Transcript genetics
EDWARD M. SANTOS, MD Department of Pediatrics UERMMMC Identify the Condition Identify HUMAN GENETICS The Molecular Basis of Genetic Disorders The Molecular Diagnosis of Genetic Disorders Patterns of Inheritance Chromosomal Clinical Abnormalities Gene Therapy Genetic Counselling Newborn Screening A. THE MOLECULAR BASIS OF GENETIC DISORDERS THE HUMAN GENOME Approximately 38,000 genes – individual units of heredity for all traits Haploid – one copy (reproductive or germ line cells) Diploid – 2 complete copies (somatic cells) Genes organized into long segments of DNA during cell division are compacted into intricate structures with proteins CHROMOSOMES Somatic cell – 46 chromosomes Germ cells (eggs and sperm)- 23 chromosomes Human Genome Project HUMAN GENOME PROJECT The genome is very lumpy- some areas have functional genes packed together; other areas are composed of filler DNAs Humans may have fewer genes than expected, approximately 38,000. Many lower organisms have more genes than humans Human genes make more proteins per gene (3 on average) than many other organisms Human proteins are more complex than those of many other organisms Human Genome Project Dozens of human genes may be the result of horizontal transfer from bacteria The repetitive sequence in the human genome provide a fossil record dating back 800 million years A major component of the filler DNA has an important function The male mutation rate is approximately twice that of the female mutation rate Human Genome Project Humans (including all different racial and ethnic groups) are 99.9% identical at the functional gene level, implying that there is no genetic basis for precise racial categorization. Nevertheless, various genes and genetic markers are specific for different races. STRUCTURE AND FUNCTION OF GENES Basic purpose – production of structural proteins and enzymes Transcription, Processing, Translation Three bases in DNA code for one amino acid. The DNA code is copied to produce mRNA. The order of amino acids in the polypeptide is determined by the sequence of 3-letter codes in mRNA. Transcription Transcription is the synthesis of mRNA from a DNA template. It is like DNA replication in that a DNA strand is used to synthesize a strand of mRNA. Only one strand of DNA is copied. A single gene may be transcribed thousands of times. After transcription, the DNA strands rejoin. Translation Translation is the process where ribosomes synthesize proteins using the mature mRNA transcript produced during transcription. MUTATIONS Change in the DNA sequence Somatic vs germ cells Point Mutation Silent mutation : no change in the amino acid Missense: a different amino acid Non-sense mutation: stop codon is specified Insertions and Deletions Frameshift mutation : if the deletion or insertion is not a multiple of three Tandem repeat DNA sequences Ex: CGGCGGCGGCGGCGG Point Mutation Deletion Mutation GENOTYPE AND PHENOTYPE CORRELATIONS IN GENETIC DISEASES Genotype – genetic constitution of an individual Refers to which particular allele is present at a locus on the chromosome Phenotype – observed structural, biochemical and physiologic characteristics B. MOLECULAR DIAGNOSIS OF GENETIC DISEASES Molecular Cytogenetic Techniques FISH (Fluorescence in-situ hybridization) Subtelomeric Rearrangements Comparative Genomic Hybridization Spectral Karyotyping and Multicolor FISH Southern/Northern /Western Blotting Polymerase Chain Reaction C. PATTERNS OF INHERITANCE Genetic vs Familial Disorders The Pedigree Autosomal Dominant Inheritance Autosomal Recessive Inheritance X-linked Recessive Inheritance X-linked Dominant Inheritance Multifactorial Inheritance Non-traditional Patterns of Inheritance GENETIC VS FAMILIAL DISORDERS Genetic- caused partially or completely by an altered genetic material Some may occur in multiple family members; some are sporadic Familial disorders – more common in relatives of an affected individual than in the general population Some are genetic and some are caused by environmental factors PEDIGREE Diagram of the family history and establishes relationship among family