Original Genetics Flashcards
List 3 applications of cytogenetics
- Chromosomal abnormalities
- Microdeletions
- Cancer diagnostics
Give an example of a metacentric chromosome
Chromosome 2
Give an example of a submetacentric chromosome
Chromosome 18
Give an example of an acrocentric chromosome
Chromosome 21
Name two classes of chromosomal abnormalities
- Numerical abnormalities, e.g. Down syndrome
2. Structural abnormalities, e.g. Wolf Hirschhorn syndrome 4p-
What chromosome abnormality is seen in Wolf Hirschhorn syndrome?
Del 4p16
What disease are the following characteristics associated with:
- Wide spaced eyes
- Prominent nose
- Delayed growth and development
- Intellectual disability
- Seizures
Wolf Hirschhorn (del 4p16)
What disease are the following characteristics associated with:
- Distinctive high-pitched cry that sounds like a cat
- Intellectual disability and delayed development
- Widely set eyes (hypertelorism), low-set ears, small jaw, rounded face
Cri du Chat (del 5p15)
What disease are the following characteristics associated with:
- Underdeveloped chin, low-set ears, wide-set eyes, narrow groove in upper lip
- Variable clinically: Heart defects, poor immune system function
- ADHD
Di George Syndrome
What chromosome abnormality is seen in Cri du Chat syndrome?
del 5p15
What chromosome abnormality is seen in Di George syndrome?
del 22q11.2
What is the most prevalent behaviour disorder seen in children with 22q11.2 deletion syndrome?
ADHD
It is seen in 30-50% of children
What is the most common psychiatric disease that adults with 22q11.2 deletion syndrome develop?
1 in 4 develop schizophrenia
What chromosome abnormality is seen in Angelman syndrome?
del 15q11-q13
What chromosome abnormality is seen in Prader-Willi syndrome?
del 15q11-q13
What type of chromosomal abnormality is seen in the Philadelphia chromosome?
Reciprocal translocation
46, XY, t(9:22)(q34.1;q11.2)
What two chromosomes are implicated in the Philadelphia chromosome (bcr-abl)?
Chromosome 9 & 22
What kind of chromosomes are involved in a Robertsonian translocation?
Translocation between two acrocentric chromosomes.
Balanced rearrangement.
What is a carrier of a Robertsonian translocation at risk of when having a baby?
Aneuploid offspring
Polyploidy is a change in chromosome number that is an ____ _____ of the haploid chromosome number
Polyploidy is a change in chromosome number that is an exact multiple of the haploid chromosome number
What will triploid foetuses show in the womb?
Intrauterine growth retardation
What causes aneuploidy?
Non-disjunction of homologous chromosomes at meiosis
List 3 autosomal aneuploidy disorders
- Trisomy 21 (Down syndrome)
- Trisomy 18 (Edward syndrome)
- Trisomy 13 (Patau syndrome)
What are 2 common features of autosomal trisomies?
- Intellectual disability
- Congenital abnormalities
What disease are the following characteristics associated with:
- Simian crease
- Flat facial profile
- Congenital heart defects
- Thick neck skin
Down syndrome
What is Down syndrome’s most common karyotype?
47, XX, +21
If a parent has a child with Down syndrome, what is the recurrence risk for future pregnancies?
1%
Aside from aneuploidy, what other chromosomal abnormality accounts for 4% of down syndrome?
Robertsonian translocation.
Usually involves chromosome 14 or 21.
45, XY, t(14,21)
If two parents have a child with translocation Down syndrome, what is the recurrence risk in future pregnancies if the father is the carrier of the translocation?
2.5%
If two parents have a child with translocation Down syndrome, what is the recurrence risk in future pregnancies if the mother is the carrier of the translocation?
10%
What is the karyotype that accounts for 1% of down syndrome cases? (mosaic karyotype)
47, XY,+21/46, XY
What adulthood disease does Down syndrome confer a 100% risk of developing?
Early onset Alzheimer’s Disease
What gene is implicated in both Down Syndrome and Alzheimer’s Disease?
APP
Name two dosage sensitive genes
APP and DSCR1
What is the average incidence of Down syndrome?
1 in 650-700
The risk of Down Syndrome increases with….
The risk of Down Syndrome increases with age of the mother
When is Meiosis I completed?
Ovulation
In first trimester screening, what screening tests are done for Down Syndrome?
- Nuchal translucency thickness scan (NT)
- Maternal serum markers
In second trimester screening, what screening tests are done for Down syndrome?
Anomaly scan and quad test - inhibin A
In prenatal diagnosis for Down Syndrome, what procedure is done in first trimester?
