Cytogenetics Study Notes

COURSE OUTLINE

I. CYTOGENETICS

  • Definition: The branch of genetics that deals with the study of cell and structure and function of the chromosome.

  • Related Fields:

    • Genetics: Study of heredity and variation.

    • Cytology: Study of structure and function of cells and cell organelles.

  • Molecular Cytogenetics: Combines molecular biology and cytogenetics, utilizing techniques with DNA probes labeled with fluorescent tags to visualize certain genomic regions.

II. KARYOTYPING

  • Definition: Laboratory technique to analyze an individual's chromosomes, detecting genetic abnormalities.

  • Involves arranging and photographing chromosomes to examine number, size, shape, and structure.

A. Steps of Karyotyping

  1. Sample Collection:

    • Types of Samples: Blood (most common), Bone marrow, Amniotic fluid, Chorionic villus sampling.

  2. Cell Culture & Chromosome Preparation:

    • Cells stimulated to divide using mitogens.

    • Cell division halted at metaphase using colchicine.

    • Cells treated with hypotonic solution for spreading and fixed for staining (e.g., Giemsa stain for G-banding).

  3. Microscopic Examination & Analysis:

    • Chromosomes arranged in pairs (23 pairs in humans).

    • Analyzed for structural abnormalities, missing/extra chromosomes, deletions/duplications.

B. Types of Karyotyping

  • Conventional Karyotyping:

    • Uses Giemsa staining (G-banding), identifies large chromosomal changes.

  • Spectral Karyotyping (SKY):

    • Utilizes fluorescent probes to uniquely color each chromosome for detecting complex rearrangements.

  • Comparative Genomic Hybridization (CGH):

    • Detects copy number variations (CNVs), small deletions/duplications, more sensitive than conventional karyotyping.

C. Clinical Applications of Karyotyping

  • Prenatal Diagnosis:

    • Detects chromosomal disorders like Down syndrome (Trisomy 21).

  • Cancer Diagnosis:

    • Identifies chromosomal translocations such as Philadelphia chromosome in CML.

  • Infertility & Recurrent Miscarriages:

    • Detects balanced translocations or aneuploidies.

  • Genetic Syndromes:

    • Diagnoses conditions like Turner syndrome (45,X) and Klinefelter syndrome (47,XXY).

III. BANDING

A. Types of Banding Techniques

  • Principle: Banding techniques used for chromosome identification and structural abnormality detection, creating unique light and dark band patterns.

  • Common Banding Techniques:

    • G-banding (Giemsa staining):

      • Chromosomes treated with trypsin and stained with Giemsa.

      • Dark bands = AT-rich regions (heterochromatin).

      • Light bands = GC-rich regions (euchromatin).

    • Q-banding (Quinacrine staining):

      • Utilizes quinacrine dye for fluorescence visualization.

    • R-banding (Reverse banding):

      • Chromosomes treated with heat before Giemsa staining; produces opposite results of G-banding, emphasizing gene-rich areas.

    • C-banding (Centromere banding):

      • Focuses on centromeres using alkali/heat treatment.

    • T-banding:

      • Targets telomeric regions for identification of subtelomeric changes.

B. Clinical Importance of Banding Techniques

  • Diagnosing Chromosomal Disorders:

    • Deletions, trisomies (e.g., Down syndrome) and structural abnormalities (e.g., Philadelphia chromosome in CML).

  • Banding methods assist in identifying specific genetic conditions, anomalies, and cancers.

IV. ANEUPLOIDY

  • Definition: A genetic condition characterized by an abnormal number of chromosomes (extra or missing).

  • Essential Points:

    • Commonly seen in Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13).

    • Aneuploidy usually results from meiotic nondisjunction.

    • Specific Examples (Trisomy, Monosomy, etc.):

      • Monosomy (Turner syndrome): Missing one chromosome.

      • Trisomy: An extra chromosome (e.g., Down syndrome 47,XX,+21).

A. Causes and Types of Aneuploidy

  • Caused largely by nondisjunction during meiosis or mitosis.

  • Includes:

    • Monosomy: Missing one chromosome (e.g., Turner syndrome, 45,X).

    • Trisomy: Extra chromosome (e.g., Down syndrome, 47,XX,+21).

    • Tetrasomy/Pentasomy: More than two copies of a single chromosome.

B. Detection and Implications

  • Techniques include karyotyping, FISH, and NIPT.

  • Clinical implications include relation to miscarriages, developmental disorders, and increased cancer likelihood.

V. CHROMOSOMAL ABNORMALITIES

A. Types and Examples of Chromosomal Abnormalities

  • Numerical Abnormalities (Aneuploidy):

    • Trisomy, Monosomy.

    • Examples:

      • Down syndrome: Intellectual disability, distinct facial features.

      • Edwards syndrome: Severe developmental delays.

      • Patau syndrome: Miller dysphagia, organ defects.

  • Structural Abnormalities:

    • Deletions, duplications, translocations (e.g., Philadelphia chromosome t(9;22)).

VI. MONOSOMY

  • Monosomy is a type of aneuploidy where a chromosome is missing.

