whether it’s DNA or the downstream product RNA)
2) Gain of Function
3) Novel Property mutations
4) Ectopic or heterchronic (Altered Expression Mutation?)
♣ uncommon, except for cancer
