Cryptic t(4;11) encoding MLL-AF4 due to insertion of 5′ MLL sequences in chromosome 4

Abstract

The t(4;11) translocation is the cytogenetic hallmark of a subset of acute lymphoblastic leukemias characterized by pro-B immunophenotype and a dismal prognosis. This translocation fuses the MLL gene on chromosome band 11q23 and the AF4 gene on 4q21, resulting in the expression of fusion transcripts from both translocated chromosomes. The MLL-AF4 chimeric transcript is thought to mediate the leukemic transformation. The MLL genomic disruption detected by Southern blot and the RT-PCR for the MLL-AF4 chimeric transcript expression are molecular evidence of this chromosomal translocation. However, similar molecular rearrangements have also been identified in cases without the cytogenetic t(4;11). We report a 30-year-old patient with high risk ALL, a normal karyotype, and molecular evidence of MLL-AF4 fusion. Using a double color FISH assay with MLL specific PAC probes, a cryptic t(4;11) due to insertion of 5′ MLL sequences in chromosome 4q21 was demonstrated. Consequently the MLL-AF4 was encoded by der(4). This insertion mechanism precludes the genomic recombination of AF4-MLL and supports the crucial role played by MLL-AF4 in leukemogenesis. The findings of our case, along with others, show the importance of complementing the karyotype with molecular and FISH techniques. Leukemia (2001) 15, 595-600.

Cryptic insertion and translocation or nondividing leukemic cells disclosed by FISH analysis in infant acute leukemia with discrepant molecular and cytogenetic findings

“…The insertion produced a fusion of the MLL and AF4 genes that are transcribed in the same orientation in relation to the centromere. Recently, Von Bergh et al 24 reported a similar insertion of the 5 0 MLL fragment into the AF4 locus in an adult patient with ALL. Using various PAC clones, they evaluated the size of the inserted fragment as approximately 250 kb that is unidentifiable by cytogenetic analysis, and the insertion is reasonably called cryptic.…”

“…[20][21][22][23][24] Cryptic insertions were classified into two types: one has the opposite transcriptional orientations of the two genes forming a fusion in relation to the centromere, and the other has the same orientation of the two genes in relation to the centromere. [20][21][22][23][24] The inv ins(10;11)(p12;q23q23) found in the present patient (no. 1477) belongs to the former type, and the inverted insertion produced a fusion of the MLL and AF10 genes that are transcribed oppositely in relation to the centromere in the normal chromosome 10 and 11 locus.…”

MLL translocations with concurrent 3′ deletions: Interpretation of FISH results

“…A cryptic insertion involving MLL would evade detection by whole chromosome paints and subtelomeric probes but would also need to be accompanied by a 3Ј deletion in order to produce this signal pattern. However, previously reported cryptic insertions resulted in the insertion of 5Ј MLL into the partner chromosome rather than the other way, and they occurred without loss of the 3Ј MLL signal from the der(11) (Borkhardt et al, 2001;von Bergh et al, 2001;Dyson et al, 2003).…”

“…In three cases, involvement of the MLL gene was excluded, indicating a falsepositive rate of 3 of 11, or 27%, for the presence of an MLL rearrangement by this FISH assay. Although the majority of MLL rearrangements occur as a result of an established chromosomal translocation involving 11q23, it is evident that cryptic insertions and complex abnormalities can mask its involvement (von Bergh et al, 2001;Dyson et al, 2003;Watanabe et al, 2003). In conjunction with the knowledge that not all 11q23 abnormalities involve the MLL gene , the MLL status of rare 11q23 translocations needs to be confirmed.…”

Biochemistry of the Mixed Lineage Leukemia 1 (MLL1) Protein and Targeted Therapies for Associated Leukemia

“…AF4 is located at a fragile break-point region on chromosome 4 and is associated with a wide variety of chromosomal translocations. AF4 is a member of AF4/LAF4/AF5q31/FMR2 family of nuclear transcription factors (Gu et al, 1996;von Bergh et al, 2001;von Bergh et al, 2002) and also shows significant homology to the Drosophila melanogaster pair-rule gene Lilliputian (Su et al, 2001). Surprisingly, three of these family members (AF4/LAF4/AF5q31) are associated with infant leukemias involving reciprocal translocations with the MLL1 gene Ma and Staudt, 1996;Taki et al, 1999).…”