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The Predictivity Concept Peter Propping Institute of Human Genetics University of Bonn, Germany CDBI Seminar on predictivity, genetic tests and insurance Strasbourg, 3-4 December 2007
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Source: Dr. Ron Zimmern, Oxford Gene-environment Interaction: Cystic fibrosis Fragile X Duchenne muscular dystrophy Heart disease PKU Cancer Diabetes Multiple sclerosis Schizophrenia Asthma TB Obesity Alzheimer Autism Meningococcus Struck by lightning Motor vehicle accident Rheumatoid arthritis ‘Totally’ Genetic ‘Totally’ Environmental
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The Human Genome 3,2 x 10 9 nucleotide pairs not a “unique” sequence, but appreciable interindividual variation any two genomes: 99,9% DNA sequence identity, thus, 0.1 % sequence differences (3 mio). Any individual (diploid, i. e. two genomes): 6 mio differences to the reference genome.
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Modes of inheritance ? Autosomal dominant ? Autosomal recessive ? X-chromosomal
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Two major groups of genetic diseases Monogenic (= Mendelian) disorders - monocausal - clear relationship between genotype and phenotype - about 2.000 disorders clarified - most disorders are rare - therapy mostly difficult Genetically complex (multifactorial) disorders - complicated genetic structure - many of them common in the population - may be influenced by exogenous factors - therapy frequently possible
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Conceptual distinction -Prognosis: statement about the future course of a past or currently existing disorder -Prediction: probability of the onset of a disease that has not yet occurred
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Methods of prediction and prognosis in a proband -medical history -medical examinations -family history -predictive genetic diagnosis -prediction based on lifestyle
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Prediction on the basis of medical examinations Imaging techniques (CT, MRT, Ultrasound) -e.g.polycystic kidney disease hereditary brain tumors, e.g. tuberous sclerosis degenerative brain disorders Electrocardiogram -e.g.hereditary disturbance of conductivity (long QT-syndrome) Blood biochemistry -e.g.hypercholesterolemia hyperlipidemia
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I:1 60 y I:2 58 y II:1 36 y II:2 34 y II:4 21 y II:3 34 y III:1 10 y III:2 8 y III:3 5 y Genetic diagnostics in familial adenomatous polyposis (FAP)
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I:1 60 y I:2 58 y II:1 36 y II:2 34 y II:4 21 y II:3 34 y III:1 10 y III:2 8 y III:3 5 y Predictive diagnostics in familial adenomatous polyposis (FAP)
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CRC, 56y CRC, 32y HNPCC? 30y28y ?? 2y Persons at risk for Lynch Syndrome (Hereditary Nonpolyposis Colorectal Cancer, HNPCC)
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Cumulative risk in carriers of a mutation in the BRCA1 or BRCA2 gene Meta-analysis, King et al., Science 2003 BRCA1 BrCa BRCA2 BrCa BRCA1 OvCa BRCA2 OvCa Gen.pop. BrCa Gen.pop. OvCa X
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Examples for Hereditary disorders with late onset for which predictive genetic diagnosis is possible (autosomal-dominant) Treatable: Hereditary tumor syndromes: -breast/ovarian cancer -colorectal cancer -familial adenomatous polyposis Polycystic kidney disease, type 1 Hereditary deafness, several late onset forms Untreatable: Huntington disease Myotonic dystrophy Alzheimer disease, autosomal-dominant forms Spinocerebellar ataxia, several forms Facio-scapulo-humeral muscular dystrophy Retinitis pigmentosa, several late onset forms
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Concordance rates in identical (monozygotic, MZ) and fraternal (dizygotic, DZ) twins % MZDZ Coronary heart disease 4612 Hyperthyroidism47 7 Neurodermitis8328 Diabetes mellitus I45 5 Diabetes mellitus II9510 Lepra5920 Epilepsy („idiopathic“)86 4 Schizophrenia – narrow definition264-10 – wide definition4110-20
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Genetic model of a complex (multifactorial) disease: Hypertension as an example „super-normal“ slightly predisposed slightly increased definitely increased severely ill
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Relationship between genotype and phenotype in a complex disease -Predictive value of a genotype Positive Predictive Value (PPV) - Fraction of persons with a predisposing genotype who will develop the disease Negative Predictive Value (NPV) - Fraction of persons without the genotype who do not have the disease
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Positive Predictive Value (PPV): Example: Crohn disease and association with NOD2 variant NOD2 GenotypePatientsControls Wildtype / Wildtype227248 Wildtype / Ins5723 Ins / Ins201 Positive predictive value: Homozygous 20 = 0,95 Heterozygous 57 = 0,71 20 21 57 80
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Relationship between Genotype Frequency, Relative Risk and Positive Predictive Value DiseaseDiseaseGenotypeGenotypeRelative RiskPPVFrequency COPD + 0,05Pi ZZ0,000520,099,1% Narcolepsy0,0005DQB1*06020,02110,50,4% homozygosity + COPD = chronic obstructive pulmonary disease
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Predictability of affection status in the carrier of a predisposing genotype -monogenic diseases up to 100% depending on penetrance - complex (multifactorial) diseases often low eventually higher after genotypic profiling
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To what degree can multifactorial disorders be predicted ? Generally, the concordance rate of MZ twins is the upper limit of prediction; but: only cross-sectional information taken into account, no age correction possible; global concordance rates give only average data, in fact part of the cases higher degrees of heritability may exist.
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Screening approaches: -Genetic population screening newborn screening for treatable diseases e. g. preconceptual thalassemia screening on Sardinia and Cyprus preconceptual screening in certain ethnic groups, e. g. for Tay-Sachs in Jews cascade screening, e. g. for hypercholesterolemia in the Netherlands
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Screening approaches: -Ascertainment of persons at high risk through family history e. g. inherited breast/ovary cancer and Lynch syndrome (HNPCC) population-based for preconceptual testing in recessive diseases
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The Future: The “1000 Dollar Genome” -nightmare of informed consent -nightmare of interpretation
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