Presentation on theme: "Genetic disorders Dr.K.V.Bharathi. Normal karyotype Study of chromosomes karyotyping A karyotype is the standard arrangement of a photographed, stained."— Presentation transcript:
Genetic disorders Dr.K.V.Bharathi
Normal karyotype Study of chromosomes karyotyping A karyotype is the standard arrangement of a photographed, stained chromosomes pairs which are arranged in order of decreasing length
When chromosomes are preparing to divide, the DNA replicates itself into two strands called chromatids Replicating chromosomeThe same chromosome under normal conditions Centromere Telomere The two chromatids
Chromosome nomenclature Two arms –p (petite) small and q (follows p in alphabet) 1-22 = autosome numbers X, Y = sex chromosomes
Cytogenetic terminology Short arm p and long arm q Each Chromosome is divided into 2 or more regions Each region is subdivided into bands and sub-bands Total no of chromosomes is given first followed by sex chromosome and finally description of abnormality in ascending order.eg:47,XY,+21,and Xp ,XY,del(16)(p11.2 p13.1)
The normal human karyotype Somatic cells: 22 pairs of autososmes & 1 pair of sex chromosomes (46,XX or 46,XY). The normal karyotype is diploid (2 copies of each chromosome). Sperm & eggs carry 23 chromosomes & are haploid (one copy of each chromosome).
What is the difference between an Autosome and a Sex-chromosome? Autosomes are the first 22 homologous pairs of human chromosomes that do not influence the sex of an individual. Autosomes are the first 22 homologous pairs of human chromosomes that do not influence the sex of an individual. Sex Chromosomes are the 23 rd pair of chromosomes that determine the sex of an individual. Sex Chromosomes are the 23 rd pair of chromosomes that determine the sex of an individual.
Sperm determines genotypic sex by contributing either an X or a Y chromosome during fertilization.
46,XX = female 46,XY = male Giemsa banding (G-banding)
Three classes of chromosome Metacentric - centromere in middle Submetacentric - centromere distant from middle Acrocentric - centromere at end
Uses of karyotype analysis: 1.Genotypic sex ( identification of X & Y chromosomes). 2.Ploidy ( euploid, aneuploid or polyploid). 3.Chromosomal structural defects (translocation, isochromosome, deletion etc..).
Some definitions Haploid (n)- refers to a single set of chromosomes (23 in humans).Sperm & eggs are haploid. Diploid (2n)- refers to a double set of chromosomes (46 in humans). Somatic cells are diploid. Euploid- refers to any multiple of the haploid set of chromosomes (from n-8n)
Polyploid- refers to any multiple of the haploid set of chromosomes> diploid (2n). Aneuploid- refers to karyotypes that do not have multiples of the haploid set of chromosomes. Monosomy- refers to an aneuploid karyotype with one missing chromosome (XO in Turners syndrome). Trisomy- refers to an aneuploid karyotype with one extra chromosome (trisomy 21 in Downs syndrome))
Aneuploidy results from the failure of chromosomes to separate normally during cell division: Meiotic Nondisjunction
NORMAL SEPARATION NORMAL ZYGOTE First meiotic division Second meiotic division Gametes Fertilization Zygotes 4N 2N N
NONDISJUNCTION TRISOMIC ZYGOTEMONOSOMIC ZYGOTE First meiotic division Second meiotic division Gametes Fertilization Zygotes
Aneuploidy usually results from non- disjunction Chromosomes or chromatids fails to separate An error of mitotic or meiotic spindle attachment to centromere May occur in either the maternal or the paternal germ cells More commonly arises in the mother Frequency of non-disjunction increases with maternal age
Structural abnormalities of chromosomes
Six main types Deletion Ring chromosome Duplication Isochromosome Inversion –paracentric & pericentric Translocation –Robertsonian & reciprocal
Involves loss of part of a chromosome Results in monosomy of that chromosomal segment Clinical effects due to –Insufficient gene products –Unmasking of mutant alleles on normal chromosome Deletion Before deletion After deletion
Two types of deletion Interstitial Terminal
Ring chromosome Breaks occur in both arms of a chromosome. The two broken ends anneal; the two acentric fragments are lost. Results in double deletion (in p and in q).
