M1 – Human Genetics Non-Mendelian Inheritance Virginia A. Pallante, M.S. [email protected] 1 Non-Mendelian Inheritance • Mitochondrial Inheritance • Unstable Trinucleotide Repeats • Imprinting 2 1 Mitochondrial Inheritance • Nuclear DNA • Also DNA in mitochondria 3 Mitochondria contain mtDNA • Thousands of mitochondria per cell • In each mitochondria there are several circular chromosomes containing mtDNA 4 2 Mitochondrial Genome •2 rRNAs • 22 tRNAs • 13 poly- peptides: subunits of enzymes of ox-phos 5 Pedigree of Mitochondrial Inheritance With rare exception only a mother transmits her mtDNA to her offspring. 6 3 Features of Mitochondrial Inheritance • Both sexes affected • All children of a mother with the disorder will have the disorder • No children of a father with the disorder will have the disorder • Reduced penetrance, variable expressivity, and pleiotrophy 7 MERRF Myoclonus Epilepsy with Ragged Red Fibers Quadriceps muscle histology 8 4 Replicative segregation • Homoplasmy-same mutation in all mitochondria • Heteroplasmy-mutation present in only some mitochondria 9 Mitochondrial Inheritance: Homoplasmy 10 5 Mitochondrial Inheritance: Heteroplasmy 11 Anticipation and Parent of Origin Effects • Anticipation - phenomenon of disorder getting worse or having earlier age of onset in successive generations • Parent of Origin Effects – sex of parent transmitting the disorder influenced severity or age of onset 12 6 Trinucleotide Repeats Tandemly repeated trinucleotides (i.e. CGG, CTG) within or adjacent to a gene that may increase or decrease in number during formation of egg or sperm cells and thus disrupt the functioning of the gene and lead to disease 13 Disorders due to Unstable Trinucleotide Repeats • Fragile X MR Syndrome (CGG) • Huntington Disease (CAG) • Myotonic Dystrophy 1 (CTG) • Friedreich Ataxia (GAA) • Spino-cerebellar ataxia (usually CAG) • Others 14 7 Fragile X Syndrome • Moderate MR • Speech delay • Large head • Long face, prominent forehead and chin • Protruding ears • Large testes after puberty • Loose joints • Charac. behaviors 15 Fragile X Chromosome • Fragile site at Xq27.3 observed in about 10- 40% males • No longer used for diagnostic testing 16 8 Fragile X Syndrome • Gene identified and found to have a ‘CGG’ trinucleotide repeat region in the untranslated portion of exon 1 17 Fragile X Syndrome • Allele sizes – Normal: about 5-54 repeats – Premutation: about 55-200 repeats – Full: > 200 repeats • Change from phenotypically normal to affected state has only been observed following oogenesis – Expansion of premutation to full mutation may only occur during oogenesis 18 9 Inheritance of Fragile X syndrome 19 Fragile X Syndrome • Males who carry the full mutation have mental retardation • About 50% of females who carry the full mutation have mental retardation • Transmitting males (carry a premutation) – their children are not at risk for MR, but their daughters will be at risk to have an affected child 20 10 Fragile X Syndrome • About 20% females who carry a premutation will develop premature ovarian failure • About 30% of males who carry a premutation will develop Fragile X- associated tremor/ataxia syndrome (FXTAS) in adulthood 21 Fragile X Syndrome • Atypical X-linked inheritance • Due to amplified ‘CGG’ trinucleotide repeat in FMR-1 gene and subsequent methylation of this expansion • This results in silencing of FMR-1 transcription and leads to loss of FMRP • FMRP is RNA-binding protein and appears to be a nucleocytoplasmic shuttling protein 22 11 Pathophysiology of Trinucleotide Repeat Conditions 23 Fragile X Syndrome • In women who carry a premutation allele the risk of expansion to a full mutation is related to: – Number of repeats in the premutation allele – Presence of AGG triplets imbedded in the CGG repeat segment -CGGCGGAGGCGGCGGCGGCGGCGGCGG- -CGGCGGAGGCGGCGGCGGCGGCGGCGG- 25 12 Huntington Disease • Autosomal dominant with complete penetrance • Due to amplified ‘CAG” trinucleotide