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Download Chapter 13 – Genetic Mapping of Mendelian Characters
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Sequencing the Human Genome • In 1998, Celera Genomics announced plans to sequence the human genome… • …175,000 sequence reads per day, operating 24 hours a day, 7 days a week J. Craig Venter Sequencing the Human Genome • Whole genome shotgun approach vs. Clone by Clone approach • By-passes the initial work of ordering clones • Celera performed about 32 million sequence reads, each 500 – 1000 bp Sequencing the Human Genome Sequencing the Human Genome • IHGSC published sequence reads every 24 hours to prevent patenting of DNA • Celera had access to IHGSC data • Debate over whether Celera could have shotgun sequenced the genome without IHGSC data Sequencing the Human Genome • Both groups published results simultaneously • Celera – Science February 2001 • IHGSC – Nature February 2001 Sequencing the Human Genome Nature 409, 818 - 820 (15 February 2001) Sequencing the Human Genome • Controversy! Science published Celera’s sequence without requiring deposition to GenBank • Celera provides full access, with a catch… • Celera provided Science with a copy in escrow Sequencing Your Human Genome • For $500,000 you can have your DNA sequenced • Sequence 1000 individual human genomes • “Personalized” medicine J. Craig Venter Human Genome • Legal considerations – Should DNA, or genes, be patentable? • In the past, USPTO considered genes as man-made chemicals – Copy DNA region, splice it together, and propagate it in bacteria, etc Human Genome • Celera >6500 genes • Human Genome Sciences >7000 • Incyte >50,000 • Only a fraction may be awarded by USPTO, and only a fraction of these may be useful in treating human disease Human Genome • 1994 U. of Rochester scientists isolate mRNA for COX-2 and clone gene • Suggest that compounds which inhibit COX-2 might provide pain relief from arthritis • Submit patent application in 1995 Human Genome • 1998 – Celebrex – inhibitor of cyclooxygenase-2 (COX-2) introduced as arthritis medication • Developed by Pfizer/Searle • Development began in early-90’s i.e. around time of U. of Rochester discovery Human Genome • April 2000, U. of Rochester awarded patent covering COX-2 gene and inhibition of the peptide product thereof • The same day, U. of Rochester files lawsuit against Pfizer/Searle to block Celebrex sales • Claims that Pfizer/Searle infringes on their patent • They want royalties from the sale of the invention Human Genome • 2003 – U. of Rochester patent found invalid • 2004 – Invalidation upheld by higher Court • U. of Rochester patent did not provide sufficient example of what the inhibitor would be…i.e. claims too broad without a working example • How will “basic science” performed by Universities be rewarded? Human Genome • Vioxx and Celebrex in news again this year: increased risk of “cardiovascular event” i.e. heart attacks Human Genome • Gene discovery – Methods for finding genes – Easy in prokaryotes Human Genome • Gene discovery – Difficult in eukaryotes Human Genome • Gene discovery – Average gene extends over 27 kb – Average 8.8 introns – Average 145 bp • Extremes: – Dystrophin gene 2.4 Mb – Titin gene contains 178 introns, coding for a 80,780 bp mRNA Human Genome • Gene discovery – One approach is to examine “transcriptome” Human Genome • Conservation of chromosome/gene location between organisms • Synteny • Exons tend to be conserved between species Human Genome • Human vs. Pufferfish genome • Pufferfish genome about 1/7th the size of the human genome with similar number of genes Human Genome • Predictive computer programs, e.g. GENSCAN • GENSCAN predicts the location of genes based on splicing predictions, promoter regions and other criteria Human Genome • Online databases have formed to curate Human genome data • Ensembl (www.ensemble.org) Genetic Mapping of Mendelian Characters Identifying Disease-Causing Gene Variations • Linkage analysis and Positional Cloning – Clone disease gene without knowing anything except the approximate chromosomal location Recombination • Recombination during meiosis separates loci – More