  • Examples and Implications:

    • Turner Syndrome (45,X): Only viable form of monosomy, affecting females and leads to growth and fertility issues.

    • Detection Methods: Karyotyping, FISH, CGH.

VII. TURNER SYNDROME

  • A genetic disorder due to absent or altered X chromosome (45,X).

  • Associated characteristics including short stature, infertility, and certain congenital defects.

  • Diagnosed through karyotyping and FISH.

  • Management includes hormone treatments and careful monitoring of health.

VIII. CRI-DU-CHAT SYNDROME

  • Genetic condition due to deletion of the short arm of chromosome 5 (5p deletion).

  • Characterized by a high-pitched cry in infants, intellectual disability, and various physical dysmorphisms.

  • Diagnosed by karyotyping and FISH; management includes early intervention programs.

IX. STRUCTURAL ABNORMALITIES

A. Notable Examples

  1. 5q Deletion in Myelodysplastic Syndrome (MDS):

    • Characterized by macrocytic anemia and normal platelet levels.

    • Common genes involved: RPS14, EGR1, CSNK1A1.

  2. 17P Duplication in Charcot-Marie-Tooth Disease:

    • Leads to peripheral neuropathy due to PMP22 gene duplications.

  3. Chronic Myeloid Leukemia (CML) & Philadelphia Chromosome:

    • Presence of translocation t(9;22) creates BCR-ABL fusion involved in uncontrolled cell growth.

X. FLUORESCENCE IN SITU HYBRIDIZATION (FISH)

A. Applications and Advantages of FISH

  • Used for detecting chromosomal abnormalities, including aneuploidies, deletions, and translocations.

  • Offers high sensitivity and quicker results than standard karyotyping but cannot detect unknown mutations.

B. Types of FISH Probes

  • Centromeric Probes, Locus-Specific Probes, Whole-Chromosome Paint Probes for various applications in detecting genetic disorders.

XI. COMPARATIVE GENOMIC HYBRIDIZATION (CGH)

A. Principles and Applications

  • Description: Detect gains and losses of DNA across the genome, useful in cancer and genetic disorders.

B. Advantages and Limitations

  • Advantages: Detects small changes, genome-wide analysis; limitations: Cannot identify balanced translocations or rearrangements.

XII. CHROMOSOME AND GENOME ANALYSIS WITH MICROARRAYS

A. Applications in Cancer Cytogenetics

  • Used for analyzing genetic changes in tumors; significant chromosomal abnormalities linked to various cancers.

B. Prenatal Cytogenetics

  • Includes methods for detecting chromosomal abnormalities in fetuses, using techniques like amniocentesis and NIPT.

XIII. NEXT-GENERATION SEQUENCING (NGS)

A. Definition and Importance

  • A high-throughput sequencing technology for comprehensive genomic analysis.

B. Applications

  • Used in cancer research, genetic disorder detection, RNA sequencing, etc. Advantages include speed and cost-effectiveness over traditional methods.

XIV. BURKITT LYMPHOMA (BL)

A. Genetic Features

  • Characterized by chromosome translocation t(8;14) leading to MYC oncogene overexpression.

B. Clinical Implications

  • Associated with aggressive behavior and potential for cure with early intervention.

XV. EWING SARCOMA

A. Genetic Alterations

  • Primarily associated with translocations (EWSR1-FLI1 fusion).

B. Clinical Manifestations and Treatment

  • Presents with bone pain or swelling; treatment often involves chemotherapy and surgery.

XVI. GLIOBLASTOMA CYTOGENETICS

A. Chromosomal Abnormalities

  • Characterized by alterations in chromosomes, impacting treatment prognosis and strategies.

B. Genetic Mutations

  • Notable mutations include IDH1/IDH2 and TERT, influencing disease outcomes.

XVII. PRENATAL CYTOGENETICS

A. Importance and Techniques

  • Involves analysis of fetal chromosomes to evaluate for genetic abnormalities.

B. Clinical Applications

  • Used in maternal age concerns, abnormal serum screenings, and family history which may indicate increased risk.

C. Ethical Considerations

  • Discussion around consent, implications of findings, and decisions involving pregnancy based on results.

XVIII. EPIGENETICS IN FUNCTIONAL CYTOGENETICS

A. Definition and Modifications

  • Study of heritable changes in gene expression not involving DNA sequence alterations.

B. Applications to Disease

  • Impacts on cancer progression and genetic diseases; involved in understanding complex genetic conditions.

XIX. MITOCHONDRIAL CYTOGENETICS

A. Features of Mitochondrial DNA

  • Double-stranded, circular, maternally inherited with high mutation rates.

B. Common Diseases Associated

  • Includes LHON, MELAS, and others affecting high-energy demand organs.

C. Diagnostic Methods

  • Gained through muscle biopsy, genetic testing, and lactic acid tests; treatments aimed at symptom management.

D. Mitochondrial Disorders and Their Management

  • Examples include MERRF, Leigh syndrome, and Kearns-Sayre syndrome with approaches focused on supportive therapy.