Cryptic t(4;11) encoding MLL-AF4 due to insertion of 5' MLL sequences in chromosome 4

Area of Science:

• Hematology
• Molecular Biology
• Genetics

Background:

• The t(4;11) translocation is a key genetic abnormality in certain acute lymphoblastic leukemias (ALL), associated with a poor prognosis.
• This translocation involves the MLL gene (11q23) and the AF4 gene (4q21), creating MLL-AF4 fusion transcripts driving leukemogenesis.
• Standard cytogenetic methods may miss similar molecular rearrangements in ALL patients.

Observation:

• A 30-year-old patient with high-risk ALL presented with a normal karyotype but showed molecular evidence of MLL-AF4 fusion.
• Fluorescence in situ hybridization (FISH) revealed a cryptic t(4;11) translocation involving insertion of MLL sequences into chromosome 4q21.
• The MLL-AF4 fusion was encoded by the derivative chromosome 4 (der(4)) via this insertion mechanism.

Findings:

• The study identified a cryptic t(4;11) translocation in an ALL patient with a normal karyotype, demonstrating MLL-AF4 fusion.
• This insertion mechanism, rather than direct translocation, results in the MLL-AF4 fusion transcript.
• The findings highlight that the MLL-AF4 fusion is oncogenic regardless of the specific mechanism of gene rearrangement.

Implications:

• Complementing karyotyping with molecular assays like RT-PCR and FISH is essential for accurate diagnosis and risk stratification in ALL.
• Detecting cryptic MLL-AF4 fusions, even without apparent chromosomal translocations, is critical for appropriate patient management.
• This case underscores the importance of advanced molecular techniques in understanding leukemogenesis and identifying therapeutic targets.