often when they are farther apart – Less often when they are close • Recall discussion of the Genetic Map – Loci on separate chromosomes segregate independently – Loci on the same chromosome segregate as a function of recombination Recombination 13-1 13_06.jpg Linkage analysis • Linkage analysis locates the disease gene locus – Linkage analysis requires • Clear segregation patterns in families • Informative markers close to the locus – Utilize LOD analysis to verify linkage – Calculate cM distance between Loci Positional Cloning • Widely used strategy in human genetics for cloning disease genes • No knowledge of the function of the gene product is necessary • Strong for finding single-gene disorders Positional Cloning • Linkage analysis with polymorphic markers establishes location of disease gene • LOD score analysis, and other methods are employed • Once we know the approximate location… – The heavy molecular biology begins Positional Cloning • Example - Huntington’s disease – CAG… – Autosomal dominant – 100% penetrance – Fatal – Late onset means patients often have children Finding the Huntington Gene – 1981-1983 • Family with Huntington's disease found in Venezuela • Originated from a single founder - female • Provided: – Traceable family pedigree – Informative meiosis – Problem was… only a few polymorphic markers where known at the time Finding the Huntington Gene • Blood samples taken • Check for disease symptoms • Paternity verified Finding the Huntington Gene • By luck, one haplotype segregated very closely with Huntington disease • Marker was an RFLP called G8 (later called D4S10) Finding the Huntington Gene Finding the Huntington Gene • Locate the region to the tip of the short arm of chromosome 4 by linkage with G8 (D4S10) • Maximum LOD score occurred at about 4 cM distance, i.e. 4 in 100 meiosis Finding the Huntington Gene • Together this started an international effort to generate YAC clones of the 4 Mb region • More polymorphisms were found Finding the Huntington Gene • Next, find an unknown gene in an uncharacterized chromosome location • Locate CpG islands • Cross-species comparisons • Further haplotype analysis suggested a 500 Kb region 3’ to D4S10 Finding the Huntington Gene • Exon trapping was key • Compare cloned exons between normal and Huntington disease patients Finding the Huntington Gene Finding the Huntington Gene • One exon, called IT15, contained an expanded CAG repeat…. • Mapping to 4 cM – 1983 • Cloning of Huntington gene – 1993 Complex Disease and Susceptibility Single gene disorders Gene Mendelian Inheritance High penetrance Low environmental influence (but sometimes significant) Gene LOD-based linkage analysis works great Genetic heterogeneity Disease Low population incidence Complex Disease and Susceptibility Gene Gene Gene Gene Environment Disease A Disease B Disease C Multifactorial disorders Complex Disease and Susceptibility • Single gene disorders – – – – – – Huntington’s Fragile X SCA1 DMD Werner’s syndrome Cystic fibrosis • Multifactorial – – – – – – – Heart disease Cancer Stroke Asthma Diabetes Alzheimer’s Parkinson’s Genetic Component in Complex Disorders • Relative risk lr= frequency in relative of affected person Population frequency Genetic Component in Complex Disorders • Family Studies Class of relative Proportion of genes shared Examples First degree 50% Parent/child, siblings Second degree 25% Grandparent/grandchild, aunt/niece Third degree 12.5% Cousins Genetic Component in Complex Disorders Congenital Malformations Cleft lip Pyloric stenosis General population 0.001 0.001 First degree relatives X40 (0.04) X10 (0.01) Second degree relatives X7 X5 Third degree relatives X3 X1.5 • Problem of environmental impact Genetic Component in Complex Disorders Disorder Breast cancer Type I diabetes Type II diabetes Multiple sclerosis Peptic ulcer Rheumatoid arthritis Tuberculosis Monozygotic 6.5 30 50 20 64 50 51 Dizygotic 5.5 5 30 6 44 8 22 Genetic Component in Complex Disorders Disorder Alcoholism Autism Schizophrenia Alzheimer’s Dyslexia Monozygotic 40 60 44 58 64 Dizygotic 20 7 16 26 40 Genetic Component in Complex Disorders • In polygenic diseases, risk (susceptibility) alleles increase the phenotypic value • Traits may appear continuously variable • Traits may appear discontinuous Genetic Component in Complex Disorders • How to find susceptibility gene? – Four main approaches 1. 