Fish Reveal Origins of Melanoma

Although researchers have discovered numerous cancer-causing mutations, they have not determined why not all cells with these alterations develop into tumors. A study of zebrafish suggests an explanation: individual skin cells are able to grow into melanomas because they return to an embryonic state (Science 2016;351:aad2197).

The BRAF V600E mutation, a key melanoma driver, is also frequently found in benign nevi, or moles, that do not become cancerous. To find out why, Leonard Zon, MD, of Boston Children's Hospital, MA, and colleagues tracked the early events of melanoma formation in zebrafish carrying BRAF V600E and lacking p53 in their melanocytes, cells that arise from the embryonic neural crest and can develop into melanoma. Although every melanocyte carries the cancer-promoting mutations and could potentially give rise to a melanoma, each fish grows only one to three tumors during its lifetime.

Having previously observed that zebrafish melanomas reactivate expression of crestin, a gene whose expression is normally restricted to neural crest progenitor cells in the developing embryo, the researchers engineered transgenic zebrafish that express GFP under the control of crestin-regulatory elements. By tracking GFP-positive cells, they determined that only individual melanocytes that reactivated crestin initiated melanomas.

“We are seeing cancer at its beginnings, at the single-cell state,” says Zon. “The cell, to start the process of becoming a cancer, has to reprogram itself to become more neural crest–like.”

Zon and his colleagues also found that a transcription factor involved in neural crest development called Sox10 helps orchestrate this reprogramming of melanocytes to an embryonic state. When the team spurred zebrafish melanocytes to overexpress sox10, the fish developed tumors much earlier, whereas using the CRISPR/Cas9 system to delete sox10 delayed tumor onset.

To understand the mechanism whereby neural crest genes such as crestin and sox10 are reactivated, the researchers evaluated epigenetic marks at these loci and identified regions with broad enrichment of H3K27Ac histone marks known as super-enhancers. Consistent with these findings in zebrafish, the researchers also found that most human melanoma cell lines in the Cancer Cell Line Encyclopedia as well as human embryonic stem cell–derived neural crest cells express SOX10 and are distinguished by super-enhancers at this locus.

“What we've been able to identify is the initiating event” for melanoma, says Zon. Although other researchers had previously noticed that cancer cells often regress to a more embryonic state, they “haven't been able to show that it's causative,” adds Glenn Merlino, PhD, of the National Cancer Institute in Bethesda, MD, who was not connected to the study. Zon and his team are now trying to determine whether chemicals in the fish's water or other factors trigger melanocytes to make this switch.

“It's a tour de force in terms of technology application” that may lead to better diagnostic methods and techniques to halt melanoma formation, says David Fisher, MD, PhD, of Massachusetts General Hospital in Boston, who also was not connected to the study. “Once you have a system that can identify these early events, you can ask what the causes are and if there are prevention opportunities.”

Martin McMahon, PhD, of the University of Utah in Salt Lake City, gives the authors credit for “visualizing the early specification of cells that go on to become melanomas.” He emphasizes that researchers need to confirm that the mechanism also occurs in mouse models and human melanomas, not just cell lines. –Mitch Leslie

For more news on cancer research, visit Cancer Discovery online at http://cancerdiscovery.aacrjournals.org/content/early/by/section.