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Published byShavonne Ferguson Modified over 9 years ago
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Dr. Nasser A Elhawary Professor of Medical Genetics
Chromosomal Abnormalities Dr. Nasser A Elhawary Professor of Medical Genetics
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2 Chromatides Centromere
A chromosome distincts into 2 “sister chromatides”. They are linked together via a centromere
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Tjio & Levan reported it is 46 chrom. in 1956
Human Chromosomes… Contains DNA and Protein 46 chromosomes Autosomes: Pairs 1-22 Sex chromosmes Human metaphase
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Identification of Chromosomes…
The chr’s are present in pairs ‘homologs’ (one from female, the other from male). Diploid cells (2n): Cells that contain pairs of homologous chr’s. Haploid cells (n): Certain cells (sperm/egg gametes) contain only one copy of each chr. At fertilization, the fusion of haploid gametes together produces a cell carrying the diploid no. of chr’s (zygote).
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Classification of Chromosomes
The centromere (1ry constriction) divides the chr into 2 arms (petit short ‘p’-arm & long ‘q’-arm). The location of centromere in each chr is characteristic for a given chromosome. ‘Metacentric’: a chromosome with a centrally placed centromere. ‘Submetacentric’: a chromosome with a centromere close to one end than the other. ‘Acrocentric’: a chr with the centromer placed very close to one end.
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Classification of Chromosomes…
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Chromosome Preparation
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Human chromosomes are arranged in groups from A to G.
Chromosome Banding… Human chromosomes are arranged in groups from A to G. Each group is defined by chromosomal size and centromere location. In 1960s, new staining procedures were developed that resulted in banded chromosomes (‘dark’ and ‘light’). Banding protocols: G-banding, R-banding, Q-banding, C-banding, etc.
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G-banded Human Chromosomes
Chromosome Banding… A karyptype of G-banded Human Chromosomes
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R-banding…
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Q-banding
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C-banding…
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What is the importance of high resolution banding?
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Identification of Chromosomes…
Each arm is subdivided into numbered regions starting at the centromere. Thus, any region can be identified by a descriptive address such as Xq27.3/ FMR1 gene
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Karyotype Analysis… The banding pattern of the chromosomes in the human karyotype.
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Banding chromosomes… G-Banding using trypsin and Giemsa stain the chromatin in 2 main forms: Euchromatin: stains ‘light’ and consists of genes which are actively expressed. Heterochromatin stains ‘dark’ and is made up largely of inactive unexpressed repetitive DNA.
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Molecular Cytogenetics
Fluorescence In-Situ Hybridization (FISH): It is ability of a portion of ssDNA (i.e. probe) to anneal with its complement-ary target sequence on: i) a metaphase chromosome, ii) interphase nucleus or iii) extended chromatin fiber.
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FISH is widely used in clinical diagnostic purposes.
In FISH, the DNA probe is labeled with a fluorochrome + patient’s chrom visualized using fluorescent microscope.
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Types of FISH 1- Centromeric probes.
2- Chromosome-specific unique-sequence probes. 3- Telomeric probes. 4- Whole chromosome paint probes.
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Types of FISH… 1- Centromeric probes
FISH of interphase nuclei with Centromeric probes for chromosomes 18.X and Y: showing 3 signals consistent with trisomy 18.
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2- Unique sequence probes are useful for identifying tiny submicroscopic deletions & duplications.
Also, use of interphase FISH probe to identify HER2 over-expression in breast tumors. Metaphase image: Chromosome band 7q11.23 showed deletion associated with Williams syndrome. Normal chrom. has 2 signals (green) for the control probe and the ELN gene probe signal (orange), but the deleted chrom. shows only the control probe signal
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Types of FISH… Telomeric probes
Telomeric probes have been used for identifying tiny ‘cryptic’ subtelomeric abnormalities, e.g. deletions, translocations.
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Types of FISH… 3- Whole-chrom paint probes
These consists of a cocktail of probes obtained from different parts of a particular chromosome. When this mixture of probes is used together in a single hybridization, the entire chromosome fluoresces (i.e. is “painted”). Useful in rearrangements (e.g. translocations) & additional chromosomal materials.
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Types of FISH… Whole-chrom paint probes
Chrom painting showing a reciprocal translocation involving chrom 3 (red) & 20 (green).
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Types of FISH… Whole-chrom paint probes
M-FISH or Spectral karyotyping (SKY) uses pools of whole human chrom. paint probes to provide a multicolor human karyotype. Each homologous chrom. shows a unique color. These are useful to detect chromosomal rearrangements (e.g. deletions, trans-locations), ring chrom, …
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Types of FISH… Whole-chrom paint probes
M-FISH showing complex chromosome rearrangement involving chromosomes 4, 8, 13, 18, and 21
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Genomic Hybridization
Comparative Genomic Hybridization CGH was originally developed to overcome the difficulty of obtaining good-quality metaphase preparations from solid tumors. CGH enables the detection of regions of allele loss and gene amplification. Tumor ‘test’ DNA is labeled with green paint & control normal DNA with red paint. The two samples are mixed and hybridized com-petitively to normal metaphase chromosomes. watch the green-to-red ratio????
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Genomic Hybridization
Comparative Genomic Hybridization CGH Analysis showing areas of gene amplification & reduction (del) in tumor DNA. DAPI: diamidophenylindole FITC: Fluorescein isothiocyanate
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CGH limits for more than 10Mb for losses and 2Mb for gains.
CGH extended to include the analysis of single cells for prenatal diagnosis following whole-genome amplification. CGH limits for more than 10Mb for losses and 2Mb for gains. Microarray or array CGH is likely to replace metaphase CGH.
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Chromosomal Abnormalities
Numerical Structural
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Numerical Abnormalities
Aneuploidy: Loss or gain one or more chromosome Polyploidy: Addition of one or more complete haploid sets (69, 92 chromosomes). Monosomy: Loss of a single chromosome (Turner syndrome: 45,X0) Trisomy: gain of a single chromosome (e.g. Down syndrome: 47,XY,+21).
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Aneuploidy.. Numerical abnormality…
Turner syndrome Monosomy Down syndrome Trisomy 21
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A Karyotype of triploidy cell, 69,XXX
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Mechanism of Trisomy
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Structural Abnormalities
Translocation: Transfer a genetic material from one chromosome to another. Reciprocal translocation: Breakage in 2 chromosomes and exchanged. Robertsonian translocation: Breakage close to centromeres of acrocentric chrom. (e.g. Down syndrome)
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Translocation
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Origin of translocation
in mother giving Down syndrome
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Structural abnormalities…
4- Deletions: loss of part of a chromosome (e.g. Wolf syndrome ‘4p-’, cri du chat syndrome ‘5p-’). 5- Insertions: a segment of one chromosome is inserted into another chromosome. 6- Inversions: a two-break rearrangement in a chrom. which re-inserted in the inverted positions. 7- Ring chromosome: 2 breaks leaving sticky ends which re-unit to give ring chromosome. 8- Isochromosomes: from loss of one arm of the chrom. with duplication of the other.
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Cri du chat syndrome (5p–)
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Insertion
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Ring chromosome (1926)
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Inversion: A) Pericentric inversion B) Paracentric inversion
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Mosaicism & Chimerism Mosaic: Presence of two or more cell lines in an individual, but derived from the same zygote. Mosaic produces from non-disjunction in an early embryonic mitotic division (mosaic trisomy 21). Chimerism: Presence of two or more cell lines in an individual, derived from more than one zygote. Blood Chimeras & Dispermic chimeras.
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Mosaicism & Chimersim mechanism
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