Week 2 Lecture- Genetic Disorders Flashcards
1
Q
What are the 3 types of genetic diseases discussed?
A
- Mendelian disorders
- Chromosomal abnormalities
- Complex multigenic disorders
2
Q
What are Mendelian disorders?
A
- Monogenic disorders
- Usually rare
- Single gene defect causes the disorder
- Has to be a survivable defect
3
Q
What are chromosomal abnormalities?
A
- Extra chromosomal material or structural defect in a chromosome
- Uncommon
4
Q
What are complex multigenic disorders?
A
- Complex interaction of multiple genetic defects with environmental factors (predisposition)
- Genetic defect predisposes you, then an initiating event causes the disease
- Far more common
5
Q
What does autosomal mean?
A
Located on chromosomes 1-22 rather than X or Y
6
Q
What is nondisjunction?
A
Failure of 2 homologous chromosomes (or 2 sister chromatids) to separate at metaphase of meiosis I (or meiosis II/mitosis)
- Problem with separation
7
Q
What is penetrance?
A
Whether or not the variant genotype can be inferred on the basis of defined phenotypic criteria
- Likelihood that we can determine the patient has the genetic abnormality based on appearance of the phenotype (symptoms)
- Another way to think about it is–> How likely is the altered genetic pattern to cause symptoms
8
Q
Overview Mendelian disorders
A
- Single mutation (monogenic) so follow Mendelian inheritance
- High penetrance
- Contribute to understanding of physiology- ex: chloride channel function in the lung
- Rare unless selective pressure- ex: Malaria
9
Q
List the Mendelian disorders we are focusing on
A
- Neurofibromatosis
- Ehlers-Danlos Syndrome
- Phenylketonuria
- Sickle Cell Anemia
- Fragile X Syndrome
10
Q
What specific type of monogenic disorder is neurofibromatosis?
A
Autosomal dominant disorder- genes are located on chromosomes 1-22, one mutation in one gene copy produces disease
11
Q
How many types of neurofibromatosis are there and what are the differences?
A
- Type I (mutation in neurofibromin)- common (1:4000), Highly variable course
- Type II (mutation in merlin)- Rare (1:40,0000/50,000), range of tumors
Both are involved in the formation of growths/tumors
12
Q
How does neurofibromatosis present?
A
- Neurofibromas
*Peripheral nerve myelin sheath tumors- overgrowth of cells in myelin sheath
*Plexiform- infiltration in towards nerve fivers themselves; can produce disruptions in nerve transmissioin
AND ALSO
*Solitary- growing out and do not infiltrate - Optic nerve gliomas
- Lisch nodules- pigmented nodules of the irus
- Cutaneous hyperpigmented macules (spots on skin)- very dark; not freckles or moles
13
Q
What protein is mutated in type I neurofibromatosis?
A
Neurofibromin I (NF1)
14
Q
Describe the effect of NF1.
A
- Mutated in neurofibromatosis I
- Higher expression in nerve cells with low expression in all other cell types
- GTPase that inhibits Ras–> responsible for regulation of growth and differentiation
- Loss of neurofibromin results in unregulated cell growth
-Absence of NF1= excessive Ras activity
*Signaling to Ras is associated with increased proliferation or survival of cells, even in the presence of death signals
15
Q
Why is Ehlers-Danlos Syndrome called a syndrome?
A
There are multiple types
16
Q
Overview of Ehlers-Danlos Syndrome
A
- Multiple types- still considered monogenic because each type is associated with a different mutation
- Defect in fibrillar collagen in CT
- Synthesis or Structure
17
Q
What are the symptoms of Ehlers-Danlos Syndrome?
A
- Elastic skin
- Bruises easily
- Overly flexible joints
18
Q
Mutations in which types of collagen are known?
A
I, III, and v
19
Q
Name 2 proteins that are required for processing of collagen into fibrillar form that have known mutations.
A
- Lysyl Hydroxylase (Kyphoscoliosis- deformities in spinal cord)
- Procollagen N- Peptidase (Dermatosparaxis- excessively loose skin)
20
Q
What is fibrillar collagen?
A
- Collagen that undergoes structural modification to become a fibrillar collagen, which gives it greater tensile strength
21
Q
Fibrillar collagen is a major component of what areas of the body?
A
- Bone
- Tendon
- Cartilage
- Skin
- Blood vessels
22
Q
Fibrillar cartilage is important for….