2. 3. 4. Candidate gene Parametric linkage analysis Non-parametric linkage analysis Population association studies Candidate gene • Before searching the whole genome, think about what genes may be involved – Eg., Type I diabetes – Some genes involved in cell-mediated immunity are located on chromosome 6 (Human leukocyte antigen region) – Linkage between Type I diabetes and HLA was closely examined • After a small genomic region is isolated, determine best candidate gene Parametric Linkage Analysis • Standard LOD score analysis, as used for single-gene disorders Parametric Linkage Analysis • Eg., breast cancer susceptibility genes • Collect family history of >1500 breast cancer patients – Some family histories showed multiple cases occurring at early ages – could be a Mendelian allele segregating – Best model suggested a dominant single-gene allele with a population frequency of 0.0006 – this suggested about 5% of total breast cancers Parametric Linkage Analysis • Eg., breast cancer susceptibility genes • Collect family history of >1500 breast cancer patients – Now, look for families with multiple breast cancer cases with early onset – Genotype family members and look for linkage – Linkage (significant LOD score) to breast cancer was found to a marker on 17q21 Parametric Linkage Analysis • Eg., breast cancer susceptibility genes • Collect family history of >1500 breast cancer patients – The gene involved was cloned, like other single-gene disorders – Breast cancer (BRCA) 1 gene– tumor suppressor gene involved in genomic stability – LOH leads to high penetrance of breast cancer, as well as ovarian cancer Parametric Linkage Analysis • Eg., breast cancer susceptibility genes • Collect family history of >1500 breast cancer patients – However, examination of BRCA1 mutations outside of affected families suggests lower penetrance Parametric Linkage Analysis • Other successes in finding Mendelian risk factors in polygenic diseases – HNPCC – non-polyposis colon cancer • MSH1, MLH1, PMS1, PMS2 – FAP – familial polyposis colon cancer • APC – Premature heart disease - hypercholesterolemia • Mutation of the LDL receptor Parametric Linkage Analysis • Familial hypercholesterolemia – Autosomal dominant Parametric Linkage Analysis • Familial hypercholesterolemia • 200 mg/dl - 350 mg/dl - dietary, common • 400 mg/dl - 600 mg/dl - heterozygous, uncommon • >600 mg/dl - homozygous, rare Parametric Linkage Analysis • Familial hypercholesterolemia • Autosomal dominant; allele frequency about 1:150 Parametric Linkage Analysis • Spectacular misfires as well: – Bi-polar disease (manic depression) – Initial linkage to HRAS and INS on chromosome 11 – LOD scores of 4.08 and 2.63 – Two individuals in extended family misdiagnosed – Lowered LOD score to 1.03 and 1.75 Non-parametric Linkage Analysis • Genomic regions surrounding risk alleles will be inherited from a common ancestor in affected individuals to a greater frequency than by chance – also called autozygosity mapping • Search for commonly inherited regions by polymorphic microsatellites, SNP’s, etc. • High throughput analysis critical Non-parametric Linkage Analysis • Common to use Affected Sib-Pairs (ASP) • Collect genotypic data for 100’s of ASP • 300+ microsatellite markers genotyped for 10cM coverage • Look for significant IBD (>chance occurrence) Non-parametric Linkage Analysis • IBD: if parental alleles differ at locus, then sibs that have both alleles in common are identical by decent • IBS: if parental alleles are not know, then we can only say sibs are identical by state Population association studies • Association studies are carried out on populations • Look for alleles that segregate with the disease in a whole population – Direct causation – Natural selection – Linkage disequilibrium Population association studies • Linkage disequilibrium • Combination of alleles at two closely linked loci occur more often than expected by chance from population frequencies • Recombination reduces linkage disequilibrium Population association studies • Linkage disequilibrium vs. Linkage Mapping – Mapping is performed on families with few informative meiosis; LD is determined on populations after many generations – Mapping will show linkage over large distances; LD is visible only over short distances Genetic Component in Complex Disorders • How to find susceptibility genes? – Four main approaches 1. 