Genetic Diseases.pdf

Genetic Diseases Ana Yuil-Valdes, M.D. Department of Pathology and Microbiology  Learning Objectives Understand the definitions of congenital and hereditary conditions Classification of genetic disorders Understand the modes of transmission of Mendelian traits (...

Genetic Diseases Ana Yuil-Valdes, M.D. Department of Pathology and Microbiology  Learning Objectives Understand the definitions of congenital and hereditary conditions Classification of genetic disorders Understand the modes of transmission of Mendelian traits (autosomal dominant, autosomal recessive, and X-linked) Understand the risk factors and etiology of chromosomal abnormalities Be able to give instances in which chromosomal analysis is indicated  Introduction Almost every disease has a genetic component HTN, DM, cardiovascular disease, predisposition to cancer A large percentage of miscarriages/abortions are due to fetal chromosomal abnormalities Human Genome Project: 30,000 genes now sequenced Central Dogma of Genetics  Introduction Definitions: – Congenital– a defect or condition that is present at birth (birth defects), DOES NOT AUTOMATICALLY MEAN GENETIC (i.e. congenital syphilis). “Born with” – Hereditary– disease/defect/condition that has a genetic basis. Are derived from one's parents and are transmitted in the germline through the generations and therefore are familial.  Genetic Diseases Arise from a mutation (permanent change) in a person’s DNA Mutations that affect germ cells are transmitted to the offspring and can give rise to inherited diseases. Mutations that arise in somatic cells understandably do not cause hereditary diseases but are important in the genesis of cancers and some congenital malformations.  Classification of Genetic Disorders Disorders related to mutations in single genes with large effects Cause the disease or predispose to the disease (hemoglobinopathies) Highly penetrant, meaning that the presence of the mutation is associated with the disease in a large proportion of individuals Chromosomal disorders Structural or numerical alteration in the autosomes and sex chromosomes Complex multigenic disorders More common Caused by interactions between multiple variant forms of genes and environmental factors (multifactorial disorders) Atherosclerosis, diabetes mellitus, hypertension. SINGLE GENE DEFECTS  Single Gene Defects Types of single gene mutations: – POINT– A point mutation is a change in which a single base is substituted with a different base. – FRAMESHIFT MUTATION INSERTION – addition of nucleotide(s) DELETION– loss of nucleotide(s) The entire DNA sequence following the mutation will be read incorrectly. – TRINUCLEOTIDE REPEAT– amplification of a set of three e.g. fragile X syndrome – FRAXA gene; Xq27.3; CGG repeat.  Single Gene Defects -ACC-CAG-AGG-CTC- (normal) -ACC-CGG-CGG-CGG- CGG-CGG-CGG-CGG- CGG-CGG…- -ACG-CAG-AGG-CTC- (trinucleotide (point) repeat) -AC(_)C-AGA-GGC-TC...- (deletion)  Point mutation-Sickle cell disease Point mutation in the B-globin chain of hemoglobin Amino acid glutamate (normal) replaced with valine (mutant) Laurentino, M.R., Parente Filho, S.L.A., Parente, L.L.C. et al. Non-invasive urinary biomarkers of renal function in sickle cell disease: an overview. Ann Hematol 98, 2653–2660 (2019). Insertion mutation  Trinucleotide Repeat Disorders Caused by expansion of trinucleotides. Repeat number at a locus remains stable below a certain threshold. Beyond that threshold, the locus becomes unstable (in this range, the allele is considered a premutation). After further expansion, the allele can become a full mutation.  TRINUCLEOTIDE REPEAT National Library of Medicine (US). Genetics Home Reference [Internet]. Bethesda (MD): The Library; 2020 August 17. Available from: https://ghr.nlm.nih.gov/.  