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Why cancer recurs

Some cancers seem to recur despite aggressive treatments that eliminates every tumor cell that exists in the cancer patient. University of Toronto researchers say they may have discovered why, at least for a form of leukemia.

Leukemia is a group of cancers that strike at the blood-forming tissues. The researchers, John Dick and colleagues, say they showed that cancer cells can continue to arise from leukemia "stem cells" that self-renew like their normal blood-stem cell counterparts. 

Stem cells can duplicate themselves, but also have the unique ability to spawn progeny that change into more specialized cells (other cells only divide into twins). Dick found that acute myeloid leukemia (AML) leukemic stem cells generate progeny with genetic flaws, which then give rise to the appearance of the characteristic 'leukemic' cancer cell.

The acute leukemic stem cells identified by Dick's group were not identical. Different leukemic stem cells repeatedly generated offspring with different characteristic defects. This is best explained by the hypothesis that in AML, the developmental 'program' of the stem cell is damaged; AML is thus not caused by normal stem cells whose progeny later acquire a defect during their development into myeloid cells. 

This distinction alters the goal of treatment for AML - the patient must be cleared not only of leukemic cells, but also of the stem cells from which they arose.

The findings appear in the July issue of the research journal Nature Immunology.