Epilepsy, mental retardation and craniofacial abnormalities
Isochromosome Mirror image chromosome Loss of one arm with duplication of other Loss of p-armDuplication of q-arm
Inversion Two breaks in one chromosome The fragment generated rotates 180 o and reinserts into the chromosome Pericentric - involves p and q arm Paracentric - involves only one arm
Translocation - exchange of chromosomal material between two or more chromosomes Reciprocal Robertsonian If no essential chromosome material lost or genes damaged then the individual is clinically normal However, there is an increased chance of chromosomally unbalanced offspring
Reciprocal Translocation Involves two chromosomes One break in each chromosome The two chromosomes exchange broken segments Before translocationAfter translocation
Robertsonian translocation Named after W. R. B. Robertson who first identified them in grasshoppers in 1916 Most common structural chromosome abnormality in humans –Frequency = 1/1000 livebirths Involves two acrocentric chromosomes Two types –Homologous acrocentrics involved –Non-Homologous acrocentrics involved
Homologous acrocentric, i.e. chromosome 14 + = lost Non-homologous acrocentric, i.e. chromosomes 14 & 21 += lost
A balanced chromosome 14 & 21 Robertsonian translocation
What is mutation? A mutation may be defined as a permanent change in the DNA. These structural DNA changes affect protein expression & function.
Mutations affect protein synthesis Transcription: Mutated DNA will produce faulty mRNA leading to the production of a faulty protein.
Somatic & Germ cell mutations Mutations that occur in somatic cells such as skin cells or hair are termed Somatic. Germline mutations occur only in the gametes. These mutations are more threatening because they can be passed to offspring.
Germline mutations can be transmitted to future generations. Those that occur in somatic cells may contribute to the pathogenesis of neoplasia. Drugs, chemical & physical agents that increase the rate of mutation act as carcinogens.
Mutagens are agents that cause mutations. They include: 1. High Temperatures 2. Toxic Chemicals (pesticides, etc) 3. Radiation (nuclear and solar)
Types of mutations Chromosomal mutation: affecting whole or a part of a chromosome Gene mutation: changes to the bases in the DNA of one gene
Major types of genetic mutations 1.Point mutations: Single base substitutions. 2.Frameshift mutations: base pair insertions or deletions that change the codon reading frame. 3.Large deletions: can result in loss of gene or juxtapose genes to create a hybrid that encodes a new fusion protein. 4.Expansion of trinucleotide repeats: can arise in genes that have repeated sequences. Affected patients can have 100s or 1000s of repeats (normal:10-30).
Gene Mutations: DNA base alterations Point mutation- eg:sickle cell anemia Insertion Deletion Inversion Frame Shifts
Point mutation - when a base is replaced with a different base. CGG CCC AAT to CGG CGC AAT Guanine for Cytosine Insertion - when a base is added CGG CCC AAT to CGG CGC CAA T Guanine is added Deletion - the loss of a base CGG CCC AAT to CGG CCA A T loss of Cytosine
Frame Shift mutations A frame shift mutation results from a base deletion or insertion. Each of these changes the triplets that follow the mutation. CGG CCC AAT to CGG CGC CAA T Frame shift mutations have greater effects than a point mutation because they involve more triplets. This in turn changes the amino acids of the protein!
