PowerPoint PresentationHuman disease genes summarySome examples of single-gene diseasesFind disease genesSlide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 16Slide 17Slide 18Slide 19LessonsSlide 21Human disease genes summary1. Goals: discover the basis for disease, understand key processes, and develop diagnostics and cures.2. Finding human disease genes -- OMIM3. Sickle Cell Anemia 4. Inheritance and linkage5. RFLPs and chromosome “walking”6. Huntington’s disease -- Scientific suicide7. FutureSome examples of single-gene diseasesCommon?Find disease genesAt OMIM (Online Mendelian Inheritance in Man) http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIMThis database catalogs human genes and genetic disorders. The database contains textual information and references. It also contains links to MEDLINE and sequence records in the Entrez system, and links to additional related resources.Sequencing HbS proteins revealed a single change: Glu6Val in the chain. Fiber formation (R) at low [O2] causes sickling of RBCs (center).Gene finding 1. Sequence candidate genes or proteinsAutosome: not a sex chromosomeX, Y: sex chromosomes= Male (XY)= Female (XX)InheritanceMarkers separated by 1 centimorgan have a 1% chance of being separated in meiosis.1 centimorgan corresponds to ~750,000 bp in humans! Linkage--Recombination during meiosis separates genes1. Genes on different chromosomes assort independently2. Genes on the same chromosome are linked3. This linkage is not absoluteGene finding 2. RFLP analysisLook for restriction fragment length polymorphism (RFLP) that correlates with the inheritance pattern of the disease. Fig. 9-46. Three alleles of a RFLP on chromosome 5 in 14 individuals in 3 generations. Each lane corresponds to the individual above it.Can a gene be located by RFLP linkage? A “crazy” approach:1. Collect DNA from 100s of related individuals with and without the disease.2. Establish their pedigrees without errors.3. Digest their DNA with various restriction enzymes.4. Probe Southern blots with RANDOM probes.5. Look for an RFLP that is inherited with the same pattern as the disease.Markers separated by 1 centimorgan have a 1% chance of being separated in meiosis.1 centimorgan corresponds to ~750,000 bp in humans! For a “fully penetrant”, single-gene disease:Linkage of a RFLP to a disease in 99/100 patients impliesthe RFLP may be within 750 kbp of the disease mutation. In practice, many more patients are needed to get reliable linkage statistics.Linkage mapping requires large patient populationsJim Gusella commits “scientific suicide” 1980: Gusella starts his first faculty job at Massachusetts General Hospital with the aim of finding an RFLP marker for Huntington’s disease.No one had ever found an RFLP marker for an unmapped disease gene.The approach was to screen for RFLPs using random human DNA probes. As many as 300 probes might be needed to cover the genome.At the time, there were two RFLP markers mapped in the entire human genome. The largest accessible HD family had 27 members--too few to establish tight linkage.David Botstein, an originator of the RFLP concept, estimated it would take 10 years to find a marker linked to the HD gene!More patients: HD families in Venezuela1952: Biochemist and physician, Dr. Americo Negrette diagnoses Huntington’s disease at Lake Maracaibo in Venezuela.1963: Negrette published Corea de Huntington: Estudio de una sola familia a través de várias generaciones (Huntington’s Chorea: Study of a Single Family Through Several Generations)1972: Dr. Ramon Avila-Giron, a student of Negrette’s, attended the Centennial Symposium on HD in Columbus, OH. He showed the 146 participants from 14 countries a startling 20-minute, black-and-white film of several communities around Lake Maracaibo ravaged by HD.Patient advocacy: funding to collect DNA in Venezuela1981: Nancy Wexler leads a US/Venezuelan project to define pedigrees and collect blood samples from HD families in the towns on Lake Maracaibo in western Venezuela.--Genetically isolated--Large families--High HD incidence--All cases are believe to arise from asingle “founder” individual who settled in the area in the 1870s. PanamaColombia VenezuelaLinking genotype and phenotypeMarch 10, 1983: “…The meeting room in the modulo takes on a slightly carnivalesque atmosphere as people from the barrio drift in, children darting underfoot, staring over shoulders, while the adults shoo them outside, where they peer through the doorway or huddle at the windows. . . Taped around the walls of the room is the pedigree chart, a computer-generated system of lines, circles and squares, like a Mondrian mural, that traces the relationships of all the local families with Huntington’s. . . Alice Wexler, Mapping FateThe 12th probe, G8, is linked to HDApril 1983: Ginger Weeks, a technician in the Gusella lab at MGH, developed a new human DNA probe. The probe comprised a unique 17.6-kb fragment from an unknown location in the human genome.G8 showed an RFLP in HindIII-digested DNA. The RFLP gave a 65:1 chance of being linked to the HD gene in an Iowa family of 27 members. July 1983: G8 revealed a 106:1 chance of being linked to the HD gene in an analysis of RFLPs in a pedigree of 75 individuals from Lake MaracaiboNovember 1983: Results reported in Nature, Gusella appears on the Today Show.HD gene: Ten years after1984-1992: 6.2 Mb of DNA from the short arm of chromosome 4 is cloned and mapped.February 1993: HD gene sequenced. Gusella names the protein Huntingtin.3144 amino
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