CALM-AF10 Positive T-ALLs Show a Pattern of Expression Similar to MLL-Translocated Acute Leukemias.

The t(10;11) translocation is recurrent in T-ALL and AML. The AF10 gene on chromosome 10 is rearranged either with MLL or CALM located on chromosome 11. CALM-AF10 fusion gene is found in T-ALLs in immature (IM) and TCRγδ-expressing (TCRGD+) T-ALLs. We compared 6 CALM-AF10+ T-ALLs cases (4 IM, 2 TCRGD+) to 17 CALM-AF10 negative T-ALLs cases (14 IM, 3 TCRGD+) using Affymetrix U133A microarrays.

44 genes were significantly overexpressed in CALM-AF10+ T-ALLs, the most significant being HOXA9, a homeobox gene overexpressed in MLL-translocated acute leukemias (MLL-t AL), BMI1, a polycomb family member whose function in regulation of HOX genes expression is opposite to Trithorax genes (whose MLL belongs), SOX4, a frequent insertion site in retroviral-induced leukemogenesis, SFRS6 and COMMD3 (p≤0.001). Only two other HOX genes, HOXA5 and HOXA10, were significantly increased. 89 genes were significantly underexpressed in CALM-AF10+ T-ALLs, the most significant being GGH, ARL6IP4, NBS1, OGFR and TUBB (p≤0.001).

An independent analysis of the expression of HOXA5, HOXA9, HOXA10 and BMI1 genes was done by quantitative RT-PCR in 10 CALM-AF10+ T-ALLs and 27 CALM-AF10 negative T-ALLs. These were compared to 19 MLL-translocated acute leukemias (2 MLL-AF10, 5 MLL-AF4, 3 MLL-AF6, 5 MLL-AF9, 3 MLL-ELL and 1 MLL-ENL), since HOXA9 overexpression had been previously associated with MLL-t AL.

HOXA5, HOXA9 and HOXA10 expression were higher in CALM-AF10+ T-ALLs than in CALM-AF10 negative T-ALLs (p<0.001), confirming the microarray results. HOXA5 and HOXA9 expressions in CALM-AF10+ T-ALLs were similar to those detected in MLL-t AL and lower for HOXA10 in CALM-AF10+ T-ALLs as compared to the values of MLL-t AL (p=0.008). BMI1 expression in CALM-AF10+ T-ALLs was significantly higher than in CALM-AF10 negative T-ALLs and MLL-t AL (p<0.001).

Additionally, MEIS1 expression was determined as this gene was associated in MLL-t AL with the overexpression of HOXA9. As for BMI1, MEIS1 expression was significantly higher in CALM-AF10+ T-ALLs compared to CALM-AF10 negative T-ALLs and MLL-t AL (p<0.001 and p=0.019, respectively).

In summary, we demonstrated here the association between CALM-AF10 in T-ALLs and overexpression of HOXA5, HOXA9, HOXA10, BMI1 and MEIS1 genes. Overexpression of BMI1 is restricted to CALM-AF10+ T-ALLs. Although no obvious similarities are apparent between MLL and CALM proteins, the activation of HOXA and MEIS1 genes represent a highly recurrent pattern of expression in CALM-AF10+ T-ALLs and MLL-t AL. Consequently, the leukemias resulting in the activation of HOXA9 (MLL-t AL, CALM-AF10+ AL, NUP98-HOXA9 AML) should be seen as an independent group of acute leukemias and may benefit from common therapeutic protocols.