Transmission Patterns of Single-Gene Disorders Follow MENDELIAN rules (aka “MENDELIAN DISORDERS”) Autosomal dominant Autosomal recessive X-linked disorders  Autosomal Dominant Disorders Manifested in the heterozygous state Structural proteins are involved One parent is usually affected Male and females are affected Both can transmit the condition  Autosomal Dominant n = normal Affected Normal D = mutant Father Mother Only one copy of the mutant allele (D) results in Dn nn disease manifestation. Dn nn Dn nn Affected Normal Affected Normal 50% chance of child being affected  Autosomal Dominant Disorders Nervous System Huntington’s Disease (also triplet repeat) Neurofibromatosis Tuberous Sclerosis Urinary Adult Polycystic Kidney Disease Musculoskeletal Ehlers-Danlos (elastin) Marfan (fibrillin) Osteogenesis imperfecta (collagen) Metabolic Familial hypercholesterolemia Hematopoietic Von Willebrand’s disease Modified from Robbins, Table 7.2- p.228)  Marfan’s syndrome Marfan’s syndrome: a defect in fibrillin, a structural protein that is found in elastic fibers in various tissues - cause of chest pain/sudden death in young adults due to aortic dissection  Autosomal Recessive Disorders Largest category of Mendelian disorders Both alleles are mutated Almost all inborn errors of metabolism  Autosomal Recessive N = normal r = recessive mutant Two copies of the mutant allele are needed to be affected. 25% chance of child being affected  Autosomal Recessive Often caused by : – A defect in an enzyme in some metabolic pathway– most are METABOLIC problems – A defect in a protein that makes it function less well (enzyme 1) (enzyme 2) A B C “Disease” manifestations related to a build-up of “A” or a lack of “B” or “C”.  Cystic Fibrosis Autosomal recessive inheritance 1:25 Caucasian adults are carriers – 1:3200 live births Mutations in CFTR channel – Over 1000 mutations described. – Most common (~66%) is ΔF508: 3-base deletion of codon 508 resulting in a CFTR protein that lacks phenylalanine and is rapidly degraded. Figure 7-29 (pg. 265)  Autosomal Recessive Diseases Metabolic Cystic fibrosis Phenylketonuria (PKU) Alpha-1-antitrypsin deficiency Hemochromatosis Wilson’s disease Hematopoietic Sickle cell disease Endocrine Congenital adrenal hyperplasia Nervous Friedreich ataxia (triplet repeat disorder) (Modified from Robbins: Table 7-3, p.229)  Cystic Fibrosis Organs affected – Lungs: Bronchial obstruction, recurrent and severe pneumonias, especially with P. aeruginosa – Pancreas – malabsorption, pancreatitis. – Small intestine: obstruction (meconium ileus). – Reproductive tract: Male infertility in 95%. – Skin: excessive NaCl excretion. Diagnosis – Elevated sweat chloride – Genetic testing for the common mutations http://www.hgen.pitt.edu/counseling/public_health/cystic_fibrosis.htm  X-Linked Disorders Sex-linked disorders are X-linked Males with mutations affecting the Y-linked genes are usually infertile, and hence there is no Y-linked inheritance. These mutations may be recessive or dominant X-linked dominant disorders are very rare Almost all are recessive Males are hemizygous for the X chromosome Disorders are expressed in the male An affected male does not transmit the disorder to his sons Evidence of X-linked recessive inheritance Generally, only males are affected by the trait All daughters of affected males are carriers. All sons of affected males are unaffected and are not carriers.  Evidence of X-linked recessive inheritance Only males are affected by the trait 50% of children receive the mutant allele from a carrier mother. – 50% of daughters of a carrier mother are carriers. – 50% of sons of a carrier mother are affected.  Evidence of X-linked recessive inheritance Men only have one X-chromosome to begin with. So they only need one non-working (or mutated) gene to show symptoms of an X-linked condition. Women have an extra X-chromosome. Even if a woman has a mutation on one X-chromosome, she’ll probably have a working version on the other chromosome. Usually, enough of a woman’s cells express the working version of the gene to keep her healthy. Common X-linked recessive disorders Musculoskeletal Duchenne muscular dystrophy Blood Hemophilia A and B G6PD deficiency Immune Agammaglobulinemia Wiskott-Aldrich Metabolic Diabetes insipidus Lesch-Nyhan syndrome Nervous Fragile X syndrome (Modified from Robbins, Table 7-4, p.229)  Fragile X Syndrome X-linked recessive disorder Common cause of familial mental retardation Clinical presentation: – Males – Moderate to severe mental retardation – Physical phenotype: large mandible, large everted ears, macroorchidism May be subtle  Fragile X Syndrome Trinucleotide repeat disorder – Caused by expansion of trinucleotides (CGG in FMR1 gene) – Repeat number at a locus remains stable below a certain threshold. – Beyond that threshold, the locus becomes unstable (in this range, the allele is considered a premutation). – After further expansion, the allele can become a full mutation.  