CVS
In prenatal diagnosis for Down Syndrome, what procedure is done in second trimester?
Amniocentesis
What is the sample used for analysis in Non invasive prenatal screening?
Cell free DNA (cfDNA) which includes foetal DNA.
Blood can be taken at 9 weeks.
What is the incidence of Edward syndrome?
1 in 5000
What is the mortality of Edward syndrome in the first 2 years?
90% mortality
What disease are the following characteristics associated with:
- Prominent occiput
- Overlapping fingers
- Rocker bottomed feet
Edward syndrome (Trisomy 18)
What disease are the following characteristics associated with:
- Microphthalmia
- Cleft lip & palate
- Post axial polydactylyl
Patau Syndrome (Trisomy 13)
What % of infants with Patau syndrome survive past 1 year?
10%
Why are imbalances of sex chromosomes better tolerated than autosomal imbalances?
X chromosome inactivation
When does X chromosome lyonisation occur?
2 weeks after fertilisation
What is the dense mass of sex chromosome that arises from X chromosome lyonisation?
Barr body
How many Barr bodies does a normal female have?
1
How many Barr bodies does a normal male have?
None
What is another name for an inactive X chromosome?
Barr body
What is the karyotype for Turner syndrome?
45, X
What is the incidence of Turner syndrome?
1 in 2000
What chromosome disorder are the following characteristics associated with:
- Wedding neck
- Coarctation of aorta
- Infertility
- Broad chest and widely spaced nipples
Turner Syndrome
What are the clinical features of Turner syndrome due to?
Haploinsufficiency of certain genes on X chr e.g. SHOX
What treatment helps improve growth and breast development in Turner Syndrome?
Hormonal therapy
What is the incidence of Klinefelter syndrome?
1 in 1000
What chromosome disorder are the following characteristics associated with in a min:
- Infertility
- Tall stature
- Breast development
- High pitched voice
- Reduced body and facial hair
Klinefelter Syndrome
Name 4 types of common variants
- Nonsense
- Missense
- Frameshift
- Large deletion
Give an example of a disease that is affected by a missense point mutation
Sickle cell anaemia
Give an example of a disease that is affected by a nonsense point mutation
Beta thalassaemia
List the 3 stop codons
UAA
UAG
UGA
What specific frameshift deletion is a common cause of autosomal recessive congenital deafness?
35delG in connexin 26 gene
What is the most common mutation in cystic fibrosis?
del F508
Phenylalanine at position 508
What are the consensus nucleotide sequences involved in splicing? (i.e. all introns start and end with these nucleotides)
Almost all introns start with GU and end with AG
What is the recurrence risk of autosomal dominant disorder?
50%
What complication is associated with familial hypercholesterolaemia?
Myocardial infarction
What receptor has an abnormality in familial hypercholesterolaemia?
LDL receptor defect causes increase levels of plasma cholesterol
What disorder are these clinical features associated with:
- formation of distinctive cholesterol deposits(xanthomas) in skin and tendons
- develop premature atherosclerosis leading to coronary heart disease
Familial hypercholesterolaemia
Name a disorder that arises from haploinsufficiency
Familial hypercholesterolaemia
What chromosome is the LDL receptor gene located on? (Familial hypercholesterolaemia)
Chromosome 19p
What do mutations in familial hypercholesterolaemia cause?
Mutations reduce number of functioning LDL receptors
Heterozygotes for mutant allele in familial hypercholesterolaemia have ____ fold increased plasma cholesterol levels, while homozygotes have _ fold increased levels.
Heterozygotes for mutant allele in familial hypercholesterolaemia have 2-3 fold increased plasma cholesterol levels, while homozygotes have 5 fold increased levels.
What treatment is used for familial hypercholesterolaemia?
- Statins
- Gene therapy
What enzyme do statins inhibit for the treatment of familial hypercholesterolaemia?
HMG CoA reductase
What gene is mutated in Marfan syndrome, and what chromosome is it on?
FBN1 gene on chr 15
encodes fibrillin-1 protein
What 3 body systems are defects found in in Marfan syndrome?
- Ocular
- Skeletal
- Cardiovascular
What term describes when a gene has effects on multiple aspects of physiology or anatomy?
Pleiotropy
What pattern of inheritance does Marfan syndrome follow?
Autosomal dominant
What disorder are the following clinical features associated with:
- Unusually tall stature, long limbs with long spidery fingers (arachnodactyly), scoliosis
- Dislocation of the lens
- Mitral valse prolapse
Marfan Syndrome
What is the main type of mutation found in Marfan syndrome?
Most mutations are missense which act in a dominant negative fashion
What pattern of inheritance does Achondroplasia follow?