A
- Wound healing
- Scar formation
23
Q
What are some of the complications of Ehlers-Danlos Syndrome?
A
- Skin is stretchable, fragile, and more vulnerable to trauma–> compromised wound healing
- Lack of tensile strength leaves gaping wounds
24
Q
What are the serious internal complications of Ehlers-Danlos?
A
- Colon rupture
- Large blood vessel rupture
- Ocular (cornea rupture, retinal detachment)
- Diaphragmatic hernia
25
What is phenylketonuria?
- Metabolic disorder
- Autosomal recessive
- Produces severe intellectual disability due to accumulation of phenylalanine, which happens because there is a failure in amino acid catabolism
*Patients cannot break down phenylalanine
- Detected in urine using iron compound--> add ferric chloride to urine and it turns olive green
- Infants are tested shortly after birth because early dietary intervention is important
26
Describe phenylalanine catabolism.
- Phenylalanine is converted to tyrosine by phenylalanine hydroxylase (PAH)
- Mutation in the PAH enzyme causes PKU
- Absence of PAH causes tyrosine (Tyr) to become an essential aa
- Normal patients can break down Phe themselves to make Tyr
- Patients with PKU do NOT have a functional PAH and need to get tyrosine from the diet
27
How many disease- associated PAH alleles have been discovered worldwide?
500
28
What is the most severe phenotype of PKU associated with?
Null mutations
- Patients don't express any PAH
- They get massive accumulation of Phe because they cannot process it at all except to use it in making proteins or excrete it through urine
-
29
What are the 2 types of mutations associated with PKU?
1. Null mutations
2. Missense mutations
30
How can missense mutations in PKU be treated?
- Treated with chaperones that prevent degradation of misfolded enzyme
- Patients can make the protein, but it does not fold or work quite right
- For these patients, you can increase the likelihood that the misfolded protein will be degraded
- Symptoms not as severe
31
Describe the clinical presentation of PKU.
- Postnatal growth retardation
- Moderate-severe intellectual disability
- Recurrent seizures
* CNS symptoms due to Phe accumulation
* Inhibits AA transpor across the blood-brain barrier
* Secondary effects- protein synthesis, energy production, neurotransmitter homeostasis
- Hypopigmentation (pale)
* Product of Phe catabolism is a melanin substrate
- Eczematous skin rashes
32
What is the current treatment for PKU?
- Dietary--> reduce Phe intake
- Alternatives include gene therapy
33
Can an infant suffer symptoms of PKU even if they do not possess the genetic mutation?
YES
- If the mother has PKU, the infant is getting excessive Phe from the mother's blood during pregnancy
- The fetus will suffer symptoms at birth even though it does not have the disease itself
- Discourage breastfeeding after birth (formula)
34
What is the clinical presentation of sickle cell anemia?
- Hemolytic anemia
- Chronic hypoxia
* Generalized impairment of growth and development
* Spleen, heart, kidney, and lung damage
- Vaso-occlusive crisis
* Hypoxic injury and infarcts
- Increased susceptibility to infection (has to do with damage to the spleen)
35
What is hemolytic anemia?
Decrease in RBCs because RBCs are being lysed
36
What is the major issue caused by anemia?
Decreased O2 in the blood
37
What protein is affected by sickle cell anemia?
Hemoglobin
- Mutation in the beta subunit of the protein
38
How do we go from a mutation in the beta2 subunit to hemolytic anemia?
- Mutation produces a sticky patch
* Mutation goes from serine to a valine, producing a hydrophobic (sticky) patch on HbS that wants to bind to things so it is not exposed to aqueous environment
- Binds to deoxygenation site (aggregation)--> polymerization of Hb
- Normal HbA will terminate aggregation--> stops polymerization
- The amount of deoxygenated sickled hemoglobin is what determines the severity of disease
39
How does sickle cell disease cause hemolysis?
- HbS aggregates are insoluble
- Form long, helical fibrils (longer than the RBCs are wide)
- Twist RBC into characteristic sickle shape
- More fragile and likely to rupture, especially if moving at high rates through smaller blood vessels--> hemolytic anemia
* In the spleen
* In small blood vessels
- More likely to adhere to endothelial surface and one another due to change in expression of adhesion molecules
* Possible vaso-occulusion
40
Name the primary effects of sickling.
