METHYLTHIOADENOSINE PHOSPHORYLASE CDNA TRANSFECTION ALTERS SENSITIVITY TO DEPLETION OF PURINE AND METHIONINE IN A549 LUNG-CANCER CELLS

Methylthioadenosine phosphorylase (MTAP), an enzyme involved in purine and methionine metabolism, is present in all normal tissues but is fr equently deficient in a variety of cancers. It has been suggested that this metabolic difference between normal and cancer cells may be expl oited to selectively treat MTAP-negative cancers by inhibiting de novo purine synthesis and by depleting L-methionine. However, these therap eutic strategies have only been tested in naturally occurring MTAP-pos itive and -negative cell lines, which might have additional genetic al terations that affect chemotherapeutic sensitivity. Therefore, it is o f importance to examine the feasibility of enzyme-selective treatment using paired cell lines that have an identical genotype except for MTA P status. MTAP-negative A549 lung cancer cells were transfected with e ukaryotic expression vectors encoding MTAP cDNA in sense and antisense orientations. The resultant stable transfectomas were treated with in hibitors of de novo purine synthesis such as methotrexate, 5,10-dideaz atetrahydrofolate, and L-alanosine and by methionine depletion, The A5 49 cells transfected with an antisense construct (antisense transfecto ma) expressed no MTAP protein and were more sensitive to both purine a nd methionine depletion than were cells expressing MTAP protein (sense transfectoma). Methylthioadenosine was able to completely rescue the sense transfectoma but not the antisense transfectoma from growth inhi bition by depletion of purine and methionine. These results prove that MTAP deficiency contributes directly to the sensitivity of cancer cel ls to purine or methionine depletion. Inhibition of de novo purine syn thesis, combined with methionine depletion in the presence of methylth ioadenosine, is a highly selective treatment for MTAP-negative cancers .