Molecular Genetics Flashcards
THE CENTRAL DOGMA OF MOLECULAR BIOLOGY
The flow of genetic information: DNA to RNA to protein
Exceptions to the central dogma
Retroviruses and prions
THE HUMAN GENOME
First complete sequence in 2000 (rough draft), 2003 (essentially complete).
2 x 22 autosomes + 2 gonosomes (X or Y) + mitochondrial DNA (mtDNA)
~ 3 billion base pairs
~ 20,000 protein-encoding genes (only)
2% coding DNA
MUTATIONS
Mutations are permanent changes in DNA
Types of mutations:
- Point mutations, frameshift mutations, repeat expansions (e.g. trinucleotide expansions).
- Sporadic and familial mutations.
- Autosomal and gonosomal mutations.
- Recessive and dominant mutations.
- Somatic and germ-line mutations.
MENDELIAN INHERITANCE
Autosomal recessive
Autosomal dominant
X-linked recessive
X-linked dominant
Autosomal recessive
usually both parents are carriers of the mutant gene but are not clinically affected
disease occurs in homozygous state
- each child has a 25% chance of being affected
both males and females are affected
- complete penetrance is common
new mutations are rarely detected clinically
enzyme proteins are affected by the mutation
if mutation is rare, disease may occur through consanguinity.
Diseases include cystic fibrosis, phenylketonuria, galactosemia, Tay-Sachs disease, mucopolysaccharidoses
Autosomal dominant
usually one parent is affected
both males and females are affected, either can transmit the condition
disease occurs in heterozygous stat
new mutations are detected clinically
clinical features can be modified by reduced penetrance and variable expressivity
regulatory (i.e. receptor) and structural proteins not enzymes are affected by the mutation.
Diseases include: Familial Hypercholesterolemia, Marfan Syndrome, Ehler-Danlos Syndrome.
X-linked recessive
incidence of trait is higher in males than females
- heterozygous females are often unaffected but may express the condition due to X-inactivation
mutant gene is transmitted from affected males to all daughters but not sons
for a heterozygous female, each son has a 50% chance of being affected and each daughter has a 50% chance of being a heterozygous carrier
X-linked dominant
affected males (with normal mates) have no affected sons and no unaffected daughters
affected heterozygous females have 50% risk of passing the mutant gene to both sons and daughters
ATYPICAL OR NON-MENDELIAN FORMS OF INHERITANCE
Triplet repeat expansions
Mitochondrial inheritance
Genomic imprinting
Prions
Triplet repeat expansions
Reported in many disorders, all of which are associated with neurodegeneration
may involve any part of the gene (introns, exons, untranslated regions); often include CGs
Result in loss of protein function or gain of (toxic) function
Mutation is dynamic.
- When a certain threshold is met for ‘normal number’ of triplet repeats (referred to as premutation alleles) then trinucleotide repeat expansion occurs during gametogenesis and leads to a full mutation.
- Threshold for converting a premutation to full mutation differs with each disorder
- amplification occurs in either oogenesis or spermatogenesis depending upon disorder.
Polyglutamine expansion disorders: Huntington’s disease, Kenney’s disease, Spinocerebellar Atxias, etc.
- Polyglutamine diseases are a large group of inherited neurodegenerative disorders caused by the expansion of the CAG trinucleotide
- CAG triplet expansions in DNA result in polyglutamine expansions in disease proteins.
- Same mutation in the context of different genes/proteins results in different diseases.
Fragile X syndrome
Fragile X syndrome: characterized by mental retardation and an abnormality in the X chromosome at q27.3. This syndrome results from mutation in FMR1 gene.
Triplet repeat expansions - CGGs in Fragile X Syndrome
Result in loss of protein function (mutation in 5’ untranslated region: Fragile X Mental Retardation)
Mutation amplification occurs in oogenesis
Fragile X syndrome differs from classical X-linked recessive inheritance in that:
- Some males who carry the mutation (male carriers) are clinically normal. These carrier males transmit their mutation to grandsons through their phenotypically normal daughters
- About 50% of female carriers are mentally retarded
- These differences are attributed to the dynamic nature of the CGG expansion. In the normal population, less than 52 CGG repeats are present whereas the full mutation has 201-4000 CGG repeats. The full mutation arises through a premutation stage (52-200 CGG repeats). Both males and females (ie. carriers) have premutations but they can only be expanded to full mutations during oogenesis. These expansions can in turn be transmitted to either sons or daughters of the carrier female
- Premutations can also have a direct clinical affect on carriers. About 1/3 of female carriers have premature ovarian failure and ~1/3 of male premutation carriers develop a neurodegenerative syndrome later in life.
Huntington’s disease
Triplet repeat expansions - CAGs in Huntington’s disease
Result in gain of (toxic) function (mutation in coding region – abnormal protein: Huntington’s disease)
Mutation amplification occurs in spermatogenesis
Is a polyglutamine expansion disorders
The abnormal CAG expansion causes the expansion of the polyglutamine (polyQ) region in the protein huntingtin.
- PolyQ-expanded huntingtin misfolds, aggregates and causes the dysfunction and death of specific neurons (primarily neurons in the striatum but also cortex).
- The length of the polyQ-expansion determines the age of onset and the severity of the disease.
Mitochondrial inheritance
Mitochondria encode enzymes involved in oxidative phosphorylation. Mutations are rare.
Ova have many mitochondria, spermatozoa have few.
- Zygotes have only maternally derived mitochondria.
Mothers transmit mitochondria to all their offspring (sons and daughters) but only daughters transmit mitochondrial DNA to their offspring
Examples of disease caused by mitochondrial mutations (generally extremely rare): Leber Hereditary Optic Neuropathy (LHON), Myoclonic epilepsy and ragged red fibers (MERRF).
Genomic imprinting
Epigenetic modification (such as DNA methylation; histone modifications, such as methylation, acetylation, ubiquitylation) of genes that results in differential expression of genes inherited from mother vs. those inherited from father, ie. ‘parent-of-origin’ effects
Maternal imprinting refers to silencing of maternal allele and paternal imprinting to silencing of paternal allele. For imprinted genes, only one functional copy exists in the individual so loss of the functional allele leads to disease
Imprint is reset during gametogenesis and is stably transmitted to all somatic cells derived from zygote
Not all chromosomes have imprinted regions
Examples of diseases caused by aberrant imprinting:
- Prader-Willi Syndrome (PWS) and Angelman Syndrome (AS)
- PWS and AS result from disruption of genes on the paternally inherited and maternally derived chromosomal region, respectively.
- The disruption can result from deletion (up to several megabases of DNA), uniparental inheritance (both chromosomes from one parent), or abnormal methylation imprint.
- AS also results from mutation in UBE3A ligase gene which is paternally imprinted (i.e. expressed from maternal allele).
- In families with one affected child, the chance of having another child with AS or PWS is dependent upon the specific underlying molecular cause identified in the affected child.
