New oncogene, potential treatment for melanoma

Researchers at Children’s Hospital Boston and collaborators from several other institutions have used a zebrafish model of melanoma to identify two important mechanisms that promote the growth of this aggressive human skin cancer, which begins in pigment-producing melanocytes.

In the first of two papers featured on the cover of the 24 March issue of Nature, the scientists established a new oncogene in melanoma: SETDB1, the first identified using a zebrafish model of the disease. In the second paper, the same Children’s researchers used the model to determine that the combination of an existing arthritis drug and a new drug under study for late-stage melanomas with mutations in the gene BRAF may hold promise for treating some types of melanoma.

The zebrafish melanoma model was developed about six years ago, said Leonard Zon, the Grousbeck Professor of Pediatrics and director of the hospital’s Stem Cell Program and senior author of both papers, “to find new genes that caused human melanoma and support the development of new drugs against a disease for which there are very few treatment options.”

The disease has a poor prognosis when diagnosed late. In the United States, there were about 68,000 new cases and 8,700 deaths in 2010.

Mutations in BRAF, a core characteristic of the Zon lab’s model, are present in about 50 to 60 per cent of human melanomas. But BRAF mutations are found in benign moles as well and are by themselves insufficient to cause cancer; other mutations must also be present. The researchers set out to pinpoint other candidates in a region of chromosome 1 called 1q21 in which a stretch of 54 genes are amplified in about 30 per cent of melanoma patients. Of those 54, SETDB1 stood out: it was the only gene in this region that worked with BRAF to fuel tumour development.

By the numbers

“Along with being a creative approach, it was truly a brute force scientific effort to home in on SETDB1,” noted Zon, a Howard Hughes Medical Institute investigator. “We looked at each of the genes in this region one by one, and between discovery and validation, ultimately assayed more than 2,100 tumours from more than 3,100 fish.”

SETDB1 encodes an enzyme that helps turn other genes on or off and is overactive in breast, ovarian, liver and numerous other tumours. Because in the model the level of a tumour’s malignancy rose with the level of SETDB1 activity, the gene could be a valuable target for prognostic testing, Zon said, or for designing new melanoma treatments.

BRAF abnormalities also led to the recognition that zebrafish on the way to developing melanoma harbor excess numbers of immature “embryonic” cells called neural crest cells, raising the fishes’ risk of later cancer formation. Zon and a second research team used the zebrafish to screen 2,000 chemicals for candidates that would suppress these excess cells. Zon and his colleagues found that a compound that interfered with dihydroorotate dehydrogenase (DHODH), an enzyme involved in neural crest cell development, showed promise.

The team turned to leflunomide, a DHODH-inhibitor approved to treat arthritis. In the zebrafish model, leflunomide knocked down expression of a number of genes overexpressed in both melanomas and neural crest cells, while in rats it prevented neural crest stem cells from renewing themselves. Leflunomide also stopped the growth of cultured cells from human melanomas and caused regression of human tumours transplanted into nude mice.

According to Zon, “We realised that a combined blockade of DHODH and BRAF would cooperate to suppress melanoma growth by targeting both the fate and the growth of melanoma precursors.”

The researchers then tested in mice a combination of leflunomide and a BRAF inhibitor developed by Plexxikon that is in late-stage clinical trials. The combination of drugs led to an almost complete abolition of tumour growth; 40 per cent of the mice experienced a near complete tumour regression.

“The combination of the two drugs was more effective than either drug alone, and allowed us to use lower doses of each,” said Zon, who has begun planning a clinical trial. “It will be interesting to put them into the clinic together.”

(Source: Harvard Medical School: Nature)

More information

For more information on skin cancer including common types of skin cancers, and its treatment and preventative measures, as well as some useful videos, see Skin Cancer.