Classification of genetic disorders 1.Gross chromosomal abnormalities 2.Diseases with multifactorial inheritance 3.Disorders related to mutant genes of large effect
Cytogenetic disorders involving autosomes
Common types of trisomy
Trisomy 21 - Down's Syndrome - karyotype 47, XX +21 or 47, XY+21 - frequency about 1 in 600 births
Trisomy 18 - Edward's Syndrome - karyotype 47, XX +18 or 47, XY+18 - frequency about 1 in 8,000births
Trisomy 13 - Patau's Syndrome - karyotype 47, XX +13 or 47, XY+13 - frequency about 1 in 10,000 births
Sex chromosome trisomies - 47, XXY (Klinefelter Syndrome), 47,XXX, 47,XYY Triploidies of other chromosomes – Rare – usually incompatible with life
- Polysomy X e.g. XXXX –- Frequency about 1 in 1000
Trisomy 21(Downs syndrome) The most common malformation Incidence: 1 per 660 live births, closely related to maternal age Mother s age<30 year risk:1 per 5000 Mother s age>35 year risk:1 per 250
Clinical findings Flattened face Mental retardation Congenital heart disease:50% endocardial cushion,ASD,AV malformation,VSD 10 to 20 fold increased risk of developing leukemia Infection are common Premature aging all patints older than 40 will have Alzheimer disease(degenerative disorder of brain) Musculoskeletal problems
Cytogenetic diorders involving sex chromosomes They cause chronic problems relating to sexual development and fertility They are often difficult to diagnose at birth,and many are recognised at the time of puberty Higher the number of x chromosomes, greater the likelihood of mental retardation
Lyon hypothesis : In somatic cells of a female only one of the X chromosomes is active X-inactivation –Occurs early in embryonic life –Is random either paternal or maternal X –Is complete –Is permanent –Is clonally propagated through mitosis Mary Lyon
Y chromosome Regardless of the number of X chromosomes, the presence of single Y determines male sex The gene that indicates testicular development is sry gene (sex determining region Y gene) Located on distal arm of Y chromosome
Turner syndrome Partial monosomy of X chromosome Hypogonadism in phenotypic females Karyotype:45,X Mosaic patients with 45,X /46,XX Cystic hygromas, Congenital heart disease (coarctation of aorta and bicuspid aortic valve), failure to develop secondary sexual characterstics Mental status is usually normal
Klinefelter syndrome 47,XXY Results from meiotic nondisjunction The discovery of the karyotype of Klinefelter was the first demonstration that sex in humans is determined by the presence of the Y rather than the number of X chromosomes Male hypogonadism
Klinefelter syndrome Lower IQ than sibs Lower IQ than sibs Tall stature Tall stature Poor muscle tone Poor muscle tone Reduced secondary Reduced secondary sexual characteristics Gynaecomastia Gynaecomastia (male breasts) Small testes/infertility Small testes/infertility
Plasma gonadotropin levels( FSH) and estrodiol is elevated Testosterone levels are decreased Testicular tubules are totally atrophied Some shows primitive tubules
Hermaphroditism Genetic sex is determined by the presence or absence of Y chromosome Gonadal sex is based on histological characteristics of gonads Phenotypic sex is based on the appearance of external genitalia
True hermaphrodite implies the presence of both ovarian and testicular tissue Pseudohermaphrodite represents disagreement between the phenotypic and gonadal sex (eg:female pseudohermophrodite has ovaries but male external genitalia)
Transmisson patterns of single gene disorders Autosomal dominant Autosomal recessive X-linked
Autosomal Traits Genes located on Autosomes control Autosomal traits and disorders. Genes located on Autosomes control Autosomal traits and disorders. 2 Types of Traits: Autosomal Dominant Autosomal Dominant Autosomal Recessive Autosomal Recessive
Autosomal Dominant Traits If dominant allele is present on the autosome, then the individual will express the trait. If dominant allele is present on the autosome, then the individual will express the trait. A = dominant a = recessive A = dominant a = recessive What would be the genotype of an individual with an autosomal dominant trait? What would be the genotype of an individual with an autosomal dominant trait? –AA and Aa (Heterozygotes are affected)
Autosomal Dominant Inheritance Are manifested in heterozygous state Are manifested in heterozygous state One parent of an index case is usually affected One parent of an index case is usually affected Both males and females are affected and both can transmit the condition Both males and females are affected and both can transmit the condition 50% chance of affected heterozygote passing gene to children 50% chance of affected heterozygote passing gene to children A new mutation in the gene resulting in the offspring being first affected and then may be inherited in a dominant fashion A new mutation in the gene resulting in the offspring being first affected and then may be inherited in a dominant fashion Dominant genes may exhibit lack of penetrance, which is an all or none phenomenon; either the gene is expressed or not expressed Dominant genes may exhibit lack of penetrance, which is an all or none phenomenon; either the gene is expressed or not expressed May show variable expressivity with different family members showing different manifestations of the trait May show variable expressivity with different family members showing different manifestations of the trait
Autosomal Recessive Traits If dominant allele is present on the autosome, then the individual will not express the trait. If dominant allele is present on the autosome, then the individual will not express the trait. In order to express the trait, two recessive alleles must be present. In order to express the trait, two recessive alleles must be present.
