Detection of Rearrangements of Mixed Lineage Leukemia Gene and Its clinical relevance

Background: The mixed lineage leukemia (MLL) gene located on chromosome 11 band q23 normally functions as a transcription regulator of the HOX genes and is essential for normal mammalian development and hematopoiesis. Chromosomal translocations involving MLL gene represent frequent cytogenetic abnormalities found in aggressive acute leukemia, both lymphoblastic and myeloid. AL MLL rearrangements have the unique clinical, hematological and prognostic features. We aimed to study the incidence and the types of fusion genes and the clinical relevance.

Results: Study samples were from 60 acute leukemia (AL) patients from Sep. 2003 to Dec. 2005. There were 28 males and 32 females. The ages were from 3 months to 54 years old. 17 patients were less than 15 years old and 43 patients were more than 15 years old. All patients were diagnosed based on FAB diagnostic criteria. The patients included 30 with ALL (10 of ALL-L1 patients, 15 of ALL-L2, 5 of ALL-L3), 28 with AML (2 of AML-M1 patients, 5 of AML-M2, 5 of AML-M4, 13 of AML-M5, 1 of AML-M6, 2 of AML-M7), 1 with mixed cell leukemia (AMLL), and 1 with NK cell leukemia. 59 patients were newly diagnosed and 1 patient was refractory. The rearrangements of MLL gene were detected by fluorescence in situ hybridization (FISH) and 6 types of common fusion genes (MLL / AF4, MLL / ENL, MLL / AF9, MLL / ELL MLL / AF6, MLL / AF10) resulting from the rearrangements of MLL gene were detected by nested RT-PCR. There arrangements of MLL gene was found in 7 out of 60 AL patients, (11.67%). Among these 7 patients, 2 were diagnosed with AML- M5 and 5 patients were diagnosed with B-ALL. The fusion genes of the 2 AML-M5 patients who had the rearrangements of MLL gene were MLL/AF9. Among 5 B-ALL patients, 2 patients were confirmed to express MLL/ENL, 1 patient was confirmed to express MLL/AF4, and the other 2 patients did not express the fusion genes. 1 of 2 AML-M5 MLL fusion gene positive patients had invasion of leukemia cell in the left leg and CR was achieved after the first course of chemotherapy. The central nervous system leukemia was got after 1 year and died. The other achieved CR with the third of chemotherapy. 1 of 5 patients died from DIC and the cerebral hemorrhage on the second day after diagnosis B-ALL of MLL fusion gene positive. 2 of 5 patients were not in remission and died from multi-organ failures and infection 3 weeks after diagnosis with positive MLL fusion gene. 1 of 5 patients B-ALL of MLL fusion gene positive got invasion of leukemia cell in thoracic and achieved CR after the second course of chemotherapy. 1 of 5 patients of B-ALL of MLL fusion gene positive achieved CR after the first course of chemotherapy and was relapse 1 year after the bone marrow transplant and then died.

Conclusions: We conclude that nested RT-PCR is convenient and feasible method to detect the types of fusion genes resulting from the rearrangements of MLL gene. The clinical features of AL with MLL fusion gene include high white blood cell, invasion of multiple organs, resistant to conventional chemotherapy, easy to relapse after remission, and poor prognosis. The detection of MLL gene rearrangement is of great importance in predicting prognosis and guiding therapy in AL.