1. Chronic hypoxia
2. Hemolytic anemia
3. Bone marrow hyperproliferation to compensate for loss of RBCs
- Affects bone growth (characteristic deformation)
4. Decreased nitric oxide (NO), a vascular dilator
- Released Hb binds to NO, thereby depleting it
- Decreases NO levels in blood, causing vasoconstriction
41
Describe the mechanism of vaso-occlusion.
- Sickled RBCs block microvasculature
- Reduced NO causes constriction of blood vessels--> increases risk of blockage (occlusion)
- Increased expression of adhesion molecules produces aggregates of RBCs
- Vicious cycle
* Blockage causes localized hypoxia
* Hypoxia induces HbS aggregation, more hemolysis, and more blockages
* Vaso-occlusive crisis results from the vicious cycle because the first blockage induces more blockages
42
What causes pain crisis in sickle cell?
- Pain associated with hypoxic injury
- Local infarct--> tissue will signal damage to nervous system, which we detect as pain
- Young children--> can be masked as an osteomyelitis (bone infection)
- Sickle cell patients are also prone to certain types of infections
43
Why are sickle cell patients at increase risk of bacterial infections?
- Specific to encapsulated bacteria--> host phagocytes do not recognize encapsulated bacteria
- Immune tissue in spleen participates in opsonization
* Opsonization: binding antibodies and complement molecules to allow recognition by phagocytes
- Damage to spleen from injury or sickle cell may cause compromised ability to opsonize bacteria= more prone to infections by certain encapsulated bacteria
44
Why is splenic injury more of an issue in children and not so much in adults?
- Children have not fully developed their immune systems yet
- If there is a splenic injury in adulthood, the body can still handle most immunological issues because the immune system is fully mature.
45
Can someone who is heterozygous for HbS still have symptoms?
Yes, less severe
46
What is the benefit of being heterozygous for HbS?
- 70-90% protection against severe malaria
- 50% protection against uncomplicated malaria--> 50% less likely to develop the disease
- Precise mechanism unknown
* Protected against establishment of parasitemia
* Protected against progression to symptomatic disease
47
What is the phenotype of fragile X syndrome?
Intellectual disability
- Characteristic physical appearance is not always present
* Can be subtle
* Long face, large jaw
- Frequency:
* Males: 1 in 1550
* Females: 1 in 8000
- 30-50% of carrier females are affected, which is a high percentage
48
What is the name of the mutated gene that is associated with Fragile X Syndrome?
FMR1- familial mental retardation 1
- Mutation not actually in the coding region of the gene
- When this protein is expressed, the protein produced is normal
- Mutation is in the untranslated region upstream of the coding region in 5' UTR
* Multiple tandem CGG repeats in the 5' UTR
- Normal population: 6-55 CGG repeats, avg. 29
- Carriers: 50-200
- Affected: 200-400
- CGG repeats promote DNA methylation
49
What does DNA methylation do?
Represses DNA expression by reducing availability for transcription
- If you get methylation in CpG island, it increases the likelihood of histone deacetylase (HDAC) attaching to the area
* This then increases the likelihood of chromatin packaging of the gene
- Because of the high # of CpG islands in this region of this gene, this gene is very likely to be wrapped around a histone
* This restricts the ability to transcribe the gene
- When FMR protein is made, it is normal, but made at much lower levels
50
Why is it called Fragile X?
The chromosome is physically fragile in the area with so many CGG repeats
- The chromosome may break
- Cytogenetic abnormality is constriction of the long arm of the X chromosome
* Cleavage of the long arm
51
What is FMRP and how does it result in intellectual disability?
FMR1 product is FMRP (FMR protein)
- FMRP is cytoplasmic protein highly expressed in brain and testes
- RNA binding protein associated with polysomes
* Neurons produce lots of protein, have lots of ER and ribosomes (polysomes) to make protein--> specifically at synapses
- Regulates protein synthesis at synapses
- Excessive methylation represses FMRP expression
* Reduced expression of FMRP affects ability to synthesize proteins at synapses
- Altered synaptic function leads to intellectual disability
52
Which diseases did we discuss under the category of chromosomal abnormalities?
1. Trisomy 21 (Down Syndrome)
2. DiGeorge Syndrome
53
Describe chromosomal abnormalities as a whole.
- Structural or numerical alteration in the chromosomes
- Rare
- High penetrance
- Karyotyping to analyze chromosomes
54
What is karyotyping?