2. 3. 4. Candidate gene Parametric linkage analysis Non-parametric linkage analysis Population association studies Alzheimer’s Disease (AD) • North America – 0.1% at 60, 10% at 80, 30% at 90 • Early onset: <60 • Neurofibrillary tangles in the cerebral cortex and amyloid plaques in the brain • Neuronal apoptosis occurs in the hippocampus and cerebral cortex – memory and learning Alzheimer’s Disease (AD) • Neurofibrillary tangles – polymerized tau protein • Amyloid plaques – deposition of the b-amyloid protein Alzheimer’s Disease (AD) • Apoptosis of neuronal cells – Sometimes called “Programmed cell death” – Energy-utilizing program of orderly selfdestruction – Organized dismantling of the cell to avoid autoimmune reaction Apoptosis Apoptosis • Activation of proteases (cysteine-aspartic acid specific; called Caspases) • Cascade of “irreversable” proteolysis • Activation of endonuclease – chops up the cells DNA – no going back now! Apoptosis • Apoptosis occurs: – During development – Removal of immunological cells – In cells with DNA damage – Defeated in cancer cells • Neuronal cells maintain survival by exposure to “neurotrophins” Search for Susceptibility Alleles for Alzheimer’s Disease • Some clues as to causative agents of AD – Down syndrome individuals develop clinical features of AD when they live >30 years – Suggested that chromosome 21 may be involved in AD – Parametric linkage analysis located a locus on chromosome 21q in early-onset familial AD Causative genes in AD • Amyloid precursor protein (APP) overabundant in Alzheimer’s and Down syndrome individuals • Amyloid precursor protein gene mapped to chromosome 21 • Trisomy 21 causes a over-expression of genes from chromsome 21, including APP Causative genes in AD • APP – a causative agent of AD and involved in pathology of Downs syndrome • Large transmembrane protein processed by a, b or g-secretase • a-secretase generates Aa40 protein – non-toxic and the main protein in normal brain Causative genes in AD • b and g-secretase generates Ab42 protein – toxic and insoluble – which forms plaques • After APP was found by parametric linkage, mutations were found • In familial AD, mutations in APP increased the amount of Ab42 cleavage Causative genes in AD • More parametric linkage analysis within families of early-onset AD – Presenilin I and II were discovered on chromosome 14 and 2 – Presenilin I is a g-secretase – leading to increased Ab42 secretion Causative genes in AD • 1% of AD is familial, and shows strong Mendelian inheritance of altered Ab42 generation • What about risk alleles in sporadic AD? – 99% of cases Causative genes in AD • Non-parametric linkage analysis was performed on Affected Pedigree Member (APM) • 32 families in which 87 of 293 members showed AD • Linkage with locus on chromosome 19 Causative genes in AD • In this region was the gene for Apolipoprotein E. • ApoE was found in plaques and tangles – Good candidate • A population association study was performed • Three alleles of ApoE were identified: – ApoE2 (6%), ApoE3 (78%) and ApoE4 (16%) • Strong LD was found for allele ApoE4 and several nearby SNP’s Causative genes in AD • ApoE4 is a risk factor Alzheimer’s disease ApoE4 dose 0 1 2 % affected 20 46.6 91.3 Relative Risk 1 2.84 8.07 Age of onset 84.3 75.5 68.4 Summary • Family, adoption and twin studies provide evidence of genetic component to complex disease • Risk of disease is the combined effect of polygenes influenced by environment, thus termed multifactorial • Combined affect of many common alleles each providing a small effect, or of a few uncommon alleles with large effect • Candidate gene, parametric and non-parametric linkage analysis, and population association analysis are used to find risk factors for multifactorial disease