A = dominant a = recessiveA = dominant a = recessive What would be the genotype of an individual with an autosomal recessive trait?What would be the genotype of an individual with an autosomal recessive trait? –aa What would be the genotype of an individual without the autosomal recessive trait?What would be the genotype of an individual without the autosomal recessive trait? –AA or Aa –Aa – called a Carrier because they carry the recessive allele and can pass it on to offspring, but they do not express the trait.
Autosomal Recessive Traits Heterozygotes are Carriers with a normal phenotype. Heterozygotes are Carriers with a normal phenotype. Most affected children have normal parents. (Aa x Aa) Most affected children have normal parents. (Aa x Aa) Two affected parents will always produce an affected child. (aa x aa) Two affected parents will always produce an affected child. (aa x aa) Close relatives who reproduce are more likely to have affected children. Close relatives who reproduce are more likely to have affected children. Both males and females are affected with equal frequency. Both males and females are affected with equal frequency. Pedigrees show both male and female carriers. Pedigrees show both male and female carriers. Complete penetrance is common Complete penetrance is common Onset is early in life Onset is early in life
X-Linked Inheritance Involves particular genes located on the X chromosome Involves particular genes located on the X chromosome Disorders more commonly affect males Disorders more commonly affect males Heterozygote female will pass the gene to 50% of her sons who will express the trait, and to 50% of her daughters who will be carriers for the trait Heterozygote female will pass the gene to 50% of her sons who will express the trait, and to 50% of her daughters who will be carriers for the trait Affected males pass the gene to all of their daughters and none of their sons Affected males pass the gene to all of their daughters and none of their sons Hallmark is absence of male to male transmission Hallmark is absence of male to male transmission
Marfan syndrome (defect in the structural proteins) Is a disorder of connective tissues, manifested by changes in skeleton,eyes and cardiovascular system Autosomal dominant
Pathogenesis Marfan syndrome results from inherited defect in extracellular glycoprotein –fibrillin-1 Fibrillin is the major component microfibrils These fibrils form a basement on which tropoelastin is deposited to form elastic fibers Microfibrils are abundant in aorta, ligaments,and ciliary zonules of lens Mutations of FBN1 are mapped on the chromosome 15q21.
Morphology Cardiovascular System: Dilatation of ascending aorta due to cystic medial necrosis, mitral vale insufficiency,aortic dissection Eyes: Dislocation of lens (usually outward and upward) called as ectopia lentis, severe myopia
Musculoskeletal: exceptionally tall with long extremities and tapering fingers and toes – The ratio of upper segment to the lower segment of the body is lower than normal – Joint ligaments of hands and feet are lax;typically thumb can be hyperextended back to the wrist – The head is dolicocephlic(long headed) with bossing of frontal eminences – Pectus excavatum deformity, scoliosis
Marfan Syndrome Subluxation of the lens
Ehlers-Danlos Syndrome A family of disorders with defect in synthesis and structure of fibrillar collagen characterized by hyperextensibility of skin, joint hypermobility, early bruisability Mode of inheritence show all three types of Mendelian patterns Orthopaedic problems: joint instability, joint laxity, arthralgia and scoliosis
Lysosomal storage disorders(defects in enzymes) Key component of intracellular digestive tract Composed of acid hydrolases that catalyse the breakdown of macromolecules Inherited deficiency- catabolism of macromolecules is incomplete accumulation of partially degraded macromolecules cell organelles become large lysosomal storage disease
Cause of Tay-Sachs Hex-A Accumulation of GM2 in neurons Absence of Involvement of CNS, ANS and retina common The absence of a vital enzyme called Hexosamindase A (Hex-A)
Gene Location Chromosome 15 showing location of the syndrome
Characteristics Birth: Appear normal 6 months: Development slows 2 years: Seizures and deteriorating mental functions 3 years: Blindness, mentally retardation, paralysis and non-responsiveness. Cherry red spot in the macula Common in Jews
Microscopy: neurons are ballooned with cytoplasmic vacuoles having lysosomes filled with gangliosides EM: Whorled configuration with lysosomes composed of onion skin layer of membranes
In Summary Tay-Sachs is a genetic disorder that causes Hex-A, an enzyme important to the function of nerve cells, not to be produced. Babies with Tay-Sachs often appear normal at birth, but develop severe symptoms in the first few years of life. There is genetic counseling as well as support groups available for carriers of Tay-Sachs or parents with an affected child.
