Genetic Pathology PDF

Pathology Concepts III Principles of Genetic Disorders BMS 100 Week 9 Genetic Mutations 1. Gene mutations 1. 2. 3. 4. Frameshift Mutation Point mutation Trinucleotide-repeat mutations Mutations within noncoding sequences 2. Complex multigenic disorders 3. Chromosomal disorders Genes and Disea...

Pathology Concepts III Principles of Genetic Disorders BMS 100 Week 9 Genetic Mutations 1. Gene mutations 1. 2. 3. 4. Frameshift Mutation Point mutation Trinucleotide-repeat mutations Mutations within noncoding sequences 2. Complex multigenic disorders 3. Chromosomal disorders Genes and Disease • genotype commonly contributes to disease ▪ Estimated that lifetime frequency is 670/1000 ▪ 50% of spontaneous abortions/miscarriages have a chromosomal abnormality ▪ 5% of those under age 25 develop a disease with a significant genetic component ▪ … and these are just the diseases we understand Classification of mutations • Point mutations ▪ 3 types of point mutations: all involving changes at one (or very few) nucleotides • SUBSTITUTION ▪ THE FAT CAT ATE THE RAT ▪ THE FAT HAT ATE THE RAT • INSERTION ▪ THE FAT CAT ATE THE RAT ▪ THE FAT HCA TAT ETH ERA T • DELETION ▪ THE FAT CAT ATE THE RAT ▪ THE FAT ATA TET HER AT • C is missing Classification: Substitution Substitutions can be: -Transitions -interchanges of purines (A, G) or of pyrimidines (C, T) - involve bases of similar shape (both one ring or both two ring) -Transversions -interchanges of purine for pyrimidine bases - involve exchange of one-ring and two-ring structures Classification of mutations • Insertions or deletions of nucleotide(s) ▪ Insertions or deletions of single nucleotides can lead to frameshift mutations – all of the triplets are off by one • These are often called “frame-shifting indels” • Often results in total loss of function of the protein: ▪ “O” blood type results from a frameshift mutation and loss of function of the red blood cell antigen ▪ Tay-Sachs disease ▪ If a multiple of three nucleotides are inserted or deleted, then the reading frame is preserved • These are often called non-frameshifting “indels” • Example – most common cystic fibrosis mutation Nucleotide deletions One nucleotide deletion – frameshift • Protein is no longer functional Three nucleotide deletion – non-frameshift, but loss of an amino acid • Most common mutation in cystic fibrosis Classification of mutations • Point mutations ▪ Silent or conservative missense point mutation = little or no change in function ▪ nonconservative missense point mutation = significant change in function • Example – sickle cell anemia ▪ If the nucleotide triplet being changed becomes a stop codon, then premature ending of translation → truncated (shortened) protein = non-sense mutation • Example – some types of thalassemia Codon Chart Adenine (A) Cystosine (C) Guamine (G) Thymine (T) Point mutations Non-conservative missense • sickle-cell anemia Nonsense • beta-thalassemia Mutations aren’t all bad • Due to a mutation, some people can’t be infected by the HIV virus ▪ HIV uses a chemokine receptor, CCR5, to enter cells; a deletion in the CCR5 gene thus protects from HIV infection • Sickle-cell trait – protects against malaria ▪ RBCs that have some sickle-cell hemoglobin are not good hosts for the parasite that causes sickle cell disease – thus the trait (heterozygote patient) is protective ▪ However, the homozygote (all hemoglobin is sickle-cell hemoglobin) is more vulnerable to the disease than rest of the population Genetic disorders • Mendelian Disorders ▪ Autosomal mutations • Dominant: structural proteins ▪ Marfan Syndrome (structural protein defect) • Recessive: enzyme defects ▪ Lysosomal storage diseases ▪ X-Linked disorders • Hemophilia • Pedigree Drawing Mendelian disorders • Due to mutations in single genes that have large effects ▪ Thought that everyone has 5 – 8 non-beneficial gene mutations • Most of these have relatively small effects on phenotype ▪ 80 – 85% familial, rest are new mutations • this can differ depending on the type of disorder – 80% of those with achondroplasia (defect in elongation of the bone growth plate) are new mutations ▪ Traits can be dominant, recessive, or codominant Autosomal dominant disorders • Manifested in heterozygous or homozygous state • Usually have at least one parent with the disorder ▪ Exception is if a spontaneous mutation occurs ▪ New mutations more common when father is older • Usually manifests in each generation Autosomal dominant disorders • Penetrance = how likely the mutated gene is to be expressed ▪ So, if something is autosomal dominant but has a 50% penetrance, a heterozygote may only have a 50% chance of showing the disease phenotype • Expressivity = how “much” the disorder-causing gene is expressed ▪ All heterozygotes still show the trait ▪ The “intensity” of the trait differs from person to person, though • Unsure of the molecular mechanisms behind this variability for most conditions Autosomal dominant disorders • Most mutations lead to a protein that has reduced function, or is produced less (loss of function mutations) ▪ Autosomal dominant disorders tend to involve genes that are part of metabolic pathways or regulation of these pathways ▪ Some involve defects in structural proteins • Disorders due to insufficient production of an enzyme tend to be recessive ▪ Why? • Gain of function mutations are rare, but can be autosomal dominant Marfan syndrome - basics • disorder of connective tissues, manifested principally by changes in the skeleton, eyes, and cardiovascular system • Epidemiology: prevalence of 1 in 5000 • Etiology: ▪ Disorder due to a defect in gene for fibrillin-1 • 75 – 85% are familial; the rest are new mutations • Autosomal dominant ▪ chromosome 15 ▪ 600 distinct mutations – most are missense Marfan syndrome • Pathophysiology ▪ Fibrillin is an important component of elastic connective tissue, provides a “scaffold” for elastic fibre deposition ▪ Loss of fibrillin-1 explains many findings • i.e. aneurysm formation, ligamentous laxity, defects in eye structure • Others are more difficult to explain • Thought that increased skeletal growth is due to increased bioavailability of TGF-beta, which is affected by fibrillin levels (TGF-beta can also impact smooth muscle development) Marfan syndrome • Clinical findings: ▪ Tall, with very long extremities and lax ligaments ▪ Dislocation of the lens ▪ Cardiovascular changes: • Mitral valve prolapse – malformed and “weak” heart valve • Weakness in the muscular layers of the aorta, which can lead to aortic valvular incompetence and development of serious aneurysms ▪ Variable expressivity – some individuals may be lacking certain clinical findings • i.e. skeletal findings with no ocular findings • Prognosis: Variable, main cause of mortality and morbidity are aneurysms and valvular defects ▪ Surgical repair of aneurysms, heart valves Autosomal Dominant Disorders: Marfan Syndrome Autosomal recessive disorders • Largest category of Mendelian disorders • Basic rules of Mendelian inheritance apply • As well: ▪ The expression of the defect tends to be more uniform than in autosomal dominant disorders. ▪ Complete penetrance is common. ▪ Onset is frequently early in life. ▪ Although new mutations associated with recessive disorders do occur, they are rarely detected clinically, since the individual with a new mutation is an asymptomatic heterozygote ▪ Many of the mutated genes encode enzymes • In heterozygotes, equal amounts of normal and defective enzyme are synthesized • Usually the natural “margin of safety” ensures that cells with half the usual complement of the enzyme function normally Consequences of Enzyme Defects • Accumulation of a substrate ▪ Sometimes the substrate can be toxic in high concentrations • Blockade of a metabolic pathway • Failure to inactivate another enzyme or substrate ▪ i.e. alpha-1 anti-trypsin deficiency Lysosomal storage diseases • A wide variety, but only Gaucher disease will be discussed today • Lysosomal storage disorders can be from a range of problems with lysosomal enzymes: ▪ Lack of the enzyme, leading up to a build-up of a substrate within a cell that is toxic ▪ Misfolding of the lysosomal enzyme ▪ Lack of a protein “activator” that binds to the substrate and improves the ability of the enzyme to act on it Pathophysiogy of lysosomal storage diseases • In the example shown, a complex substrate is normally degraded by a series of lysosomal enzymes (A, B, and C) into soluble end products • deficiency or malfunction of one of the enzymes (e.g., B) → incomplete catabolism → insoluble intermediates that accumulate in the lysosomes → “primary storage” problem ▪ huge, numerous lysosomes interfere with cellular function • secondary storage problem = toxic effects from defective autophagy ▪ autophagy = “cellular housecleaning” Gaucher Disease • Most common lysosomal storage disease ▪ Between 1 in 20,000 and 1 in 40,000 live births ▪ Autosomal recessive inheritance • Defect in the gene for glucocerebrosidase ▪ Enzyme cleaves the glucose residues from ceramide, found in cell membranes • glucosylceramide accumulates in lysosomes ▪ Metabolites accumulate mainly within macrophages and other phagocytic cells as they phagocytose dying cells and metabolize the membranes • This can lead to the activation or loss of function of the phagocytes Gaucher Disease • Type I – involves organs outside the central nervous system – 99% of cases ▪ Findings are mostly within the spleen and bone • Enlargement of the spleen and liver • Weakened bones → frequent fractures ▪ Often relatively mild course • Type II – involves the CNS as well as other organs ▪ Hepatosplenomegaly and rapid neurological deterioration, with death in early childhood ▪ CNS macrophage activation → production of toxic signals by macrophages → neuronal death X-Linked Disorders • All sex-linked disorders are X-linked, and the vast majority are recessive ▪ Males with mutations affecting the Y-linked genes are usually infertile, and hence there is no Y-linked inheritance • Features: ▪ An affected male does not transmit the disorder to his sons, but all daughters are carriers. • Sons of heterozygous women have a 1 in 2 chance of receiving the mutant gene ▪ The heterozygous female usually does not express the full phenotypic change because of the paired normal allele X-linked disorders • Gene carried on the X chromosome and usually only manifests in males ▪ A male with a mutant allele on his single X chromosome = hemizygous for the allele • X-linked recessive inheritance: ▪ transmitted by healthy heterozygous female carriers to affected males ▪ affected males to their obligate carrier daughters • consequent risk to male grandchildren through these daughters • affected males can’t transmit to sons X-linked recessive disorders – a few examples X-linked recessive – Hemophilia A • Loss of function of a coagulation factor necessary for clotting ▪ Affects over 20,000 men in North America ▪ Different mutations confer different bleeding risk – thousands of mutations have been identified with variable impacts on coagulation • Clinical Features ▪ Bruising and prolonged bleeding with minimal trauma ▪ Mucosal bleeding, hematomas in joint spaces (hemarthrosis) Pedigree drawing – how genetic diseases “appear” in families • The person being “examined” (usually the one with a genetic condition) is known as the proband ▪ Position of the proband in the family tree is indicated by an arrow ▪ A complete family history is then taken “centered” around the proband • Diagram developed is called a pedigree Pedigree drawing Pedigree – autosomal dominant • Frequent appearance of the disease throughout generations ▪ may not show typical 50% chance of transmission (remember reduced penetrance) Affects both males and females Pedigree – autosomal recessive • The risk of autosomal recessive disorders manifesting increases if there is consanguinity ▪ if a homozygote has offspring with a heterozygote, can also look deceptively frequent • Often parents of affected proband not affected Pedigree – X-linked recessive Only males appear affected Trait is never passed from father to son May see “knight’s move” pattern of transmission