CRISPR Screen Reveals New Targets In Squamous Cell Carcinomas

Transcription factor p63 has multiple isoforms that show unexpected functional diversity in both tumor formation as well as development. During the early stages of embryonic development, lack of p63 can lead to birth defects like cleft palate, fused fingers or even missing limbs. However, in an adult genome, its activation can result in tumor formation. It has been known that more than half of all squamous cell carcinomas are driven by p63.

A team of Researchers at the University of Colorado Cancer Center has elucidated p63 activity in lung squamous cell carcinoma, constraining a possible discovery of drug against this known cause of cancer. The study has been published in the journal Cell Reports.

“The question that initiates this study is what is this oncogene, p63, doing to drive cell proliferation and why too much of it would cause cancer,” Espinosa says.

In order to find a solution to this question, Espinosa and colleagues determined the expression of thousands of genes involved in squamous cell carcinomas using CRISPR screen technology. They at first investigated lung squamous carcinoma cells and found that it required Np63a protein, an oncogenic product of p63 gene, for its proliferation. On silencing Np63a in these cells, these cancer cells could not proliferate further.

Based on this finding, Espinosa and colleagues hypothesized that Np63a restrains the expression of key-tumor suppressor genes and in absence of Np63a these tumor suppressor genes are activated thereby preventing further cell division of cancer cells.

“It’s an example of the classic tug-of-war between oncogenes and tumor-suppressor genes – based on this balance, you either have cancer or you don’t,” Espinosa says.

To recognize which tumor suppressor genes got inactivated in presence of Np63a, researchers investigated thousands of genes in a single experiment using CRISPR screen.

“We screened the entire genome and there was one molecular pathway, super clean and super clear, that Np63a needed to turn off to drive the growth of squamous cell carcinoma cells,” Espinosa says.

Based on the experiment they discovered that TGFβ2 and RHOα are the main tumor suppressor genes in this pathway. They also viewed Cancer Genome Atlas at published data of 518 samples of lung squamous cell carcinoma and found that TGFβ2 and RHOα were inactivated in cancers where Np63a was activated.

“Np63a shuts off TGFB2 and RHOA to promote cancer progression, and this is clearly a widespread phenomenon in squamous cell carcinomas,” Espinosa says.

According to the research conducted, it was found that activating TGFβ2 and RHOα seems to be a therapy against Np63a and thereby for most of the squamous cell carcinomas.

“If there was a way to deliver something that mimics TGFB2, perhaps we could stop proliferation of squamous cell carcinoma,” Espinosa says.

Or, RHOA is an enzyme that can switch between active and inactive forms.

“So if you can find drugs that lock RHOA in its active form, that would shut down cell proliferation, as well,” Espinosa says.

“This is a potentially druggable pathway that is driving the progression of squamous cell carcinomas,” Espinosa says. “The challenge now is to exploit this knowledge for therapeutics, to find a way to reactivate the TGFB/RHOA pathway to save the lives of patients with cancers.”