members 3 generation pedigrees Proband – affected individual where the family is ascertained Pedigree AUTOSOMAL DOMINANT INHERITANCE Vertical transmission Any child of an affected parent has a 50% risk of inheriting the disorder Phenotypically normal family members do not transmit the condition to their offspring Males and females are equally affected A significant proportion of cases are due to new mutation Other Features Male to male transmission (vs X-linked) Variable expressivity Reduced penetrance Somatic Mosaicism or germ line mosaicism New mutations Advanced paternal age (>40 yr) Autosomal Dominant Disorders Neurofibromatosis 1 Osteogenesis imperfecta Achondroplastic dwarfism Marfan’s syndrome Apert’s syndrome Crouzon’s syndrome Neurofibromatosis 1 Incidence: 1 in 3,000 Findings: Multiple café au lait spots Neurofibromas Axillary or inguinal frecklings Optic glioma Lisch nodules Osseous lesions Osteogenesis imperfecta Incidence: 1 in 25,000 Findings Increased fragility of the bones Small face with frontal and temporal bossing Laxity of the joint capsules and ligaments Blue sclerae Hypoplasia of the dentine/enamel of the teeth Hearing impairment Achondroplasia Incidence: 1 in 25,000 Findings Generalized skeletal dysplasia Disproportionate dwarfism Large head Typical facial dysmorphism Characteristic xray findings Marfan’s Syndrome Incidence: 1 in 66,000 Findings Tall stature Marked deficit of fatty tissue Long, narrow face with high palate and narrowly spaced teeth Signs of connective tissue weakness Eye defects (lens dislocation) Aortic aneurysms Apert Syndrome Incidence: Low Findings Acrocephaly ( high “full” forehead, flat occiput) Facial dysmorphism Extensive symmetrical syndactily Crouzon Syndrome Incidence: Low Findings Acrocephaly Exophthalmos Maxillary hypoplasia with parrot-beaked nose AUTOSOMAL RECESSIVE INHERITANCE Horizontal pattern in pedigrees Males and females are equally affected Parents of an affected child are asymptomatic heterozygous carriers of the gene Recurrence risk for siblings of an affected child is 25% Pedigree The child of 2 heterozygous parents = 25% chance of being homozygous Males and females are affected with equal frequency Affected individuals are almost always born in only 1 generation of the family Children of the affected person are all heterozygotes The children of a homozygote can be affected only if the spouse is a heterozygote Parents of the affected person may be genetically related (consanguinity) Autosomal Recessive Disorders Phenylketonuria Tay Sachs disease Canavan disease Fanconi anemia Gaucher disease Cystic fibrosis Sickle cell disease Phenylketonuria Incidence: 1 in 20,000 Findings Mental deficiency Microcephaly Retarded growth Increased incidence of structural defects seizures Tay Sachs disease Common among Ashkenazi Jewish population Carrier rate: 1/25 Infantile form- most common Findings Loss of motor skills Increased startle reaction Macular pallor and cherry red spots Fanconi Anemia Heterozygote frequency: 1/100 to 1/300 1000 reported cases Findings Hyperpigmentation and café au lait spots Skeletal abnormalities Short stature Integumentary and organ abnormalities Gaucher Disease Most common lysosomal storage disease Most prevalent genetic defect among Ashkenazi Jews Incidence among Ashkenazis: 1 in 1,000 Carrier frequency= 1/18 Findings Thrombocytopenia Anemia Hepatosplenomegaly Bone pain Cystic fibrosis Incidence 1/3500 white live births 1/17000 black infants 1/90000 Asian infants Findings Pulmonary Gastrointestinal Mutation Long arm of chromosome 7 Sickle cell disease Incidence: 1 in 625 live births to African Americans Findings Hemolytic anemia Acute sickle dactylitis Acute painful episodes Mutation Hb S = result of single base pair change X-LINKED INHERITANCE Associated with altered genes on the X chromosome Most are recessive A heterozygous female will produce 50% of the normal amount of gene product An affected male who inherits the disorder is hemizygous and will express the condition X-linked Recessive Inheritance Incidence of the condition is much higher in males than in females Heterozygous