Autosomal dominant
What disorder are the following clinical features associated with:
- Common cause of dwarfism
- Large head with prominent forehead, shortened limbs
- Severely dwarfed (adults ~ 4ft), normal IQ
- Respiratory problems can occur in infants and children due to narrowed nasal passage
Achondroplasia
What gene is mutated in Achondroplasia and what chromosome is it found on?
FGFR3 on chromosome 4p.
Majority of mutations are G to A transition.
Is the mutation of FGFR3 in Achondroplasia loss or gain of function?
Gain of function mutation - has implications for gene therapy
Name a disorder that has 100% penetrance
Neurofibromatosis type 1
What is the recurrence risk with autosomal recessive disorders?
25%
What pattern of inheritance do disorders affecting metabolic pathways usually have?
Autosomal recessive
Name 2 genes that may be mutated in Albinism
- TYR
- TYRP1
(chromosome 11p)
What do 10% of babies with cystic fibrosis present with at birth?
Meconium ileus
List 2 methods used for the diagnosis of cystic fibrosis
- Sweat test with increased Na+
- Newborn screening program
What gene is mutated in cystic fibrosis and what chromosome position does it occupy?
CFTR (7q31.2)
What other receptor does CFTR work in conjunction with? (especially in the sweat gland epithelium)
ENaC
List 4 TYPES of mutations that may arise in cystic fibrosis
- Missense
- Nonsense
- Splicing
- Frameshift
What is the most common 3bp frame deletion in cystic fibrosis?
F508del
What is the R117H variant affected by?
Poly T tract in intron 8
Which T variant of R117H promotes exon 9 splicing?
5T variant promotes exon 9 splicing
List 2 treatments for cystic fibrosis
- Antibiotics
- Kalydeco
- Physiotherapy
What is the complication of phenylketonuria if undiagnosed and untreated?
Develop hyperphenylalaninemia which impairs brain development
What gene is mutated in phenylketonuria?
PAH gene resulting in failure to convert phe to tyrosine
What test is performed at birth to screen for Phenylketonuria?
Guthrie test (heel prick test)
If a mother has phenylketonuria and takes excess phe during pregnancy, what does that excess phe act as on the foetus?
Excess phe acts as teratogen on developing foetus
Name two clinical presentations found among offspring of mothers with phenylketonuria who did not maintain diet during pregnancy?
- Dysmorphic features
2. Microcephaly
What is the most common mutation in Hereditary Haemochromatosis?
C282Y in exon 4
List 2 disorders seen in travelling community
- Galactosemia
2. Hurler syndrome
What term describes a person with the following:
“because X inactivation is random, carrier females have varying proportion of cells expressing mutant allele and hence can occasionally exhibit signs of disorder”
Manifesting heterozygote
What gender are X-linked disorders more commonly seen in?
Males.
X-linked rec diseases are transmitted by phenotypically normal heterozygous females to sons. But NO male to male transmission.
Name 2 common X-linked disorders.
- Duchenne muscular dystrophy
2. Becker muscular dystrophy
What enzyme is increased in Duchenne muscular dystrophy?
Increased creatine kinase.
What gene is mutated in both Duchenne- and Becker Muscular Dystrophy, and what chromosome is it on?
Dystrophin gene at Xp21
What type of mutation is usually seen in Duchenne muscular dystrophy?
Frameshift
What molecular genetic tool is used for the initial genetic screening of Duchenne muscular dystrophy?
MPLA or multiplex PCR.
Followed up by full sequencing is MPLA negative.
Give 3 examples of mitochondrial disorders.
- Leigh syndrome
- MELAS
- MERRF
What does MELAS stand for? (Mitochondrial disorder)
Mitochondrial encephalomyopathy, lactic acidosis, stroke-like episodes
When is the onset for MELAS syndrome?
Childhood
Is mitochondrial inheritance paternal or maternal inheritance?
Maternal inheritance
Name 4 complications in inheritance patterns1
- Penetrance
- Expressivity
- Mosaicism
- Anticipation
Name mutated genes that commonly show reduced penetrance
BrCa1 and BrCa2
Name a disease that shows variable expressivity.
Neurofibromatosis
What is one explanation of why parents who are apparently normal on genetic testing, can have more than one affected offspring with a dominant genetic disorder?
Germline mosaicism
What term describes the following:
“Tendency for some disorders to begin at an earlier age and with increased severity with successive generations eg Huntington’s Disease & Myotonic Dystrophy”
Anticipation
What is one possible explanation for genetic anticipation?
