Cytogenetics 2024 Lecture Notes PDF

Cytogenetics Dr. Madona Akhobadze 2024  8. Principles of Clinical Cytogenetics. Introduction to Cytogenetics. Chromosome Abnormalities. Studies of Chromosomes in Human Meiosis. Mendelian Disorders with Cytogenetic Effects Reading: Ch. 5...

Cytogenetics Dr. Madona Akhobadze 2024  8. Principles of Clinical Cytogenetics. Introduction to Cytogenetics. Chromosome Abnormalities. Studies of Chromosomes in Human Meiosis. Mendelian Disorders with Cytogenetic Effects Reading: Ch. 5 - Thompson & Thompson Genetics in Medicine, Robert L. Nussbaum, Roderick R. McInnes  CLINICAL CYTOGENETICS IS THE STUDY OF CHROMOSOMES, THEIR STRUCTURE AND THEIR INHERITANCE, AS APPLIED TO THE PRACTICE OF MEDICAL GENETICS  CHROMOSOME ABNORMALITIES—MICROSCOPICALLY VISIBLE CHANGES IN THE NUMBER OR STRUCTURE OF CHROMOSOMES—COULD ACCOUNT FOR A NUMBER OF CLINICAL CONDITIONS THAT ARE THUS REFERRED TO AS CHROMOSOME DISORDERS  CHROMOSOME DISORDERS MAJOR CATEGORY OF GENETIC DISEASE ACCOUNT FOR A LARGE PROPORTION OF ALL REPRODUCTIVE WASTAGE, CONGENITAL MALFORMATIONS, AND MENTAL RETARDATION AND PLAY AN IMPORTANT ROLE IN THE PATHOGENESIS OF MALIGNANT DISEASE  INTRODUCTION TO CYTOGENETICS TO PREPARE A SHORT-TERM CULTURE THAT IS SUITABLE FOR CYTOGENETIC ANALYSIS OF THESE CELLS, SAMPLE OF PERIPHERAL BLOOD IS OBTAINED, USUALLY BY VENOPUNCTURE AND MIXED WITH HEPARIN TO PREVENT CLOTTING THE WHITE BLOOD CELLS ARE COLLECTED, PLACED IN TISSUE CULTURE MEDIUM, AND STIMULATED TO DIVIDE  INTRODUCTION TO CYTOGENETICS AFTER A FEW DAYS, THE DIVIDING CELLS ARE ARRESTED IN METAPHASE WITH CHEMICALS THAT INHIBIT THE MITOTIC SPINDLE, COLLECTED, AND TREATED WITH A HYPOTONIC SOLUTION TO RELEASE THE CHROMOSOMES CHROMOSOMES ARE THEN FIXED, SPREAD ON SLIDES, AND STAINED BY ONE OF SEVERAL TECHNIQUES, DEPENDING ON THE PARTICULAR DIAGNOSTIC PROCEDURE BEING PERFORMED. THEY ARE THEN READY FOR ANALYSIS.  MOLECULAR KARYOTYPING THE APPLICATION OF GENOMIC TECHNIQUES TO ASSESS THE INTEGRITY AND DOSAGE OF THE KARYOTYPE GENOME-WIDE THE DETERMINATION OF WHAT APPROACHES ARE MOST APPROPRIATE FOR PARTICULAR DIAGNOSTIC OR RESEARCH PURPOSES IS A RAPIDLY EVOLVING AREA, AS THE RESOLUTION, SENSITIVITY, AND EASE OF CHROMOSOME AND GENOME ANALYSIS INCREASE  CLINICAL INDICATIONS FOR CHROMOSOME ANALYSIS PROBLEMS OF EARLY GROWTH AND DEVELOPMENT STILLBIRTH AND NEONATAL DEATH FERTILITY PROBLEMS FAMILY HISTORY NEOPLASIA PREGNANCY IN A WOMAN OF ADVANCED AGE  SKIN BIOPSY, A MINOR SURGICAL PROCEDURE, CAN PROVIDE SAMPLES OF TISSUE THAT IN CULTURE PRODUCE FIBROBLASTS WHITE BLOOD CELLS CAN ALSO BE TRANSFORMED IN CULTURE TO FORM LYMPHOBLASTOID CELL LINES THAT ARE POTENTIALLY IMMORTAL  BONE MARROW CAN BE OBTAINED ONLY BY THE RELATIVELY INVASIVE PROCEDURE OF MARROW BIOPSY FETAL CELLS DERIVED FROM AMNIOTIC FLUID (AMNIOCYTES) OR OBTAINED BY CHORIONIC VILLUS BIOPSY CHROMOSOME ABNORMALITIES Numerical or structural involve one or more autosomes, sex chromosomes, or both simultaneously  ANEUPLOIDY - ABNORMAL CHROMOSOME NUMBER DUE TO AN EXTRA OR MISSING CHROMOSOME, WHICH IS ALWAYS ASSOCIATED WITH PHYSICAL OR MENTAL MALDEVELOPMENT OR BOTH  RECIPROCAL TRANSLOCATIONS - AN EXCHANGE OF SEGMENTS BETWEEN NONHOMOLOGOUS CHROMOSOMES, ARE ALSO RELATIVELY COMMON BUT USUALLY HAVE NO PHENOTYPIC EFFECT, MAY BE AN ASSOCIATED INCREASED RISK OF ABNORMAL OFFSPRING  