Cytogenetic Analysis
Definition and Overview
- Cytogenetic analysis involves the study of chromosome number, structure, and function, typically in metaphase cells, to detect numerical and structural abnormalities.
- Provides a visual and genomic assessment of large-scale chromosomal changes, acting as a cornerstone in pediatric genetics for evaluating dysmorphic syndromes, congenital anomalies, and malignancies.
- Standard resolution for classical karyotyping is 5 to 10 Megabases (Mb), whereas molecular cytogenetic techniques like Chromosomal Microarray (CMA) offer a much higher resolution of 10 to 100 kilobases (kb).
Indications for Cytogenetic Testing
- Cytogenetic evaluation is crucial for diagnosing a multitude of genetic, developmental, and reproductive conditions.
| Clinical Category | Specific Indications |
|---|---|
| Pediatric and Developmental | Unexplained intellectual disability, global developmental delay, multiple congenital anomalies, dysmorphic features, ambiguous genitalia, and autism spectrum disorders. |
| Reproductive and Endocrine | Primary amenorrhea, infertility, recurrent miscarriages, prior history of stillbirths or neonatal deaths. |
| Prenatal Testing | Advanced maternal age, abnormal fetal ultrasound findings, unexplained fetal growth restriction, abnormal noninvasive prenatal testing (NIPT). |
| Oncology and Hematology | Tumor surveillance (especially leukemia using bone marrow aspirates) and evaluation for chromosome instability syndromes. |
Methodologies and Techniques
Conventional Karyotyping (G-Banding)
- Sample Collection: Peripheral blood lymphocytes, skin fibroblasts, bone marrow aspirates, amniotic fluid, or chorionic villi.
- Procedure:
- Cells are cultured and stimulated (e.g., with phytohemagglutinin for lymphocytes).
- Dividing cells are artificially arrested in metaphase or prometaphase using a chemical agent like colchicine or colcemid.
- A hypotonic solution disrupts the nuclear cell membrane to disperse chromosomes.
- Slides are fixed and stained with Giemsa (G-banding) to produce a unique sequence of dark and light bands.
- Resolution: A standard metaphase spread provides 450 to 550 bands, detecting anomalies down to approximately 5 Mb in size.
Molecular Cytogenetic Techniques
- Utilized to detect subtle submicroscopic abnormalities that fall below the 5 Mb resolution limit of conventional karyotyping.
| Technique | Principles and Clinical Utility |
|---|---|
| Fluorescence In Situ Hybridization (FISH) | Utilizes fluorochrome-labeled DNA probes on interphase or metaphase cells. Does not require cell culture. Used for rapid prenatal aneuploidy screening and confirmation of specific microdeletion/microduplication syndromes. |
| Chromosomal Microarray (CMA) | Differentially labels patient and reference DNA, hybridizing them to a microarray grid. Detects copy number variations (CNVs). Provides up to 50-fold higher resolution than karyotyping and does not require dividing cells. |
| Single Nucleotide Polymorphism (SNP) Arrays | A type of CMA that evaluates polymorphic variations. Highly useful for detecting uniparental disomy (UPD), regions of homozygosity (ROH), and consanguinity. |
Types of Abnormalities Detected
Numerical Abnormalities
- Aneuploidy: Includes trisomies (e.g., Trisomy 21, 18, 13), monosomies (e.g., 45,X in Turner syndrome), and sex chromosome aneuploidies (e.g., 47,XXY).
- Polyploidy: Includes triploidy (e.g., 69,XXX).
Structural Abnormalities
- Deletions and Duplications: Detects large losses or gains of genetic material, including microdeletions via FISH and CMA.
- Rearrangements: Detects balanced and unbalanced translocations, inversions (pericentric/paracentric), ring chromosomes, and isochromosomes.
- Mosaicism: Identifies mixtures of genetically distinct cell lines (e.g., 45,X/46,XX) provided a sufficient number of metaphases are analyzed.
Advantages and Limitations
| Modality | Advantages | Limitations |
|---|---|---|
| Conventional Karyotyping | Direct visualization of the entire genome. Low cost. Essential for identifying balanced structural rearrangements (translocations, inversions). | Low resolution (misses microdeletions <5 Mb). Requires dividing cells. Subjective interpretation. Time-consuming. |
| Chromosomal Microarray (CMA) | High resolution (detects 10-50 kb anomalies). Does not require dividing cells. Automated quantitative interpretation. Higher diagnostic yield (15-20%). | Cannot detect balanced chromosomal rearrangements. Cannot detect point mutations. Limited utility in detecting low-level mosaicism (<20-30%). |
Current Guidelines and Clinical Integration
- First-Tier Testing: CMA is currently recommended as the first-tier diagnostic test for children presenting with unexplained intellectual disability, autism spectrum disorder, and multiple congenital anomalies.
- Sequential Approach: Per ICMR guidelines, conventional karyotype with or without FISH is indicated initially if there is a strong clinical suspicion of classical aneuploidy or a large structural rearrangement, followed by CMA and Next-Generation Sequencing (NGS) if negative.
- Recurrence Risk: Conventional karyotyping remains mandatory for patients with structurally unbalanced anomalies (e.g., Translocation Down syndrome) to screen parents for balanced carrier status, dictating accurate genetic counselling and recurrence risk calculation.