Genetics gives clues to melanoma's development

Stanford, Calif. - Studying functional tissue has provided Stanford University researchers important insight into melanoma's development.

Scientists, until now, have been unable to establish direct causal links between multiple genetic alterations and melanoma development.

But by simulating the environment in which human melanoma naturally arises, researchers have been able to get a clear, three-dimensional picture of what happens to human melanocytes when they are invaded by cancer-causing genes.

"One of the problems with that approach is that while some of the mutations you see in those laboratory cells are ones that lead to cancer, often they are just tagging along or happen because the first cancer-causing mutation makes the genome unstable, causing secondary changes," Dr. Adams explains.

"What is unique about our research is that we take mutations that we suspect might be involved in cancer and add them to primary cells that have no other mutations. In a short period of time, we can develop a three-dimensional model of cancer."

Melanoma

Rather than making an observation and assuming that particular mutations might have an effect, Dr. Adams and colleagues were able to see firsthand if adding specific mutagens create invasive cancer. In the case of this study, it was melanoma.

After combining genetically engineered melanocytes with keratinocytes, they grafted the resulting human skin sample onto laboratory mice. They introduced mutant genes commonly found in human melanoma, including those that interfere with retinoblastoma (Rb) and p53 tumor suppressor pathways, human telomerase reverse transcriptase (hTERT), cyclin-dependent kinase 4 (CDK4), Ras, PI3K and Raf pathways.

They observed the mice for up to six months.

The research team was surprised to discover that B-Raf, a protein in the Ras pathway implicated in melanoma and a recent chemotherapy target, is not sufficient to cause melanoma.

"We suspect that while B-Raf might have an effect, it might just be a bystander. But, according to our system, it cannot, in and of itself, cause melanoma in combination with the other genes," Dr. Adams says.

They found that another protein, in combination with other genetic elements, could cause cancer. A Ras effector protein that is on a parallel pathway with B-Raf, called PI3K, was able to cause melanoma in the researchers' system.

hTERT, the riboprotein that protects the ends of the chromosomes during cell division, has been observed in different kinds of cancer but has not been widely described in melanoma, according to Dr. Adams.

"We found that when we added hTERT, it was needed for progression of cancer.

"In other words, if it was not there, the other genes could cause something that looked like a mole or something that was barely invasive below the basement membrane zone."

Checkpoints

Both CDK4 and p53 are proteins with multiple functions, but one function that they have in common is that they are checkpoints on cells' cycle regulation.

"We found that either CDK4 or p53 were sufficient in our system in combination with the other genes," Dr. Adams says. "Another important finding from our study is that often the key distinction in patient prognosis is whether melanoma breaks through the basement membrane zone and invades into the dermis and deeper structures of the skin. But if you study melanoma in a dish, you do not have that three-dimensional architecture and cannot tell if the changes that you have made actually cause invasion through the basement membrane."

Because the system that Dr. Adams and colleagues use recapitulates human skin, depicting its three-dimensional architecture, they could predict whether these genes can cause invasion.