ABNORMALITIES OF CHROMOSOME NUMBER A CHROMOSOME COMPLEMENT WITH ANY CHROMOSOME NUMBER OTHER THAN 46 IS SAID TO BE HETEROPLOID AN EXACT MULTIPLE OF THE HAPLOID CHROMOSOME NUMBER (N) IS CALLED EUPLOID, AND ANY OTHER CHROMOSOME NUMBER IS ANEUPLOID TRIPLOIDY AND TETRAPLOIDY  HETEROPLOID TRIPLOID (3N) AND TETRAPLOID (4N) - 92,XXXX OR 92,XXYY  Triploidy and Tetraploidy ► triploid infants can be liveborn, they do not survive long ► Triploidy is observed in 1% to 3% of recognized conceptions, and among those that survive to the end of the first trimester, most result from fertilization by two sperm (dispermy) Failure of one of the meiotic divisions, resulting in a diploid egg or sperm Phenotypic manifestation depends on the source of the extra chromosome set paternal chromosomes have an abnormal placenta and are classified as partial hydatidiform moles  ANEUPLOIDY TRISOMY - THREE INSTEAD OF THE NORMAL PAIR OF A PARTICULAR CHROMOSOME MONOSOMY - ONLY ONE REPRESENTATIVE OF A PARTICULAR CHROMOSOME  Aneuploidy ► 5% of all clinically recognized pregnancies ► Either trisomy or monosomy can have severe phenotypic consequences ► most common type of trisomy in liveborn infants is trisomy 21 ► karyotype 47,XX or XY,+21 ► 95% of patients with Down syndrome ► trisomy 18 and trisomy 13 ► lowest number of genes located on them ► Monosomy for an entire chromosome is almost always lethal ► exception is monosomy for the X chromosome - Turner syndrome Nondisjunction ► failure of a pair of chromosomes to disjoin properly during one of the two meiotic divisions, usually during meiosis I ► If the error occurs during meiosis I, the gamete with 24 chromosomes contains both the paternal and the maternal members of the pair ► during meiosis II, the gamete with the extra chromosome contains both copies of either the paternal or the maternal chromosome Nondisjunction can also occur in a mitotic division after formation of the zygote clinically significant mosaicism may result In some malignant cell lines and some cell cultures, mitotic nondisjunction can lead to highly abnormal karyotypes  ABNORMALITIES OF CHROMOSOME STRUCTURE LESS COMMON THAN ANEUPLOIDY - 1 IN 375 NEWBORNS CHROMOSOME REARRANGEMENT OCCURS SPONTANEOUSLY AT A LOW FREQUENCY MAY ALSO BE INDUCED BY BREAKING AGENTS - CLASTOGENS SUCH AS IONIZING RADIATION, SOME VIRAL INFECTIONS, AND MANY CHEMICALS  Abnormalities of Chromosome Structure Balanced Unbalanced Abnormalities of Chromosome Structure ► Balanced - if the chromosome set has the normal complement of chromosomal material ► Unbalanced - if there is additional or missing material  UNBALANCED REARRANGEMENTS PHENOTYPE IS LIKELY TO BE ABNORMAL BECAUSE OF DELETION, DUPLICATION, OR BOTH ANY CHANGE THAT DISTURBS THE NORMAL BALANCE OF FUNCTIONAL GENES CAN RESULT IN ABNORMAL DEVELOPMENT DELETIONS loss of a chromosome segment, resulting in chromosome imbalance The clinical consequences generally reflect haploinsufficiency - the inability of a single copy of the genetic material to carry out the functions normally performed by two copies 1 in 7000 live births DUPLICATIONS Originate by unequal crossing over or by abnormal segregation from meiosis in a carrier of a translocation or inversion less harmful than deletion duplication of all or a portion of chromosome 12p leads to Pallister-Killian syndrome, in which