Expression Profiling of ABC-Transporters in Childhood AML Reveals ABCA3 as a Potential Cause of Drug Resistance.

A major issue in the treatment of acute myeloid leukemia (AML) is the development of resistance to chemotherapeutic drugs. While more than 80% of children with AML may initially achieve complete remission with current therapeutic regimens, a large number of these patients relapse with resistant disease; even with aggressive therapy, the event free survival rate is only about 50%.

Several mechanisms of drug resistance have been identified. One of these is the overexpression of ATP-binding-cassette (ABC)-transporters that function as drug efflux pumps.

The best characterized member of this family is the P-glycoprotein (P-gp or MDR1 or ABCB1), which is an important cause of drug resistance in adult AML but not in children with AML. We could recently show that the multidrug resistance-associated protein 3 (MRP3 or ABCC3) and the breast cancer resistance protein (BCRP or ABCG2) are associated with a poor response to therapy in childhood AML.

The aim of the present study was to identify other members of this family of proteins which are involved in drug resistance. We were particularly interested in ABC-transporters which were overexpressed in AML samples compared to healthy bone marrow because this might be an important prerequisite for using such proteins as therapeutic targets.

A new microarray (DualChip human ABC) was developed for the simultaneous detection and quantification of 38 ABC transporter genes. Samples from 30 children with AML were analyzed with this microarray and the results were compared with normal bone marrow.

The expression of five genes (ABCA2, ABCA3, ABCB2, ABCC1, and ABCC10) which were overexpressed in most patient samples was then analyzed by real time PCR in 40 selected samples from children with previously untreated AML. Half of the patients had achieved complete remission after the first course of chemotherapy. The other 20 patients had also undergone the full induction therapy but had failed to achieve remission at this stage.

Only the expression of ABCA3 was significantly higher (p=0.015) in patients with a poor response to therapy. There was a 4-fold difference between the median expressions in both groups. The expression of ABCA3 was not significantly associated with the expression of P-gp, MRP3 or BCRP.

Our results show that ABCA3 is overexpressed in childhood AML compared to healthy bone marrow and that the expression of ABCA3 is associated with a poor response to therapy. Interestingly, there are two ways in which ABCA3 might influence the prognosis of AML. Like other ABC-transporters, it could be involved in drug efflux. On the other hand, it was recently shown that ABCA3 plays a role in apoptosis and that many cancer cell lines show amplifications of the ABCA3 gene.

Further studies are warranted to analyze the clinical and biological relevance of ABCA3 in leukemia.