BIOL 311 Human Genetics Fall 2006Lecture: Molecular PathologyReading: Chapter 16Lecture outline:1. Molecular approaches to disease2. Loss of function mutations3. Gain of function mutations4. Examples of mutations causing human diseases5. Triplet expansion diseasesLecture:1. Molecular approaches to diseasePathology: the study of disease.Molecular pathology: finding the underlying genetic or biochemical basis for the diseaseGenetic variation that leads to disease not always easy to distinguish from random genetic change (polymorphism).2. Loss of function mutationsMutant DNA defective RNA protein with reduced function or no functionTypes of mutations leading to loss of function- Small insertions or deletions--shift reading frame "frameshift mutants"- Nonsense mutations- Nucleotide changes that result in premature stop codons- mRNAs with premature stop codons are usually degraded- Splicing mutations- Alteration of splice junctions or "branch points"- Alter regulatory elements that regulate splicing (splicing enhancers or silencers)Usually loss of function mutations are recessive, however some show incomplete dominance or "haploinsufficiency"--where one good copy is not sufficient to restore function.Dominant negative effect--when a mutant polypeptide loses function and interferes with product of the normal allele.- Seen in proteins that form dimers and multimers, i.e. collagen, transcription factors.3. Gain of function mutation1- Gene product does something positively abnormal- Usually requires a specific change; other mutations in the same gene give a different phenotype.Single types of mutation leading to disease can be due to - Specific mechanism of change, i.e. triplet expansion.- Founder effect--a deleterious mutation in a small population is source of major type of mutation- Heterozygote selectionGain of function common in genes involved in cancer. Gain of function mutations activating cell signaling pathways often produce dominant phenotypes.Can have gain of function and loss of function mutations in same gene leading to different diseases.4. Examples of molecular bases for diseasesa. Hemoglobinopathies: diseases of hemoglobinsSingle nt substitution at amino acid #6 of beta-globin causes sickle cell anemia.Thalessemias- Not enough alpha or beta globin chains, leads to anemias- α0 thalessemia: no alpha globin- α+ thalessemia: reduced levels of alpha globinFig. 16-3 deletions in globin genes due to unequal crossoverTable 16-1 lists 11 ways to reduce or abolish production of functioning gene product.5. Triplet expansion diseases- discovered as disease mechanism in 1991- show anticipation--age of onset is lower and severity is worse in subsequent generations.- At least eight diseases show triplet expansion- Two classes of triplet expansions that cause disease (5'UTR, intron, 3'UTR)- Large expansions of repeats outside coding sequencesFragile X syndromeMental retardation syndrome(CGG)n repeat in 5'UTRstable @ 6-54 copiesunstable @ 200-1000+ copies2Other diseases:Friedreich AtaxiaMyotonic dystrophy (2 genes)Spinocerebellar ataxia (2 genes)- Modest expansion of CAG repeats within coding sequences encoding polyglutamine.- Late onset neurodegenerative disorders, all but one dominantly inheritedHuntington Disease (CAG)nStable # repeats 6-35Unstable # repeats 36-100+Polyglutamine diseases, Alzheimers, Parkinsons have in common protein