patients show characteristic craniofacial features, mental retardation MARKER CHROMOSOMES Very small, unidentified chromosomes, called marker chromosomes, are occasionally seen in chromosome preparations, frequently in a mosaic state usually in addition to the normal chromosome complement and are thus also referred to as supernumerary chromosomes or extra structurally abnormal chromosomes derive from chromosome 15 and from the sex chromosomes acquired centromere activity - contain neocentromeres RING CHROMOSOMES Many marker chromosomes lack identifiable telomeric sequences and are thus likely to be small ring chromosomes that are formed when a chromosome undergoes two breaks and the broken ends of the chromosome reunite in a ring structure When the centromere is within the ring - mitotically stable ISOCHROMOSOMES chromosome in which one arm is missing - partial monosomy and the other duplicated in a mirror-image fashion - partial trisomy long arm of the X chromosome, i(Xq), in some individuals with Turner syndrome DICENTRIC CHROMOSOMES rare type of abnormal chromosome in which two chromosome segments (from different chromosomes or from the two chromatids of a single one), each with a centromere, fuse end to end, with loss of their acentric fragments  BALANCED REARRANGEMENTS DO NOT USUALLY HAVE A PHENOTYPIC EFFECT IF THEY ARE BALANCED BECAUSE ALL THE CHROMOSOMAL MATERIAL IS PRESENT EVEN THOUGH IT IS PACKAGED DIFFERENTLY INVERSIONS when a single chromosome undergoes two breaks and is reconstituted with the segment between the breaks inverted Inversions are of two types: paracentric - (not including the centromere), in which both breaks occur in one arm; pericentric - (including the centromere), in which there is a break in each arm Pericentric Inversion ► can lead to the production of unbalanced gametes with both duplication and deficiency of chromosome segments ► producing a child with an unbalanced karyotype is estimated to be 5% to 10% ► pericentric inversion of chromosome 3 ► couple from Newfoundland married in the early 1800s ► inv(3)(p25q21) Pericentric Inversion ► Chromosome 3 duplication q21-qter, deletion p25-pter syndrome in children of carriers of a pericentric inversion ► Carriers of the inv(3) chromosome are normal, but some of their offspring have a characteristic abnormal phenotype TRANSLOCATIONS the exchange of chromosome segments between two, usually nonhomologous, chromosomes There are two main types: Reciprocal - results from breakage of nonhomologous chromosomes, with reciprocal exchange of the broken-off segments Robertsonian - involves two acrocentric chromosomes that fuse near the centromere region with loss of the short arms Robertsonian Translocations ► 13q14q and 14q21q ► 13q and 14q is found in about 1 person in 1300 ► carrier of a Robertsonian translocation is phenotypically normal ► there is a risk of unbalanced gametes and therefore of unbalanced offspring ► 14q21q - chromosome 21 are at risk of producing a child with translocation Down syndrome INSERTIONS nonreciprocal type of translocation that occurs when a segment removed from one chromosome is inserted into a different chromosome, either in its usual orientation or inverted  MOSAICISM When a person has a chromosome abnormality, the abnormality is usually present in all of his or her cells