Unstable mutation - Expansion of repeated DNA sequences
In anticipation disorders, there is a correlation between number of repeats and _____ __ ________
In anticipation disorders, there is a correlation between number of repeats and severity of disease
In anticipation disorders, there is inverse correlation between number of repeats and ___ __ _____
In anticipation disorders, there is inverse correlation between number of repeats and age of onset.
What is the pattern of inheritance for Huntington disease?
Autosomal dominant
What disease are the following clinical presentations associated with:
- Movement disorder accompanied by behavioural changes including aggression, anxiety, apathy, depression and cognitive changes affecting memory, awareness and communication
Huntington Disease
Name one early stage, middle stage, and late stage motor change in Huntington Disease.
Early stage - Restlessness
Middle stage - Trouble with balance and walking
Late stage - Inability to walk
What is the penetrance of Huntington Disease?
100% penetrant but in age-dependent manner
What part of the brain is affected in Huntington Disease?
Caudate (Severe atrophy of caudate seen in HD)
What mutation repeat is seen in Huntington Disease?
CAG repeat in exon 1 of HTT gene on chr 4
What range of CAG repeats are necessary to show full penetrance of Huntington Disease?
40-120
What range of CAG repeats indicate someone is at increased risk of developing Huntington Disease?
36-39
What disease are the following clinical presentations associated with:
- Progressive weakness
- Cataracts
- Cardiomyopathy
- Testicular atrophy
- Frontal baldness in males
Myotonic dystrophy (DM1)
What two factors complicate the inheritance pattern of Myotonic Dystrophy (DM1)?
Variable expression & anticipation
Name one complication of congenital myotonic dystrophy
Neonatal respiratory distress
What nucleotide repeat is expanded in Myotonic Dystrophy (DM1)?
Mutation is expansion of CTG normally present in 3’UTR of DMPK gene
How does Myotonic Dystrophy (DM2) differ from DM1?
DM2 does not show congenital form
What nucleotide repeat is expanded in DM2 Myotonic Dystrophy?
CCTG expansion in ZNF9 gene
How does CTG expansion cause disease?
Causes abnormal splicing
What is the most common cause of inherited mental retardation?
Fragile X syndrome
Why is the inheritance pattern of fragile X syndrome considered atypical?
Inheritance pattern not fully recessive
What is the most single genetic cause of autism spectrum disorders?
Fragile X syndrome
What nucleotide repeat is expanded in Fragile X syndrome?
Mutation is expansion of CGG in the FMR-1 gene
>200 copies in Fragile X syndrome
What is the repeat expansion in Fragile X associated with (molecularly)?
Repeat expansion is associated with Hypermethylation of DNA and gene silencing. Lack of FMR protein is responsible for signs of Fragile X.
What 2 clinical phenotypes are associated with premutation carriers of Fragile X syndrome?
- Premature ovarian failure
2. FXTAS syndrome
What can PCR be used for?
- To detect the presence or absence of a sequence
- To measure the size of a sequence
What molecular process does PCR imitate?
DNA replication
Name 4 components of PCR
- DNA
- Primer
- Nucleotides
- Taq polymerase
Name the 3 key steps in PCR and their temperatures
- Denaturing DNA (95 C)
- Annealing primers (55-60 C)
- Extending DNA (72 C)
What machine does PCR take place in?
Thermocycler
List 4 applications of PCR
- Genotyping
- Drug discovery
- Mutation screening
- Pre-natal diagnosis
Name 2 applications of multiplex PCR (amplification of multiple targets in a single PCR experiment)
- Pathogen detection
2. Exon deletion
What molecular genetic tool is used to diagnose deletion-type DMD?
Multiplex PCR
Name 2 limitations of end-point PCR (i.e. normal PCR)
- Low sensitivity
2. Not very quantitative
What type of PCR allows for the quantitative analysis of gene expression?
Real-time PCR (qPCR)
What distinguishes the qPCR components from standard end-point PCR (i.e. normal PCR)?
The addition of a fluorescent label
What molecular genetic tool is described in the following statement:
“A method based on in vitro synthesis of DNA incorporating special chain terminating nucleotides - dideoxynucleoside triphosphates”
Dideoxy method of sequencing
What general term describes modern sequencing technologies?
Next Generation Sequencing (NGS)
Is Next Generation Sequencing targeted or not targeted?
Not targeted - they sequence everything in the sample
List 2 barriers for clinical implementation of NGS
- Evidence for clinical utility
2. Data interpretation
What is the pathogenic mechanism behind Fragile X syndrome?
Loss of function due to blocked transcription
What is the pathogenic mechanism behind Fragile X associated tremor and ataxia (FXTAS)?
Gain of function by pathogenic RNA
What is the pathogenic mechanism behind Huntington Disease?
Gain of function of a protein