The biological significance of the multidrug resistance gene MRP in inversion 16 leukemias

Multidrug resistance represents an important mechanism by which leukaemic and solid tumour cells escape cell death after exposure to anthracyclines and other natural products. Acute myeloid leukaemia (AML) associated with the inversion chromosome 16: inv(16)(p13q22) has a favourable prognosis and is known to be chemosensitive. The inversion chromosome is seen in a number of FAB subclasses but is most commonly associated with acute myelomonocytic leukaemia with abnormal eosinophils, M4Eo. It results in the creation of a fusion between the myosin heavy chain gene (MYHI 11) on the short arm and the gene for a transcription factor, core binding factor β (CBFB) on the long arm. In a subset of these inv(16) AML patients, inversion also results in loss of the gene for the multidrug resistance protein (MRP) at the short arm breakpoint. This gene maps to 16p13.13, centromeric to the primary short arm breakpoint, separated from MYHII by a distance of approximately 150kb. Deletion of the MRP gene has been demonstrated by in situ hybridisation, gene dosage studies and by loss of heterozygosity of a flanking microsatellite marker (D16S405). Twenty two patients with inv(16) leukaemia were analysed for deletion of the MRP gene. Deletion of the gene was detected in seven patients, fourteen patients showed retention of the gene and in one case the findings were indeterminate. Clinical data from 13 of these patients were analysed revealing deletion of the MRP gene to be significantly associated with longer time from diagnosis until failure (death or relapse from complete remission) in these patients (p = 0.007). From this work and the growing literature concerning MRP, it appears likely that the deletion of an MRP allele, may favourably affect the biology of inv(16) AML and may have important prognostic implications.