female carriers are usually unaffected The gene is transferred from an affected man to all of his daughters, and any of his daughters’ sons has a 50% chance of inheriting the gene The gene is never transmitted from father to son The gene may be transmitted to a series of carrier females, in which case all affected males are related through the carrier females Significant proportion of sporadic cases are due to new gene mutations X-linked recessive disorders Hemophilia A Color blindness Duchenne muscular dystrophy Hemophilia A Classic hemophilia Deficiency in coagulation factor VIII Manifestations: prolonged bleeding Duchenne muscular dystrophy Incidence: 1 in 3,600 Findings Hypertrophy of the calves Progressive weakness Intellectual impairment Proliferation of connective tissue in muscle Question A patient of yours is getting married and comes to you for counselling. She has a brother with a rare X-linked recessive disease. Her mother's father also had the disease. She wants to know the probability of her being a carrier of the disease and the probability that she will pass the disease to her children. What is your advice? ANSWER Being a reasonably good human geneticist, you tell her that her mother was a carrier and that she has a one chance in two of being a carrier, depending upon which of her mother's X chromosomes she inherited. You also explain that if she is a carrier she will pass the affected X to her son one half of the time, but that her daughters will not be affected because they will always get a normal X from their father. X-linked dominant inheritance Condition is regularly expressed in the heterozygous female carriers All of the daughters and none of the sons of an affected man have the condition Both male and female offsprings of affected females have a 50% risk of inheriting the condition Affected females are about twice as common as affected males, but females have milder manifestations X-linked dominant disorders Hypophosphatemic rickets Incontinentia pigmenti Hypophosphatemic rickets Vitamin D resistant rickets Findings Bowing of the lower extremities No rachitic rosary, no Harrison groove Pulp deformities and intraglobular dentin lesions Metaphyseal widening and fraying and coarse appearing trabecular bone Incontinentia Pigmenti Very rare condition Condition is lethal in the male embryo Affects the skin, hair teeth and nails Blistering, rash, hyperpigmentation, alopecia, dystrophic nails, abnormal tooth shape, retinal vascular abnormalities Y-linked? In mammals, Y-linkage refers to when a phenotypic trait is determined by an allele (or gene) on the Y chromosome. It is also known as holandric inheritance. The Y-chromosome is small and does not contain many genes, therefore few traits are Y-linked, and so Y-linked diseases are rare. As only males have a Y chromosome, the genes are simply passed from father to son, with no interchromosomal genetic recombination. An example in humans of a y-linked trait may be hairy ears (it may also be sex-limited) MULTIFACTORIAL INHERITANCE Similar rate of recurrence among all first degree relatives The risk of recurrence is related to the incidence of the disease Some disorders have a sex predilection, as indicated by an unequal male:female incidence The likelihood that both identical twins will be affected is less than 100% The risk of recurrence is increased when multiple family members are affected Risk of recurrence is greater if the disorder is more severe Multifactorially determined disorders Neural tube defects Cleft lip Cleft lip with cleft palate Club feet Cardiac septal defects Diabetes mellitus Hypertension Stroke Schizophrenia NONTRADITIONAL PATTERNS OF INHERITANCE Genetic disorders that do not follow the usual pattern of dominant, recessive, x-linked or multifactorial inheritance Result from mutations in the mitochondrial DNA Because mitochondria are inherited virtually exclusively from the mother, these conditions are passed from mother to offspring without regard to sex of the latter Genomic imprinting Nontraditional inheritance Kearnes Sayre syndrome Leber hereditary optic neuropathy