MLL Rearranged Infant Acute Lymphoblastic Leukemia Is Characterized by Silencing of the Putative Tumor Suppressor Gene FHIT.

MLL gene rearranged acute lymphoblastic leukemia (MLL) is an aggressive type of acute lymphoblastic leukemia (ALL) with a relatively poor response to treatment, predominantly occurring in infants (<1 years of age). Since conventional chemotherapy is failing in >50% of these patients, identification of novel therapeutic targets to direct therapy against is demanded. For this, understanding this disease and finding biological and molecular features characterizing MLL may be crucial. Here we report that infant MLL is characterized by silencing of the putative tumor suppressor fragile histidine triad (FHIT) gene.

Gene expression profiling showed that FHIT expression is significantly reduced in MLL patients (n=20) as compared to patients diagnosed with conventional ALL (n=24) bearing germ line MLL genes. Using quantitative real-time PCR we confirmed FHIT mRNA expression to be low in MLL gene rearranged infant ALL (infant MLL) as compared to both infants and older children (non-infants) not carrying chromosomal abnormalities involving the MLL gene. Infant MLL patients (n=35) significantly (p<0.001) expressed ~15-fold less FHIT mRNA compared to infant ALL patients (n=8) and 11-fold less than non-infant ALL patients (n=22).

Methylation specific PCR (MSP) analysis demonstrated that the FHIT promoter region is hypermethylated in all of the 36 infant MLL cases tested, resulting in strongly reduced FHIT mRNA as well as FHIT protein expression. In contrast, the incidence of FHIT promoter methylation in both infant and non-infant ALL patients carrying germ line MLL genes was markedly lower, with a frequency of 63% (5/8) and 56% (24/43) respectively. Suppression of FHIT expression could be reversed by exposing leukemia cells bearing MLL gene rearrangements to the demethylating drug 5′-aza-2′-deoxycytidine.

These findings imply that MLL rearranged ALL is characterized by FHIT promoter hypermethylation. Currently experiments are being conducted to determine the biological significance of FHIT silencing in this type of cancer.