Zebrafish are an emerging power tool in cancer research. They can be engineered to light up when certain genes turn on-capturing the moment when a cancer is initiated. Because they breed so quickly, they can help identify tumor promoters and suppressors.

Now, as a new study in Science demonstrates, zebrafish can also help scientists dissect the intricate molecular pathways that underlie many cancers, and could help guide treatment strategies.

They focused on mucosal melanomas-rare tumors found in the skin, but in the linings of the respiratory tract, mouth, GI tract, and genitourinary tract. Mucosal melanomas tend to come to light only when very advanced.

Their biology is poorly understood, and they tend to lack the genetic mutations associated with skin melanomas. Thus, few patients benefit from targeted therapies. The five-year survival rate is only 33%. They developed a zebrafish melanoma model that faithfully simulates human melanoma tumors and is easy to manipulate genetically. 

Zebrafish offer the ability to test genetic hypotheses quite rapidly. Genetic sequencing gives you a lot of information. Our melanoma model allows us to ask these questions in large numbers of fish-and make statistically significant observations.

Modeling human cancer genes in zebrafish

Analyzing these data for point mutations and DNA deletions and duplications, the researchers saw some intriguing clues. The top alteration involved a gene called SPRED1, never before implicated in cancer. It was inactivated in 37% of mucosal melanomas, but only 5% of skin melanomas. The mucosal tumors were also likely to skin melanomas to harbor alterations in a gene called KIT, a known oncogene.

Zon's team then took the genetic observations in their zebrafish. They injected the fish in masse as embryos, so the genetic changes found in human mucosal melanoma were recapitulated in every zebrafish melanin-producing cell. This allowed them to model the effects of all the combinations of genetic alterations that occurred together with SPRED1 loss in the 43 UCSF patients.

Melanomas In Fish

Within 1 to 4 months, all the combinations had been tested to see if melanomas develop in the fish. The tumors that arose resembled human mucosal melanomas, arising in non-skin locations including internal organs.

Zebrafish that also had SPRED1 deletions had more severe melanomas, indicating that SPRED1 is a previously unrecognized tumor suppressor. Loss of SPRED1 was especially damaging in fish with KIT mutations: their melanomas were even more severe.

The team then switched from zebrafish to human mucosal melanoma cell lines to delve into cancer's biology and to understanding SPRED1's role and its interplay with KIT.the cancer's biology and to understand SPRED1's role and its interplay with KIT.

When Ablain and colleagues inactivated SPRED1, the melanoma cells proliferated faster. The reverse was also true. "If you overexpress SPRED1 in a KIT-driven zebrafish melanoma model, you delay melanomas," says Ablain. "That confirms that SPRED1 acts as a  tumor suppressor ."

Skin Melanoma

Loss of SPRED1 increased the activity of a cellular pathway called MAPK, known to be associated with skin melanomas-especially in tumors driven by KIT, which is part of the same pathway.

Before, it was unclear whether the mucosal subtype of melanomas was driven by MAPK activity. Our data suggest that they are, even though the genetic alterations are different.

Treatment 

The experiments also showed that when SPRED1 is lost, tumors become resistant to compounds that inhibit KIT. This suggests that patients with SPRED1 deletions would not benefit from existing KIT inhibitors-but could potentially benefit from other drugs that inhibit the MAPK pathway downstream of KIT, such as MEK inhibitors.

To test that prediction, the plan to collaborate with DFCI to look at past clinical trials involving patients with mucosal melanoma to see if they had SPRED1 deletions and if they benefited from KIT inhibitors.

Zon's lab also plans to look for SPRED1 in other kinds of tumors. Currently, SPRED1 is not part of the panel of cancer-related genes often used to test patients, notes Ablain. For targeted therapies, you really need to understand the molecular mechanism.

In the future, patients could have their specific mutations modeled in zebrafish to guide decisions about their treatment.