Transcript Dia 1
Lesson 2 Monogenic disorders Mendelian inheritance Mendelian pedigree patterns mendelian genetic character • depends on the genotype at one single locus • expression of any human character typically depends on several/many genes and environmental factors • >10.000 mendelian characters are known • OMIM internet database (http://www.ncbi.nlm.nih.gov/Omim/) • dominant vs recessive • semi-dominant • hemizygosity Mendelian pedigree patterns Mendelian pedigree patterns Mendelian pedigree patterns autosomal dominant inheritance • one affected parent • affects either sex • transmitted by either sex • 50% recurrence risk • ex: achondroplasie, Huntington’s chorea, Steinert muscular dystrophy, neurofibromatosis, Marfan syndrome, polycystic kidneys Mendelian pedigree patterns Mendelian pedigree patterns autosomal recessive inheritance • unaffected parents • parents are unaffected carriers • parental consanguinity • affects either sex • 25% recurrence risk, 25% carrier • ex: cystic fibrosis, metabolic disorders Mendelian pedigree patterns Mendelian pedigree patterns X-linked recessive inheritance • affects mainly males • unaffected parents • mother asymptomatic carrier • affected males in maternal lineage • no male to male transmission (why?) • sons of carrier mother have a 50% recurrence risk • daughters of carrier mother have a 50% chance to be carrier Mendelian pedigree patterns Mendelian pedigree patterns X-linked dominant inheritance • affects either sex, more females • affected parent • females often more mildly affected • 50% recurrence risk for child of affected mother • 100% and 0% RR for daughters and sons resp of affected male Mendelian pedigree patterns type of inheritance??? • infer from pedigree analysis • limitations families vs experimental animals single pedigrees – few affected family ascertainment bias ‘informed guess’ for rare conditions consequences for genetic counseling unravel molecular pathology Mendelian pedigree patterns further complications non penetrance (NP) Mendelian pedigree patterns further complications variable expression Mendelian pedigree patterns further complications imprinting Mendelian pedigree patterns further complications germinal mosaicism I II III III- 1 new mutation II-1 new mutation II-1 germline mosaicism I-2 germline mosaicism Mendelian pedigree patterns further complications anticipation = phenotypic severity increases with each generation I II III Age of onset grandmother < father Affected fetus diagnosed prenatally Mendelian pedigree patterns molecular pathology main classes of mutation • deletions 1bp up to Mbs • insertions including duplications • single base substitutions missense: AA change nonsense: stop codon splice site mutation • frameshifts • dynamic mutations Mendelian pedigree patterns molecular pathology mutation nomenclature Amino acid substitutions one or three-letter codes for AA number/position of the AA eg: R117H or Arg117His Nucleotide substitution ATG initiator codon is +1 1162G>A Mendelian pedigree patterns molecular pathology Loss of function mutations • recessive phenotypes: 50% of the normal level is sufficient • haploinsufficiency: 50% reduction leads to phenotype dominant • dominant negative: nonfunctional product interferes with function of normal protein eg: fibrillar collagen, proteins that dimerize Mendelian pedigree patterns molecular pathology Loss of function mutations (typically more heterogeneous) • deletions, insertions, • unstable expanding repeats • gene disruption by translocation or inversion • promotor inactivation by mutation or methylation Mendelian pedigree patterns molecular pathology Loss of function mutations (typically more heterogeneous) • mRNA destabilisation by polyadenylation site mutation • mRNA destabilisation by nonsense-mediated RNA decay • epigenetic modification DNA methylation/imprinting changes in chromatin configuration Mendelian pedigree patterns molecular pathology Loss of function mutations • mutations influencing splicing i.e. inactivating donor splice site, inactivating acceptor splice site, activating a cryptic splice site • frameshift, nonsense, missense • prevent posttranscriptional processing • prevent correct cellular localisation of product Mendelian pedigree patterns molecular pathology Loss of function mutations cystic fibrosis • most frequent autosomal recessive disorder • prevalence: 1/2500 newborns • carrier frequence 1/25 Mendelian pedigree patterns molecular pathology Loss of function mutations CF • clinical symptoms: viscid mucus in lungs pancreatic insufficiency meconium ileus male infertility • chronic disorder • life expectance 1955: < 5 yr present: 30-35 yr Mendelian pedigree patterns molecular pathology Loss of function mutations CF, putative protein function • imbalance in water and ion transport in secreting epithelia • excessive salt loss in sweat • ‘84 normal efflux of chloride ions across epithelial cell membranes in response to cAMP increase is deficient Mendelian pedigree patterns molecular pathology Loss of function mutations CF, gene identification • locus assignement to 7q31 using linkage analysis • cloning through physical mapping • genomic sequence 250 kb coding sequence 6.5 kb • almost exclusively expressed in epithelial cells • 3bp deletion in exon 10 in 70% in CF patients • CFTR = cystic fibrosis conductance regulator CF, gene function ion channel belonging to gene family involved in active transport across the cell membrane ABC (ATP binding casette) gene family CF, gene function protein structure 2 hydrophobic transmembrane domains 1-2 nucleotide binding folds (bind and cleace ATP) R-domain: target for PKA mediated serine phophorylation phosphorylation of R-domain binding of ATP opening of the chloride channel CF, mutation spectrum Mendelian pedigree patterns molecular pathology Unstable expanding repeats • first discovered in 1991 • triplet repeats very large expansions of repeats outside coding sequences FRAXA Xq27.3 5’UT CGG stable 6-54 unstable 200-1000 Mendelian pedigree patterns molecular pathology Unstable expanding repeats Mendelian pedigree patterns molecular pathology Unstable expanding repeats triplet repeats modest expansions of CAG repeats within coding sequences HD 4p16.3 coding (CAG)n stable 6-35 unstable 36- >100 polyglutamine tracts lead to aggregations and cell death Mendelian pedigree patterns molecular pathology Mendelian pedigree patterns molecular pathology gain of function mutations • less common • possible effects are overexpression receptor ‘on’ new substrate ion channel open Mendelian pedigree patterns molecular pathology one gene genetic variability: severity of phenotype depends upon type of mutation different mutations cause different (related) syndromes Mendelian pedigree patterns molecular pathology different mutations cause different (related) syndromes Mendelian pedigree patterns molecular pathology one gene loss of function vs gain of function RET gene = receptor lof - Hirschsprung’s disease gof - fam. medullary thyroid ca MEN2 Mendelian pedigree patterns molecular pathology one gene intrafamilial variability due to modifier genes tyrosinase deficiency causes ocular albinism R402Q common variant in association with MITF mutation causes OA Mendelian pedigree patterns molecular pathology one phenotype, several genes profound deafness compound heterozygosity