Prader-Willi syndrome Angelman syndrome Prader-Willi Syndrome Long arm of chromosome 15 Findings Severe hypotonia at birth Obesity Short stature Small hands and feet Hypogonadism Mental retardation D. CHROMOSOMAL CLINICAL ABNORMALITIES CHROMOSOMAL CLINICAL ABNORMALITIES Nomenclature Karyotype- visual display of chromosomes Normal karyotype: 46XX, or 46XY Cell Division Mitosis and meiosis Methodology Karyotyping In situ hybridization Comparative Genomic hybridization CHROMOSOMAL ABNORMALITIES Abnormalities of chromosome number Aneuploidy and polyploidy Trisomies Abnormalities of chromosome structure Deletions, translocations, inversions, duplications and insertions Sex chromosome anomalies Chromosomal breakage syndromes Mosaicism Trisomies Trisomy 13 (Patau Syndrome) Trisomy 18 (Edwards Syndrome) Trisomy 21 (Down Synrome) Trisomy 8 (mosaicism) Trisomy 13 (Patau Syndrome) Incidence: 1/10,000 births Findings Cleft lip often midline Flexed fingers with polydactyly Ocular hypotelorism Low set malformed ears Small abnormal skull Cerebral malformations Cardiac malformations Visceral and genital anomalies Trisomy 18 (Edwards syndrome) Frequency: 1/6,000 births Findings Low birthweight Closed fists with overlapping fingers Narrow hips an short sternum Rockerbottom feet Microcephaly Cardiac and renal malformations MR Trisomy 21 (Down syndrome) Incidence: 1/600-800 births Findings Hypotonia Upward and slanted palpebral fissures and epicanthic folds Speckled irises (Brushfield spots) Varying degrees of mental and growth retardation Cardiac malformations Simian crease Cri-du-chat syndrome Deletion of the short arm of chromosome 5 (5p-) Findings Hypotonia Short stature Characteristic cry Microcephaly Skeletal abnormalities Moonlike face MR Turner syndrome Incidence: 1 /4000 Complete or partial absence of the x chromosome 45x Findings Phenotypically female Short stature Underdeveloped gonads Klinefelter syndrome Male karyotype with an extra X chromosome 47XXY Findings Relatively tall Gynecomastia Delayed secondary sex development Azoospermia, small testes infertile E. GENE THERAPY GENE THERAPY Introduction of nucleic acids into a tissue to prevent, inhibit, or reverse a pathologic process Restricted for somatic cell therapy Gene transfer strategies Transferring DNA into target tissues to add expression of the exogenous gene that encodes a protein missing or supply a novel protein with a desired pharmacologic effect Inserting a nucleic acid to correct a mutation in chromosomal DNA Gene Therapy Vectors – viral or non-viral Disease Targets Immune response Replacement of tumor suppressor genes Gene induced toxicity Replication lytic viruses Transfection Agents F. GENETIC COUNSELLING GENETIC COUNSELING Advanced parental age Child with congenital anomalies Consanguinity or incest Family history of heritable disorders Pregnancy screening abnormality Stillborn with congenital anomalies Teratogen exposure or risk Requirements for Counseling Accurate diagnosis Complete family history Understanding the genetic and clinical aspects of the disorder Management of Genetic Disorders Modification of the Environment Control of the external environment Regulation of ingested food Coenzyme supplementation Substitution/replacement Modification of the internal environment Chemical modification Pharmacologic modification Endocrinologic modification Surgery Management of Genetic Disorders Genetic Engineering Protein replacement Enzyme induction and repression Transformation and transduction of the gene Newborn Screening Definition “it is the process of testing newborn babies for treatable genetic, endocrinologic, metabolic and hematologic diseases” 2004 – Republic Act 9288 : Newborn Screening Act of 2004 An act promulgating a comprehensive policy and a national system for ensuring newborn screening Requires that every baby born in the Philippines be offered an opportunity for newborn screening Newborn Screening PKU, CAH, CH, GAL, G6PD Republic Act 9288: Newborn Screening Act of 2004 Done after the 24th HOL and not later than 72 HOL Quo vadis? 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