A US research team on Wednesday urged strong caution over a prototype gene treatment that has generated billions of dollars in investment as a hoped-for cure for cancer, diabetes and other diseases.

The team said they had made the surprising discovery that the therapy has a generalised effect of blocking blood-vessel growth, a phenomenon that could harm a wide range of tissues.

The method, rooted in a 1998 breakthrough that earned the Nobel Prize for Medicine in 2006, is one of several to use so-called gene silencing.

Under it, tiny synthetic molecules called small-interfering RNA (siRNA) are designed to act as switches that "turn off" genes, the key protein-controlling stretches of DNA.

By tailoring siRNA to specific genes that are known culprits in disease, a disorder could be blocked or reversed. Cancer, kidney disease, Lou Gehrig’s and Parkinson’s disease are among the disorders deemed good candidates for this therapy.

Investigators led by Jayakrishna Ambati, an Indian-born professor of ophthalmology at the University of Kentucky, carried out research on mice using siRNA strands designed to tackle age-related macular degeneration (AMD).

AMD is a form of blindness that strikes late in life and affects around 50 million people worldwide. Excess blood-vessel cells build up in the retina, preventing it from functioning correctly.

Two US companies, Sirna Therapeutics and OPKO Health, are conducting advanced trials aimed at fixing AMD by injecting siRNAs into the eye. The idea is that the molecules enter cells and switch off the flawed gene, known as Vegfa, that causes the problem.