Genomics Flashcards
What are monogenic diseases?
Caused by mutations in a single gene; single mutation is both necessary and sufficient
What is genetic linkage?
DNA regions that are located in close proximity are more likely to be co-inherited than DNA regions originating further apart
What is the LOD score?
Detects presence of linkage; LOD = log10 (L(linkage)/L(no linkage)); more than 3 –> linkage; less than -2 no linkage
Which things can act as confounders in single pedigrees?
Non penetrance and phenocopies
What is Hajdu Cheney syndrome?
severe osteoporosis; NOTCH2 heterozygous mutation; autosomal dominant
What is a confounding factor in linkage analysis performed in multiple pedigrees?
Genetic heterogeneity
What is the difference between polymorphism and mutation?
Polymorphism: common DNA variant
Mutation: pathologic DNA variant
What is transition and transversion in single nucleotide substitutions?
Transition: substitution which conserves the base chemistry (G–>A; T–>C), twice as common as transversions
Transversion: substitution which changes the base chemistry (G–>T; A–>C)
What are the consequences of variants in splice sites?
Exon skipping, use of cryptic splice site (in exon or intron), intron retention, combination of these
What are the rarer types of functional variation?
Variants in promoters, untranslated regions (myotonic dystrophy and fukuyama muscular dystrophy), polyadenylation signals
What are the types of functional DNA variation defined by Muller?
Loss of function: amorph (complete loss of function), hypomorph (partial loss of gene function)
Gain of function: hypermorph (increase in normal gene function), antimorph (dominant alleles that act in opposition to normal gene activity), neomorphic (dominant gain in gene function that is different from the normal function)
What is NMD?
non-sense mediated decay
When do ‘loss of function’ alleles exhibit dominant forms of inheritance?
- Haploinsufficiency
- “Dominant negative” effect: homodimeric complex disrupts function of its normal counterpart
- Somatic second hits: cancer gene (recessive at cellular level but dominantly inherited in families)
What is infantile onset epilepsy in Amish?
Autosomal recessive due to nonsense mutation in SIAT9 encoding GM3 synthase –> less production of gangliosides (membrane stability); seizures starting at 6 months of age
What are the differences between phenotypic and genetic heterogeneity?
Phenotypic: one gene, mutations with different clinical outcomes (phenotypes)
Genetic: different genes but same clinical outcomes
What is penetrance?
The proportion of carriers who manifest phenotypic signs of the condition
What is Marfan syndrome?
Tall stature, long limbs, chest abnormalities, scoliosis, arachnodatcyly (long fingers), dislocated lens, risk of aortic aneurism
What is expression?
Gene carriers can be affected to different degrees; extent of clinical manifestation
What are examples of diseases with de novo mutations?
Achondroplasia, marfan syndrome, neurofibromatosis I and II
What is germline mosaicism?
A parent carries a small portion of gametes that harbour the same mutation (achondroplasia, osteogenesis imperfecta, duchenne muscular dystrophy)
How does cosanguinity affect the risk of recessive disorders in the offspring?
increases risk
What are compound heterozygosity and homozygosity?
Heterozygosity: someone who has different allelic mutations at the same locus, i.e. cystic fibrosis
Homozygosity: same allele on both chromosomes, i.e. sickle cell anemia
How can females be affected with X-linked recessive disorders?
- Non-random inactivation leading to chance expression in certain tissues
What is X inactivation?
One X in each cell is switched off before blastocyst implants in female embryos
What are lines of Blaschko?
Territories of clonal cell populations (incontinentia pigmenti, goltz focal dermal hypoplasia)
What are multifactorial disorders?
Polygenic, many different genes and environmental factors contribute to disease susceptibility; asthma, breast cancer, heart disease, depression, scizophrenia, autism, arthritis, migraine, obesity, diabetes, stroke…
What are the major genes in breast cancer?
BRCA1 and 2; mutations in these genes are rare with many different mutations in each gene; high penetrance, lower penetrance for ovarian cancer, BRCA2 has lower penetrance than BRCA1
What are the genes involved in Alzheimer’s disease?
Three rare, high risk genes (APP, PSEN1, PSEN2): young onset AD, high risk families
Moderate risk genes: APOE allele e4, TREM2 gene
Many common, low risk variants identified through association studies
What are the three alleles in APOE gene and what is their importance?
e4: increased risk of AD
e3: baseline risk
e2: protective against AD
Which statistical test is used in case-control association studies?
Chi-square test: test whether distribution of genotypes or alleles is the same in cases and controls
Which are the genes involved in rheumatoid arthritis?
PTPN22: encodes protein tyrosine phosphatase which interacts the negative regulatory kinase Csk to inhibit T cell signalling and activation
What are the measures used to communicate the contribution to disease in Mendelian and polygenic disorders?
Mendelian: absolute risk
Polygenic: relative risk (odds ratios)
What are the genetic tests used in diagnosis?
- Standard chromosome analysis
- Fluorescent in situ hybridisation (FISH)
- Comparative genomic hybridisation array (Array CGH)
- Single gene testing
- Next generation sequencing
- Methylation analysis (imprinting disorder)
What is VATER (VACTERL) association?
Vertebral Anal atresia Cardiac Trachea-oesophageal atresia/fistula Renal abnormalities Limb deformities (radial ray defect)
What is velocardiofacial syndrome?
CATCH 22; del22q11.2 syndrome; CHD (conotruncal anomaly, celft lip and/or palate, absent thymus, absent parathyroid glands, dysmorphic features, learning difficulties
What is comparative genomic hybridisation array (aCGH)?
Powerful tool for identify small genomic imbalances
What is del16p11.2?
Learning difficulties, ASD, seizures, schizophrenia, bipolar disorder, depression, obesity in young adults, unaffected carriers
What is achondroplasia?
FGFR3; most common skeletal dysplasia, disproportionally short limbs, large head, normal intelligence, usually new dominant mutation, unaffected parents 1% RR, affected parents 50% RR
How do you calculate the risk of a child being affected in incomplete penetrance?
probability of inheriting mutant allele*penetrance
In X-linked inheritance with a female carrier, what are the chances of having an affected offspring, having unaffected daughters that are carriers, having unaffected sons that are not carriers?
affected offspring: 1/4
unaffected daughters that are carriers: 1/2
unaffected sons, not carriers: 1/2
A consuldand has a mother that is a carrier for ocular albinism (X-linked recessive), what are the chances of her having an affected child with an unaffected husband?
chances of her being a carrier: 1/2
chances of her passing on the gene: 1/2
chances of her child being a boy: 1/2
chances of having an affected child: 1/21/21/2 = 1/8
What is the Hardy-Weinberg principle?
model which helps to explain gene frequencies in a population (useful for calculating risks of autosomal recessive disorder)
p + q = 1
p^2 + 2pq + q^2 = 1
Carrier frequency = 2pq
If cystic fibrosis has an incidence of 1 in 2500, what is the carrier frequency?
q^2 = 1/2500
q = 1/50
because q is small, carrier frequency = 2*(1/50) = 1/25