Niemann –pick disease (type A and B) Deficiency of sphingomyelinase accumulation of sphingomyelin
Type A More severe infantile form with extensive neurological involvement Marked visceral accumulation of sphingomyelin Progressive wasting and early death within 3 years Cherry red spot in the macula
Type B Patients have organomegaly but no CNS involvement Survive to adulthood.
Morphology Accumulation of sphingomyelin in mononuclear phagocytes Affected cell become large Innumerable small vacuoles of uniform size imparting foaminess to the cytoplasm Vacuoles stain for fat
Phagocytic foam cells widely distributed in spleen,liver, lymph node, bone marrow, tonsils, g.i.t,lungs Brain: Gyri shrunkened,sulci widened with vacuolation and ballooning of neurons EM: zebra bodies
Clinical Features: Evident by 6 months Protuberant abdomen Failure to thrive, vomiting, fever, Deterioration of psychomotor function Death by 2 yrs
Gaucher disease Autosomal recessive Mutation in the gene encoding glucocerebrosidase Most common Glucocerebroside accumulates in phagocytic cells 3 types
Type I: Chronic non-neuronopathic form Storage limited to mononuclear phagocytes throughout the body Splenic and skeletal involvement common Type II: acute neuronopathic,dominated by CNS involvement,death by 2 years TypeIII: intermediate between I and II, progressive CNS involvement
Morphology Glucocerebrosides accumulates in phagocytic cells Distended phagocytic cells (Gaucher)cells found in spleen liver,BM,LN,TONSILS,thymus and peyer patches Cells have fibrillary pattern instead of vacuolated (crumpled tissue paper )and have eccentrically placed nucleus.
Gaucher cells (Phagocytic cells with a crumpled tissue paper appearance)
Phenylketonuria Autosomal recessive disorder Deficiency of phenylalanine hydroxylase hyperphenylalaninemia Common in scandinavian people Normal at birth By 6 months severe mental retardation Seizures,decreased pigmentation of hair and skin Mental retardation can be avoided by restriction of phenylalanine intake early in life
Galactosemia Autosomal recessive disorder Deficiency of galactose -1-phosphate uridyl transferase Galactose -1-phosphate accumulates in liver, spleen, kidneys, lens of eye, cerebral corex Alternative metabolic pathways activated, leading to the production of galacitol
Clinical features Failure to thrive Vomiting, diarrhea Hepatomegaly Opacification of lens (cataracts) Aminoaciduria
Diagnosis can be suspected by demonstration in the urine of reducing sugars Many morphological changes can be prevented by early removal of galactose from diet
Oochronosis (alkaptonuria) First human inborn error of metabolism to be discovered Autosomal recessive Lack of homogentisic oxidase blocks metabolism of phenylalanine-tyrosine at the level of homogentisic acid Homogentisic acid accumulates in the body Large amount is excreted,imparting a black color to the urine if allowed to stand
Morphology The retained homogentisic acid selectively binds to collagen in connective tissues, tendons,cartillage imparting blue black pigmentation Most evident in the ears, nose and cheeks. Wear and tear erosion of abnormal cartilage leads to denudation of subchondral bone degenerative arthropathy
Mucopolysaccharidoses Result from genetic deficiency of enzymes involved in the degradation of mucopolysaccharides Progressive disorder chacterised by involvement of multiple organs like liver,spleen, heart and blood vessels Most are associated with coarse facial features,joint stiffness and mental retardation The accumulated mucopolysaccharides are grnerally found in mononuclear phagocytic cells,endothelial cells,smooth muscle cells and fibroblsts.
Glycogen storage diseases Hereditary deficiency of one of the enzymes involved in the synthesis and breakdown of glycogen. Hepatic form: an inherited deficiency of hepatic enzymes involved in glycogen metabolism leads to storage of glycogen in liver and also hypoglycemia.eg :deficiency of glucose-6-phosphatase
Myopathic form:in muscles glycogen is mainly used as a source of energy If the enzymes that fuel glycolytic pathway are deficient, glycogen storage occurs in muscles. eg: deficiency of muscle phosphofructokinase, muscle phosphorylase