Sometimes, however, two or more different chromosome complements are present in an individual this situation is called mosaicism numerical or structural detected by conventional karyotyping but can also be suspected on the basis of interphase FISH analysis or array CGH  MOSAICISM NONDISJUNCTION IN AN EARLY POSTZYGOTIC MITOTIC DIVISION ZYGOTE WITH AN ADDITIONAL CHROMOSOME 21 MIGHT LOSE THE EXTRA CHROMOSOME IN A MITOTIC DIVISION AND CONTINUE TO DEVELOP AS A 46/47,+21 ADDITIONAL PROBLEM IS THAT THE PROPORTIONS OF THE DIFFERENT CHROMOSOME COMPLEMENTS SEEN IN THE TISSUE BEING ANALYZED MAY NOT NECESSARILY REFLECT THE PROPORTIONS PRESENT IN OTHER TISSUES OR IN THE EMBRYO DURING ITS EARLY DEVELOPMENTAL STAGES  IN LABORATORY STUDIES CYTOGENETICISTS ATTEMPT TO DIFFERENTIATE BETWEEN TRUE MOSAICISM, PRESENT IN THE INDIVIDUAL, AND PSEUDOMOSAICISM, IN WHICH THE MOSAICISM PROBABLY AROSE IN CELLS IN CULTURE AFTER THEY WERE TAKEN FROM THE INDIVIDUAL THE DISTINCTION BETWEEN THESE TYPES IS NOT ALWAYS EASY OR CERTAIN  INDIVIDUALS WHO ARE MOSAIC FOR A GIVEN TRISOMY SUCH AS MOSAIC DOWN SYNDROME OR MOSAIC TURNER SYNDROME ARE LESS SEVEREL AFFECTED THAN NONMOSAIC INDIVIDUALS  Incidence of Chromosome Anomalies THREE AUTOSOMAL TRISOMIES: TRISOMY 21, TRISOMY 18, AND TRISOMY 13 FOUR TYPES OF SEX CHROMOSOMAL ANEUPLOIDY: TURNER SYNDROME (USUALLY 45,X), KLINEFELTER SYNDROME (47,XXY), 47,XYY, AND 47,XXX TRIPLOIDY AND TETRAPLOIDY ACCOUNT FOR A SMALL PERCENTAGE OF CASES, PARTICULARLY IN SPONTANEOUS ABORTIONS Live Births ► 1 in 160 births (0.7%) ► diagnosed at birth, but most sex chromosome abnormalities, with the exception of Turner syndrome, are not recognized clinically until puberty ► Balanced rearrangements are rarely identified clinically unless a carrier of a rearrangement gives birth to a child with an unbalanced chromosome complement and family studies are initiated ► unbalanced rearrangements are likely to come to clinical attention because of abnormal appearance and delayed physical and mental development in the chromosomally abnormal individual.  Spontaneous Abortions ► Because the overall spontaneous abortion rate (about 15%) is known, ► as is the overall incidence of specific chromosome defects in both abortuses and live births, ► one can estimate the proportion of all clinically recognized pregnancies of a given karyotype that is lost by spontaneous abortion Genomic Imprinting ► the expression of the disease phenotype depends on whether the mutant allele or abnormal chromosome has been inherited from the father or from the mother ► Differences in gene expression between the allele inherited from the mother and the allele inherited from the father are the result of genomic imprinting ► Imprinting takes place during gametogenesis, before fertilization, and marks certain genes as having come from the mother or father ► After conception, the imprint controls gene expression within the imprinted region in some or all of the somatic tissues of the embryo ► The imprinted state persists postnatally into adulthood through hundreds of cell divisions so that only the maternal or paternal copy of the gene is expressed ► Control over this conversion process appears to be governed by DNA elements called imprinting centers that are located within imprinted regions throughout the genome გმადლობთ, ყურადღებისთვის!!!