Steven Katz, MSIV Genetics Terms Basic Terms (Review) Gene: A hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism. Trait: A distinguishing feature, a genetically determined characteristic or condition. Allele: Versions of a gene Genotype: Genetic makeup, distinguished from the physical appearance. (G for genetic and genotype) Phenotype: The observable physical or biochemical characteristics as determined by both genetic makeup and environment Genetics Terms (cont.) High Yield Terms: Classical Dominance: Dominant allele is expressed if present Incomplete Penetrance: Not all individuals with a mutant genotype display the phenotype (many genetics dz’s but good example is NF1) Variable Expression: Nature and severity of phenotype changes between individuals Co-dominance: Neither of two alleles is dominant (e.g. blood types) Anticipation: Severity of disease worsens or age of onset is earlier in succeeding generations (e.g. Huntington’s Dz) Genetics Terms (cont.) High Yield Terms (cont.) Loss of heterozygosity: When a tumor suppressor gene is mutated or deleted, the complimentary allele must be lost before a cancer develops. Not true with oncogenes! Dominant negative mutation: a non-functioning protein also prevents a normal protein from functioning appropriately (e.g Marfan’s syndrome) Heteroplasmy: Both NL and mut mtDNA results in variable expression in mitochondrial inherited dz’s Uniparental disomy: offspring receives 2 copies of a chromosome from 1 parent and none from the other Imprinting Definition: At a single locus, only one allele is active, the other is inactive; can also occur as a result of uniparental disomy Phenotype depends on origin of mutation paternal v. maternal Both syndromes due to inactivation or deletion of genes on chromosome 15 Prader-Willi: Deletion of normally active PATERNAL allele Mental retardation, obesity, hypogonadism, hypotonia Angelman’s syndrome (aka “Happy Puppet Syndrome”): Deletion of normally active MATERNAL allele Mental retardation, seizures, ataxia, innapropriate laughter Modes of Inheritance Autosomal Dominant: Affects both males and females in all generations. Presents clinically after puberty and FH is essential for diagnosis. Examples: Achondroplasia, Huntington’s dz, Neurofibromatosis types 1 & 2, and many many more! Modes of Inheritance Autosomal Recessive: only offspring of 2 carrier parents can be affected. Usually only seen in one generation, usually due to enzyme deficiencies. Commonly more severe than dominant disorders, presents in childhood Examples: Albinism, Cystic Fibrosis, PKU, Wilson’s dz, and many more! Modes of Inheritance X-linked recessive: only sons of heterozygous mothers can be affected, no father to son transmission. Examples: Fragile X, Lesch-Nyhan, Hemophilia A and B Females may rarely be affected due to random inactivation of X chrom (e.g. Lyonization) Modes of Inheritance X-linked dominant: Transmitted through both parents, males and females can be affected, but all females of affected fathers are affected. Example○ Hypophosphatemic rickets: increased phosphate wasting at proximal tubule Modes of Inheritance Mitochondrial: Transmission ONLY through the mother. All offspring of affected mothers are affected. Variable expression due to heteroplasmy Autosomal Dominant Dz’s Achondroplasia Genetics and Cell Level: ○ Defect in Fibroblast Growth Factor receptor 3 Causes abnormal cartilage development Phenotypic Traits: ○ Dwarfism: short limbs, head and neck nl size Misc info: ○ Associated with advance paternal age ○ AD so if one parent affected then 50% of children affected ○ Homozygotes die either before or shortly after birth Autosomal Dominant Dz’s APKD (adult polycystic kidney dz) Genetics and Cell Level: ○ 90% due to mut in APKD1 on chromosome 16 Phenotypic Traits: ○ Bilateral enlargement of kidney due to multiple cysts Clinical Presentation: b/l flank pain, hematuria, HTN, progressive renal failure ○ Usually presents in adulthood (hence the name!) Misc info: ○ Associated with polycystic liver dz, berry aneurysms, MVP APKD Autosomal Dominant Dz’s Familial Adenomatous Polyposis Genetics and Cell Level: ○ Deletion on chromosome 5q21-22 (APC gene) Phenotypic Traits: ○ Colon covered with polyps after puberty that progress to cancer if not resected Clinical Presentation: anemia, melena, changes in bowel habits Misc info: ○ Will need colonoscopies early and often FAPCC Autosomal Dominant Dz’s Familial hypercholesterolemia (HLP type 2A) Genetics and Cell Level: ○ Defective or absent LDL receptor ○ Heterozygotes (1:500) ~ 300 mg/dl ○ Homozygotes (very rare) ~ 700+ mg/dl Phenotypic Traits: ○ Xanthelasma palpebrarum, tendon xanthomas (classically on the Achilles tendon), severe atherosclerotic dz, MI may develop early Familial Hypercholesterolemia Autosomal Dominant Dz’s Huntington’s Disease Genetics and Cell Level: ○ Gene located on Chromosome 4, trinucleotide repeat disorder (CAG)n ○ Decreased levels of GABA and Ach in the brain Clinical Presentation: depression, progressive dementia, choreiform movements, caudate atrophy ○ Usually presents between the ages of 20 to 50 Misc info: ○ Age of onset is variable but typically the more repeats you have the earlier the onset of the disease ○ Watch out for ethical issues! Autosomal Dominant Dz’s Marfan’s Syndrome Genetics and Cell Level: ○ Mutation in the fibrillin gene (Chrom 15) Phenotypic Traits: ○ Connective tissue disorder affecting skeleton, heart, and eyes Clinical Presentation: tall with long extremities, pectus excavatum, hyperextensive joints, and long tapering fingers and toes Misc info: ○ Cystic medial necrosis of the aorta leads to aortic incompetence and dissecting aortic aneurysms ○ Floppy mitral valve ○ Subluxation of lenses Marfan’s Syndrome Autosomal Dominant Dz’s Multiple Endocrine Neoplasia (MEN) Type 1 Type 2a Type 2b Eponym Wermer’s syndrome Sipple Syndrome MEN 3 (old name) Clinical Pancreatic tumors, Parathyroid adenoma, Pituitary hyperplasia Parathyroid hyperplasia, Medullary thyroid carcinoma, phechromocytoma Medullary thyroid carcinoma, phechromocytoma, marfanoid habitus, mucosal neuromas Gene MEN1 RET proto-oncogene RET protooncogene Misc Spontaneous mutation rate ~50% Autosomal Dominant Dz’s Neurofibromatosis 1 (NF1/von Recklinghausen’s dz) Genetics and Cell Level: ○ Mutation on chromosome 17q11 (long arm of 17) Clinical Presentation: café-au-lait spots, neural tumors, Lisch nodules (pigmented iris hamartomas) Misc info: ○ Increased incidence of pheochromocytomas, susceptibility to tumors, and skeletal disorders NF1 Autosomal Dominant Dz’s Neurofibromatosis 2 (NF2) Genetics and Cell Level: ○ Mutation on chromosome 22q12 Clinical Presentation: bilateral acoustic neuromas on CN8, juvenile cataracts ○ Tumors may cause tinnitus, HA, hearing loss, balance problems, vertigo, etc. Autosomal Dominant Dz’s Tuberous Sclerosis Genetics and Cell Level: ○ Incomplete penetrance, 2/3 of new cases arise from spontaneous mutations Clinical Presentation: facial lesions (adenoma sebaceum), hypopigmented “ash leaf spots”, cortical and retinal hamartomas, seizures, mental retardation, renal cysts and angiomyolipomas, cardiac rhabdomyomas, increased incidence of astrocytomas Misc: ○ Needless to say presentation is VERY variable Tuberous Sclerosis: “Ash Leaf Spot” Autosomal Dominant Dz’s Von Hippel-Lindau disease Genetics and Cell Level: ○ Deletion of VHL gene (tumor suppressor) on chromosome 3, results in expression of HIF and activation of angiogenic growth factors Phenotypic Traits: ○ Hemangioblastomas of retina/cerebellum/medulla ○ About ½ of affected develop multiple b/l renal cell carcinomas and other tumors Clinical Presentation: miscellaneous can be discomfort from growing tumors or blindness 2/2 tumors in retina Autosomal Recessive Dz’s a1-antitrypsin deficiency Genetics and Cell Level: ○ Serine protease inhibitor important