By Ben Hirschler

LONDON, Jan 17 (Reuters) - A previously unknown virus may be to blame for a rare but deadly form of skin cancer, opening the prospect of new ways to treat and prevent the condition, scientists said on Thursday.

Merkel cell carcinoma (MCC) mainly affects older people and those whose immune system has been compromised by AIDS or organ transplant drugs, raising suspicions it could have an infectious cause.

Now researchers at the University of Pittsburgh Cancer Institute believe they have found the culprit, after identifying viral DNA sequences present in 80 percent of Merkel cell tumours.

The foreign DNA belongs to a virus, dubbed Merkel cell polyomavirus (MCV), which integrates into the tumour's own genome in a way suggesting it is a trigger for cancerous growth.

If the new virus is proved to be the cause it should give new leads for treating a particularly aggressive cancer. Experts suspect the virus is deadly because it produces a cancer-causing protein or knocks out a gene that suppresses tumour growth.

"Information that we gain could possibly lead to a blood test or vaccine that improves disease management and aids in prevention," said Dr Patrick Moore, who with colleagues published the findings in the journal Science.

Merkel cell carcinoma develops from specialised nerve cells that respond to touch or pressure. Its incidence has tripled over the past 20 years and there are now about 1,500 cases a year in the United States.

About 50 percent of patients with advanced disease live nine months or less.

Merkel cell carcinoma is not the first cancer linked to a virus, and will likely not be the last.

The best known cancer-causing virus is HPV, or human papillomavirus, which can lead to cervical cancer and is the target of two new anti-cancer vaccines, Gardasil and Cervarix, from Merck & Co Inc <MRK.N> and GlaxoSmithKline Plc <GSK.L>.

Kaposi's sarcoma, the most common malignancy in AIDS patients and the most common cancer in Africa, is also caused by a virus. (Reporting by Ben Hirschler; Editing by Janet Lawrence)

AlertNet news is provided by