- Take cells that have chromatin condensed into chromosomes
- Take chromosomes and stain with Gimsa--> histological stain with methylene blue and eosin
* This stains chromosomes in a nice banding pattern (G-bands)
- Map genes on chromosomes using G-banding patterns
55
What is the phenotype of Trisomy 21?
Down Syndrome
- Intellectual disability
- Characteristic appearance
* short stature relative to family members
* Epicanthal fold- fold of skin on inner eyelid
* Short broad hands
* Protruding tongue
56
What causes Trisomy 21/ Down syndrome?
Meiotic Non-disjunction
- Depending on where you get non-disjunction, you get different types of gametes
* Happens during meiosis I= 2 gametes with 2 copies and 2 gametes with no copies--> likely that gametes with no copies are not viable
* Happens during meiosis II= 2 gametes with 1 copy (normal), 1 gamete with 2 copies, and 1 gamete with no copies
[Note: gametes normally have 1 copy of each chromosome]
- This process may cause reduced fertility
57
What are the lesser known symptoms of Down syndrome?
- Congenital heart disease (40%)
- Digestive problems--> need special diets
- 10x-20x increased risk of leukemia
- Abnormal immune responses of unknown etiology
* Thyroid autoimmunity
* Lung infections
- Degenerative neurological disorder similar to Alzheimer's disease
58
Describe the molecular basis of Down syndrome.
Chromosome 21 has 430 genes
- Overexpression of genes is variable--> may explain why 40% have heart disease
Comparing 2 individuals with partial trisomy:
- Down syndrome critical region
- APP (associated with Alzheimer's), SOD1 (associated with Parkinson's), Ets transcription factor, et al.
Gene dosage: if someone has more copies of a gene, they are more likely to express more of the protein product because you signal for production of that protein product
- You now have 3 genes that can be activated instead of 2
59
What is DiGeorge syndrome?
- Part of a spectrum of disorders associated with chromosomal deletion
* Chromosome 22q11.2
* Other disorder is velocardiofacial syndrome
60
What are the symptoms of DiGeorge syndrome?
- T-cell immunodeficiency, hypocalcemia, cardiac malformations, mild facial anomalies
- Cardiac malformation and facial dysmorphology are also found in velocardiofacial syndrome
- This particular deletion also increases risk of:
* Schizophrenia
* Bipolar disorder
* ADHD
61
What are additional symptoms and physical changes associated with DiGeorge syndrome?
- Thymic hypoplasia
* T-cell development
* T-cell immunodeficiency
- Parathyroid hypoplasia
* Calcium mobilization by parathyroid hormone
* Hypocalcemia
- Cardiac malformations
* Different structures impacted
* Generally affect outflow tract (transient developmental structure; connects embryonic ventricles with aortic sac)
* Associated with multiple congenital heart defects
- Alterations to facial development
* Mild facial abnormalities
62
What are the characteristics of the deleted region involved in DiGeorge syndrome?
- Very large: 1.5 megabases (1.5 million bases)
- Different borders for deleted region for 2 different outcomes
- Many genes present; no specific associations yet
* 30 candidate genes
* Transcription factor TBX1 is closely associated with phenotypic features
* TBX1 targets a gene called PAX9, which is associated with the palate, parathyroid, and thrymus
63
What are complex multigenic disorders?
- Basically a catch-all for diseases that do not fit monogenic or chromosomal criteria
- Determining if a disease qualifies:
* Familial clustering
* Not monogenic or chromosomal
- Polymorphisms present in the general population
- Most phenotypical characteristics are the result of the interaction of multiple polymorphisms (ex: eye color)
- These diseases have multiple polymorphisms and it depends on the combination of polymorphisms--> some cases you have disease, in other cases you do not
64
Are polymorphisms disease specific?
- Some are unique to a specific disease
* Ex: Bruton's agammaglobulinemia- mutation in a specific tyrosine kinase that prevents B cell maturation
- Some are found in multiple diseases
* Ex: hypersensitivity (allergies) involve HLA components; so does type 1 diabetes
- Also, some diseases associate with multiple polymorphisms
65
What is a polymorphism?
- Refers to the presence of two or more variant forms of a specific DNA sequence that can occur among different individuals or populations
66
How many genes are implicated in T1 diabetes?
20-30
67
Which of the 20-30 genes associated with T1D are important?
