Question

During FISH technique, is any fluorescence detected in the der 9 chromosome? Why?

Hint:

In a standard CML patient using dual-color dual-fusion FISH probes, interphase nuclei will display 1 normal red signal (normal 9), 1 normal green signal (normal 22), and 2 yellow fusion signals (one on der 22 Philadelphia, and one on the reciprocal der 9).

Answer 1

Step 1: Concept
Fluorescence In-Situ Hybridization (FISH) uses fluorescently labeled DNA probes to detect specific chromosome translocations.

Step 2: Analyzing der 9 Fluorescence
- Yes, fluorescence is detected on the derivative chromosome 9 (der 9).
- Reason: The translocation is reciprocal, meaning chromosome 9 and 22 exchange material mutually. While the major ABL sequence moves to chromosome 22 (forming the Philadelphia chromosome), the remaining 3' end of the BCR gene on chromosome 22 translocates to the 5' end of chromosome 9 (forming der 9).
- When dual-color probes (green for BCR and red for ABL) are used, both probes hybridize at the translocation sites. This reciprocal fusion junction on der 9 generates a distinct fluorescent signal (fusion signal), confirming reciprocal genetic exchange.
Final Answer: Yes, fluorescence is detected on the der 9 chromosome because the translocation is reciprocal. A portion of the BCR gene is transferred to chromosome 9, which binds the fluorescent probes during hybridisation.