repeat • Allele sizes – Normal: less than 26 repeats – Intermediate: 27-35 repeats – Mutant: > 36 repeats –36-40 repeats: reduced penetrance –>41 repeats: full penetrance 27 Huntington Disease • Parent of origin affect – Earlier age of onset associated with inheritance from an affected father – Likely due to greater expansion during spermatogenesis • Protein, huntingtin, has polyglutamine track due to expanded CAG repeats and this confers new protein function: the protein accumulates in the nucleus of neurons and has some toxic effect 29 13 Pathophysiology of Trinucleotide Repeat Conditions 30 Myotonic Dystrophy (DM) • Most common form of MD in adults • Progressive muscle wasting & atrophy, myotonia, cataracts, develop. delay, and cardiac conduction defects • Variable expressivity • Incidence of ~ 1/8000 • Mean age of onset is 20-25 years • Autosomal dominant with late onset 31 14 Myotonic Dystrophy (DM) •DM Type 1 – Due to expanded ‘CTG’ repeat in Dystrophia-Myotonica Protein Kinase gene – Allele sizes • Normal: 5-35 repeats • Fully penetrant: > 50 repeats – ↑ number of repeats →↑earlier age of onset and ↑ severity – Neonatal onset with maternal transmission due to greater expansion during oogenesis 32 Myotonic Dystrophy Type 1 33 15 Myotonic Dystrophy (DM) •DM Type 2 – Due to expanded ‘CCTG’ repeat in the Zinc Finger Protein 9 – Need genetic testing to distinguish between DM 1 and DM 2 34 Pathophysiology of Trinucleotide Repeat Conditions 35 16 Disease Mechanisms for Conditions Caused by Unstable Trinucleotide Repeats • Expansion of trinucleotide probably results from “slippage” during DNA replication • Expansions may – Cause a loss of protein function – Confer novel properties to the RNA – Confer novel properties to the protein 36 Slipped Mispairing Mechanism 37 17 38 Imprinting The differential expression of a gene depending on the sex of the parent from which it is inherited (i.e. the parental origin of the gene 39 18 Implications of Imprinting • Sensitive period during development • Stable during mitosis • Affects gene expression • Erased in germ cell line • Only portions of the genome • Form of gene regulation 40 Imprinting: erased in germ cell line 41 19 Imprinting: only portions of the genome 42 Examples of Parent of Origin Effects • Fragile X Syndrome • Huntington disease • Myotonic dystrophy 43 20 Evidence of Imprinting in Humans • Hydatidiform moles (diploid paternal) – Absent, disorganized fetal tissue – Over-development of extra-embryonic membranes • Ovarian teratomas (diploid maternal) – Embryonic tissue – No placental tissue 44 Evidence of Imprinting in Humans • Triploidy – Dygynic (2 mat; 1 pat) • Small embryos; very small placenta – Diandric (2 pat; 1 mat) • Malformed embryos; over-development of extra- embryonic membranes • Maternal genes → embryo • Paternal genes → extra-embryonic membranes 45 21 Prader-Willi Syndrome (PWS) • Hypotonia in infancy • Mental retardation • Voracious appetite, obesity • Short stature • Small hands and feet • Hypogonadism 46 Angelman Syndrome (AS) • Mental retardation • Short stature • Spasticity •Seizures • “Happy puppet syndrome” 47 22 PWS & AS both involve chromo 15q11-13 Deletions account for ~ 70% cases of PWS & AS • If paternal deletion of 15q11-13 → PWS • If maternal deletion of 15q11-13 → AS 48 Causes of PWS and AS 49 23 PWS & AS both involve chromo 15q11-13 Uniparental disomy also causes some cases of PWS & AS • If maternal UPD (no paternal contribution at 15q11-13) → PWS • If paternal UPD (no maternal contribution at 15q11-13) → AS 50 51 24 Uniparental Disomy 52 Trisomy Rescue: a proposed mechanism for UPD 53 25 Molecular Mechanisms for Imprinting • Involves methylation of regulatory sequences of genes during gametogenesis – Methylation of C residues of CpG dinucleotides – Methylation often represses or turns off gene function 54 Imprinting 55 26 Non-Mendelian Inheritance • Mitochondrial Inheritance • Unstable Trinucleotide Repeats • Imprinting 56 27.