What is the role of Fluorescence In Situ Hybridization (FISH) panel in the setting of Chronic Lymphocytic Leukemia (CLL)?

Role of FISH Panel in Chronic Lymphocytic Leukemia (CLL)

FISH panel testing is essential in CLL for risk stratification, treatment selection, and prognosis determination by detecting specific chromosomal abnormalities that directly impact patient mortality and quality of life outcomes. 1

Diagnostic and Prognostic Value

FISH panel analysis serves multiple critical functions in CLL management:

Primary Chromosomal Abnormalities to Detect

The recommended FISH panel for CLL should include probes for:

• ATM (11q22.3) deletion
• Trisomy 12 (centromeric probe)
• 13q14.3 deletion
• TP53 (17p13.1) deletion 1

Prognostic Stratification

FISH results directly correlate with prognosis:

• Favorable prognosis: 13q14 deletion as sole abnormality
• Intermediate prognosis: Trisomy 12
• Poor prognosis: 11q deletion, 17p deletion, complex karyotype 1

Treatment Selection Guidance

FISH results directly influence treatment decisions:

• Patients with 17p deletion/TP53 mutations have poor responses to standard chemoimmunotherapy and require alternative targeted therapies
• Patients with 11q deletion may benefit from BTK inhibitors
• IGHV mutation status combined with FISH results helps determine whether chemoimmunotherapy or targeted therapy is more appropriate 1

Technical Considerations

Methodology

• Interphase FISH is the standard method due to the low in vitro proliferative activity of CLL cells
• CpG-oligonucleotide stimulation with IL-2 significantly improves detection of chromosomal abnormalities 1
• Both peripheral blood and bone marrow samples can be used 1

Additional Testing Recommendations

• IGH::CCND1 fusion probe should be included when differentiating CLL from mantle cell lymphoma, especially with atypical immunophenotype (CD23 dim/negative, CD20 bright) 1, 2
• Conventional karyotyping with CpG stimulation should be performed alongside FISH to detect complex karyotype (≥3 unrelated chromosomal abnormalities), which is an independent poor prognostic factor 1

Timing of FISH Analysis

FISH testing should be performed:

1. At initial diagnosis for risk stratification
2. Before initiating treatment to guide therapy selection
3. At disease progression or relapse as cytogenetic abnormalities can evolve over time 1

Limitations and Emerging Approaches

Limitations of FISH

• FISH can only detect abnormalities specific to the probes used
• Cannot detect balanced translocations unless specific probes are included
• May miss small deletions below resolution threshold 1

Emerging Technologies

• Chromosomal microarray analysis (CMA) provides higher resolution and can detect copy number alterations and copy-neutral loss of heterozygosity not visible by FISH 1, 3
• Next-generation sequencing approaches are showing promise for simultaneous detection of CNAs, somatic mutations, and complex karyotypes 3

Clinical Pitfalls to Avoid

• Do not rely solely on FISH: Combine with other prognostic markers like IGHV mutation status for comprehensive risk assessment
• Do not neglect repeat testing: Cytogenetic abnormalities can evolve over time, particularly at disease progression
• Do not miss MCL diagnosis: Always include t(11;14) testing when CLL diagnosis is uncertain or immunophenotype is atypical 1, 2
• Do not overlook complex karyotype: Consider conventional karyotyping with CpG stimulation alongside FISH to detect this important prognostic factor 1

By integrating FISH results with clinical staging and other molecular markers, clinicians can make more informed decisions about treatment timing, therapy selection, and prognosis in CLL patients, ultimately improving patient outcomes.