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).

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.

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.

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.

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).

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.

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%).

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.