Question

How is CML detected by FISH techniques?

Hint:

FISH is highly sensitive: it can detect translocations in non-dividing interphase cells, avoiding the need for time-consuming metaphase cell cultures required in classical karyotyping.

Answer 1

Step 1: Concept
Fluorescence In-Situ Hybridization (FISH) is a molecular cytogenetic technique that uses fluorescent probes to detect specific DNA sequences on chromosomes.

Step 2: Probe Setup for CML
To detect the BCR-ABL translocation in CML, a dual-color dual-fusion probe system is used:
- A Red fluorescent probe is designed to bind the ABL gene on chromosome 9.
- A Green fluorescent probe is designed to bind the BCR gene on chromosome 22.


Step 3: Hybridization and Visualization
The patient's blood or bone marrow cells are fixed on a slide, denatured, and incubated with these probes. Under a fluorescence microscope:
1. Normal Cells: Will show two separate red signals (on the two normal chromosomes 9) and two separate green signals (on the two normal chromosomes 22).
2. CML Cells: Because chromosome 9 and 22 have fused, the red (ABL) and green (BCR) probes hybridize adjacent to each other on the Philadelphia chromosome. This close overlap generates a distinct Yellow fusion signal.


Step 4: Clinical Significance
Finding these yellow fusion signals in interphase or metaphase nuclei confirms the BCR-ABL fusion, providing a rapid, precise diagnosis of CML. Final Answer: FISH detects CML using a Red probe for ABL and a Green probe for BCR. In normal cells, these signals remain separate. In CML cells, the translocation brings the genes together, overlapping to form a distinct Yellow fusion signal that confirms the diagnosis.