- Insulin (Chr11
- HLA region on Chr6p21
- PTPN22 (lymphoid protein Tyr phosphatase
- IL2 receptor alpha (IL2RA)- T cell activation
- Cytotoxic T-lymphocyte associated protein 4 (CTLA4)- negative reg. of immune response
* Consistent with autoimmune disease*
68
What contributes to the pathogenesis of type 1 diabetes?
Genetic susceptibility + environmental triggers
69
Explain environmental influences
- Polymorphisms may result in predisposition to disease
- Environmental exposure required for pathogenesis of disease
* Hypersensitivity reactions
* Certain autoimmune diseases
* Type 1 diabetes
70
What are the common characteristics of T1D and T2D?
- High blood glucose
- Symptoms--> excessive thirst/urination/hunger
- Insufficient insulin function
71
What is the difference between T1D and T2D?
Why the patient has a high BG level
- T2D: insulin resistance
- T1D: reduced insulin production due to autoimmune destruction of pancreatic beta cells
72
Why is there a link between people suffering infection and showing signs of T1D?
- When exposed to bacteria/viruses, immune system is activated
- If system is not working right because of polymorphisms, this could also mean you activate the cells that attack the pancreatic beta cells.
- Other possibility is that they are attacking/directed against proteins that have something in common with the protein expressed on the surface of pancreatic beta cell
-Viruses--> no direct evidence for specific virus
-Bacteria--> linked but causation unknown
73
What are environmental triggers of T1D?
- Virus
- Bacteria
- Nutrition
- Chemicals
74
Describe the controversial concept of T1D and antigen exposure.
- T1D might be like an allergy gone haywire
* Some polymorphisms predispose people to allergies; the same polymorphisms might be what predisposes them to T1D
- Cow's milk--> cross-reactivity between antibodies to bovine albumin and beta cell surface protein
- Wheat proteins (specifically gluten)--> overlap between T1D and celiac; insufficient evidence that gluten Ab's initiate T1D
* Genetic susceptibility?
75
What is amyloidosis?
- Progressive degenerative disease characterized by fibrillar protein deposits (amyloid deposits)
- Deposits are improperly folded proteins
* Type of protein determines type of disease
* Mutations determine what type of protein you find in the deposit (multiple variants of amyloidosis)
* Will impinge on neighboring structures--> interfere with function= symptoms
76
What is amyloid?
- Deposit that stains with iodine, which stains starches
- Have uniform, straight, unbranched fibrils
- 5-10 % of protein in deposits is serum amyloid P (SAP)--> found in all amyloid deposits; not the protein that is overexpressed
* Binds to motifs common to all amyloid proteins
* Calcium dependent function
* May protect amyloid from proteolysis or phagocytosis
77
What is the common mechanism of amyloidosis?
- Excessive production of precursor or production of mutated precursor
- Specific type of tertiary protein structure
* No alpha strands; form beta sheets only
* Unfortunately, no common structural feature in the protein itself has been linked to amyloidogenicity
* Thousands auto-aggregate into beta-sheet protofilaments (long fibers)
78
How do you diagnose amyloidosis?
- Amyloid is a very specific structure with specific chemical properties, which allows us to identify them on pathological samples
- Birefringent crystals diagnostic for amyloidosis
* Congo Red staining- red= protein, but not necessarily amyloid
* Polarized light microscopy- apple green= amyloid deposits
79
What are the 2 mechanisms of genetics of amyloidosis?
1. Mutation in amyloid protein
2. Mutation in non-amyloid protein
- Therefore, mutation is not always predictive
- Amyloid typing is usually based on immunohistochemistry for proteins we think are likely to be in that deposit
80
Describe variants in amyloidosis.
Nomenclature
- A for amyloid
- Second part for protein type
* AL from Ig light chain
* AFib from fibrinogen--> amyloidosis with fibrinogen in deposits
Clinically--> Systemic vs Localized
- Some specific variants will be localized to specific areas, but many variants can be either systemic or localized
81
Why do we want to ID the protein source in a patient with amyloidosis?
Treatment is based on eliminating the source- ex: AL is the most frequent type of systemic amyloidosis
- Target underlying plasma cell issue--> making excessive amounts of Ig light chain
- Chemotherapy--> to destroy plasma cells or the B cells that will differentiate into plasma cells
- Stem cell rescue--> put stem cells into bone marrow so you are no longer making the B cells that become the plasma cells that make the Ig light chain