Mitochondrial Disorders Maternal inheritance The ova contain numerous mitochondria within their abundant cytoplasm, whereas spermatozoa contain few, if any. The mitochondrial DNA complement of the zygote is derived entirely from the ovum. 100% of children of affected mothers are affected. 100% of children of affected fathers are unaffected. e.g. Leber hereditary optic neuropathy CHROMOSOMAL ABNORMALITIES  Chromosomal Abnormalities Common in live born (1 in 200) infants and first-trimester aborted fetuses (50%) Alterations in number or structure of chromosomes Can affect autosomes or sex chromosomes  Numerical Chromosomal abnormalities Euploid = a multiple of the haploid (n) number. Diploid = 2n  normal for humans is 46 Aneuploid = not a multiple of n – Monosomy – 1 copy of a chromosome (2n-1) E.g. Turner syndrome (45, X0) – Trisomy – 3 copies of a chromosome (2n+1) E.g. Down Syndrome (47, XY+21)  Structural Chromosomal Abnormalities Isochromosome – one chromosome arm lost (e.g. iXq) Deletion – Missing portion of a chromosome Translocation – 2 chromosome segments break and cross-over – Balanced reciprocal – robertsonian – acrocentric chromosomes:  Features of Chromosomal Abnormalities May be numerical or structural More severe defects with loss of chromosomal material Sex chromosome imbalances tolerated better than autosomal imbalances Most cases are de novo (normal parents and low risk of recurrence in siblings)  Chromosomal Abnormalities Most aneuploid embryos/fetuses die in utero – 50% of spontaneous abortions in first trimester have a chromosomal abnormality Only a few of these survive to live birth – Trisomy 13 (Patau syndrome) – Trisomy 18 (Edwards syndrome) – Trisomy 21 (Down syndrome) Sex chromosome abnormalities: – Turner syndrome- 45,X0 – Klinefelter syndrome- 47,XXY  Down Syndrome 1:1550 live births in moms less than 20, 1:25 if mom greater than 45 95% cases caused by Trisomy 21 (also caused by a Robertsonian translocation)  Down Syndrome 47, XX, +21 Mental retardation Epicanthic folds Flat facial profile Hypotonia Congenital heart defects Predisposition to leukemia Early Alzheimer’s  Turner Syndrome 1:2500 female live births 99% of affected fetuses abort spontaneously 57% of cases caused by monosomy X  Turner Syndrome 45, X Short stature Webbed neck Infertility Heart defects (coarctation of the aorta) Broad chest and widely spaced nipples Peripheral edema DIAGNOSIS OF GENETIC DISEASES  Karyotype View entire chromosomal complement Advantages: – Global strategy Disadvantages: – Low resolution – Only dividing cells – Requires fresh/frozen tissue  Fluorescence In Situ Hybridization DNA probes to specific chromosomal regions. Advantages: – Dividing and nondividing cells – Fresh or fixed tissue – High resolution Disadvantages: – Must know what suspected abnormalities is  Comparative Genomic Hybridization Patient DNA and reference DNA hybridized labeled with 2 different dyes and mixed; color signal corresponds to DNA signal Disadvantages: – May miss submicroscopic alterations Courtesy of: https://www.genomicseducation.hee.nhs.uk/genotes/knowledge-hub/microarray-array-cgh/  Molecular Diagnosis of Genetic Disorders Detects mutations at the levels of nucleotides Direct detection of DNA mutations by PCR analysis Indirect detection based on linkage analysis  Polymerase Chain Reaction Exponential amplification of DNA RT-PCR: RNA → cDNA → amplification Advantages: – Requires very little patient sample – Detects mutations at level of nucleotides Disadvantages: – Sequence of normal gene must be known  Indications for Genetic Analysis Prenatal genetic analysis Postnatal genetic analysis Mother of advanced age (>34 Multiple congenital years) anomalies Parent is a known carrier of a Unexplained mental balanced translocation, retardation Robertsonian translocation, or Suspected aneuploidy inversion Suspected sex chromosomal abnormality Parent with a previous child with Infertility a chromosomal abnormality Multiple spontaneous Parent who is a carrier of an abortions X-linked genetic disorder QUESTIONS