for elastase Clinical Presentation: COPD and cirrhosis in early adulthood Misc: ○ Important when presented with pt who has COPD sx’s and has only smoked for a few years PiMM: 100% (normal) PiMS: 80% of normal serum level of A1AT PiSS: 60% of normal serum level of A1AT PiMZ: 60% of normal serum level of A1AT PiSZ: 40% of normal serum level of A1AT PiZZ: 10-15% (severe alpha 1-antitrypsin deficiency) PiZ is caused by a glutamate to lysine mutation at position 342 PiS is caused by a glutamate to valine mutation at position 264 Autosomal Recessive Dz’s Cystic Fibrosis: this one is important Genetics and Cell Level: ○ CFTR gene mutation on chrom 7 DF508 classically (loss of phenylalanine) ○ Defective Cl channel Clinical Presentation: secretion of abnl thick mucus into lungs, pancreas, and liver ○ Pulm infections (P. aeruginosa and S. aureus) ○ Chronic bronchitis, bronchiectasis, pancreatic insufficiency, male infertility (absence of vas deferens) Autosomal Recessive Dz’s Cystic Fibrosis (cont.) Diagnosis: ○ increased concentration of Cl in sweat test Treatment: ○ N-acetylcysteine to loosen mucus plugs Misc: ○ If presented with . . . THINK CF! newborn with meconium ileus or failure to thrive Fat soluble vitamin deficiency Pancreatic insufficiency Autosomal Recessive Dz’s PKU Genetics and Cell Level: ○ Defect in phenylalanine hydroxylase which converts Phe to Tyr Clinical Presentation: Mental retardation, seizures, albinism, “musty odor” to urine and sweat Misc: ○ Very treatable diet low in Phe and high in Tyr ○ Newborn screening is Mandatory! Autosomal Recessive Dz’s Sickle Cell Disease Genetics and Cell Level: ○ Point mutation in Beta-globin chain Glutamic acid to Valine Clinical Presentation: ○ Heterozygotes usually clinically silent but added protection to malaria ○ Homozygotes: symptoms are complications of sickled RBC must be vaccinated against S. pneumo before loss of spleen Hyposplenism, vaso-occlusive crises, many other complications including priaism, stroke, etc. Misc: Parvovirus B19 can cause aplastic crisis Treatment: Hydroxyurea, Folic acid, pain control for vaso-occlusive crises X-Linked Recessive Dz’s Fragile X (most common inherited form of retardation) Genetics and Cell Level: ○ Expansion of CGG on chrom X (FMR1 gene), full mutation is > 200 repeats ○ Associated with chromosomal breakage (hence the name) Clinical Presentation ○ Mental retardation ranges from mild to severe ○ Also autism, elongated face, large or protruding ears, flat feet, macroorchidism, and low muscle tone ○ Fragile X = eXtra-large testes, jaw, and ears Misc: Presentation is variable but si/sx fall into six classic categories ○ ○ ○ ○ ○ ○ Intelligence and learning Physical Social and emotional Speech and language Sensory Disorders commonly associated or sharing features with Fragile X X-Linked Recessive Dz’s Hemophilia A Genetics and Cell Level: ○ Loss of Factor VIII Clinical Presentation: Increased PTT but normal PT and bleeding time ○ Bleeding can occur into many sites most common are joints, brain, muscles, and GI tract Treatment is with Factor VIII ○ If dz is caused by low levels of Factor VIII and not loss then desmopressin can be used X-Linked Recessive Dz’s Hemophilia B aka Christmas Dz Genetics and Cell Level: ○ Loss of Factor IX Clinical Presentation: Increased PTT but normal PT and bleeding time ○ Bleeding can occur into many sites most common are joints, brain, muscles, and GI tract REVIEW THE CLOTTING CASCADE X-Linked Recessive Dz’s G6PD (aka Favism) Genetics and Cell Level: ○ Defect in glucose 6-phosphate dehydrogenase Clinical Presentation: ○ Prolonged neonatal jaundice can be complicated by kernicterus ○ Acute hemolytic anemia in the presence of simple infection, fava beans, or rxn with certain medicines (antibiotics, antipyretics, and antimalarials) Misc: Look for Heinz bodies on peripheral smear in active process G6PD Muscular Dystrophies Duchenne’s Genetics and Cell Level: ○ Frame shift mutation in dystrophin gene (DMD) leads to deletion and accelerated muscle breakdown. ○ Dystrophin anchors muscle fibers, primarily skeletal and cardiac muscles Clinical Presentation: Dx by increased CPK and muscle biopsy, onset before age 5 ○ Weakness begins in pelvic girdle and progresses superiorly ○ Pseudohypertrophy of calf muscles 2/2 fibrofatty replacement of muscle Misc: Look for use of Gower’s maneuver Gower’s maneuver Muscular Dystrophies Becker’s Genetics and Cell Level: ○ Defect in dystrophin gene, less severe than Duchenne’s defect Clinical Presentation: ○ Progressive muscle weakness, onset later than Duchenne’s Misc: dx is similar to Duchenne’s Autosomal Trisomies Down Syndrome (Trisomy 21) Most common chromosomal disorder and most common cause of congenital mental retardation Diagnosis done by triple screen ○ decr. a-fetoprotein, estriol, incr. b-hCG ○ Quad screen is above plus inhibin A (incr is +) ○ U/S shows increased nuchal translucency Clinical Presentation: ○ Mental retardation, flat facies, prominent epicanthal folds, simian crease, duodenal atresia, congenital heart dz (septum primum type ASD), hypotonia Misc: increased risk of ALL and Alzheimer's dz Autosomal Trisomies Down Syndrome (Trisomy 21) (cont.) 95% of cases due to meiotic nondisjunction of homologous chromosomes ○ Associated with advanced maternal age 1:1500 at maternal age 20-24 1: 210 at maternal age 35-39 1: 25 at maternal age >45 4% of cases due to Robertsonian translocation ○ Long arm of chrom 21 is attached to another chromosome and is kept diploid during gametogenesis 1% of cases due to Down mosaicism Down Syndrome Autosomal Trisomies Edward’s Syndrome (Trisomy 18) Edward’s = Eighteen Most common trisomy in live birth after Down syndrome (1:8000) Clinical Presentation: ○ Severe mental retardation, rocker-bottom feet, micrognathia, low-set ears, clenched hands, prominent occiput, congenital heart dz Misc: Death usually within one year of age Autosomal Trisomies Patau’s Syndrome (Trisomy 13) Incidence is 1:15000 Clinical Presentation: ○ Severe mental retardation, rocker-bottom feet, microphthalmia, microcephaly, cleft lip/Palate, holoProsencephaly, Polydactyly, congenital heart dz (anyone see a theme??) Misc: Death usually within 1 year of birth Nondisjunction Cri-du-Chat syndrome Genetics and Cell Level: ○ Congenital microdeletion of short arm of chromosome 5 (46 XX or XY, 5p-) Clinical Presentation: ○ Microcephaly, moderate to severe mental retardation, epicanthal folds, cardiac abnormalities Misc: Cri-du-chat is French for cry of the cat. The disease is named this way as the children affected make a high pitched mewing/crying sound. Williams syndrome Genetics and Cell Level: ○ Congenital microdeletion of long arm of chromosome 7 (46 XX or XY, 7q-) which includes the elastin gene Clinical Presentation: ○ Distinctive “elfin” facies, mental retardation, well-developed verbal skills, cheerful disposition, extreme friendliness with strangers, cardiovascular problems 22q11 deletion syndromes Variable presentation includes Cleft palate Abnormal facies Thymic aplasia which leads to T-cell deficienies Cardiac defects Hypocalcemia 2/2 parathyroid aplasia CATCH-22 22q11 deletion syndromes Aberrant development of 3rd and 4th branchial pouches DiGeorge Syndrome: ○ Thymic, parathyroid (hypocalcemia), and cardiac defects Cardiac defects include Tetralogy of Fallot, VSD, and perisistent truncus arteriosus Velocardiofacial syndrome: ○ Palate, facial, and cardiac defects Hardy-Weinberg Genetics If a population is in HW equilibrium and p and q are separate alleles then Disease prevalence: p2 + 2pq + q2 =1 Allele prevalence: p +q = 1 2pq = heterozygote prevalence The prevalence of an X-linked recessive dz in males = q and in females is q2 Hardy-Weinberg laws 1. No mutation occurring at the locus 2. No selection for any of the genotypes at the locus 3. Completely random mating 4. No migration