Two related proteins have been implicated in the mechanisms that allow breast cancer, and potentially other tumor types, to resist therapy with tyrosine kinase inhibitors (TKIs). FAM83A and FAM83B have been identified by separate research groups reporting in parallel in the Journal of Clinical Investigation. Both teams say that if their findings can be validated, the proteins may represent promising new therapeutic targets.
Researchers at Case Western Reserve University used a validation-based insertional mutagenesis (VBIM) strategy to generate libraries of HEM1 immortalized human mammary epithelieal cells (HMECs) that carried unique, single genetic alterations, as a means to identify genes that promoted anchorage-independent growth (AIG, a hallmark of transformed cells), and tumorigenicity, and then carried out further cell-based assays to see which of the identified genes could promote tumor growth independently of RAS.
Expression of FAM83B was found to promote AIG and tumorigenesis, in naïve HME1 cells, and FAM83B-expressing cells were also shown to be capable of forming tumors in immunodeficient mice, confirming its function as a transforming oncogene. Interestingly, when the researchers looked at public microarray data they found that FAM83B expression was elevated in human breast cancer and a range of other solid tumors, relative to normal tissue. In fact, high FAM83B expression levels were associated with specific cancer subtypes, with increased tumor grade, and with decreased overall survival. For example, increased expression of FAM83B was significantly associated with estrogen receptor– (ER-) and progesterone receptor–negative (PR-negative) breast tumors, with higher grade and poor outcome.
Further in vitro assays showed that FAM83B binds with a downstream RAS effector, CRAF, and that this binding resulted in markedly increased MAPK and mTOR signaling, and reduced sensitivity to EGFR-TKIs. Effectively, write Mark W. Jackson, M.D., Rocky Cipriano, M.D., and colleagues, tumors with elevated FAM83B display self-sufficient growth signaling. Importantly, knocking out FAM83B inhibited the proliferation and malignant phenotype of tumor-derived cells or RAS-transformed HMECs.
The authors say targeting FAM38B therapeutically may increase the sensitivity of breast cancer to EGFR-TKI therapy. “Approximately 30% of breast cancers overexpress EGFR, which correlates with loss of estrogen responsiveness and poor prognosis, yet TKIs have thus far had low clinical value for breast cancer patients,” they point out. “Given the requirement for FAM83B as an activator of CRAF/MAPK in EGFR and RAS signaling, the levels of FAM83B and FAM83A may be important to consider when determining which patients receive TKI treatment … Our observation that cancer cells harboring mutant RAS were sensitive to FAM83B ablation lays the foundation for future studies aimed at identifying novel therapies capable of targeting FAM83B.” The Case Western-led researchers describe their findings in a paper titled "FAM83B mediates EGFR- and RAS-driven oncogenic transformation."
A Lawrence Berkeley National Laboratory-led team of researchers reporting in the same issue of JCI performed a screen of human breast cancer cell lines to identify genes that make cancer cells resistant to EGFR TKIs. Top of the list of candidates was FAM38A, which had coincidentally previously been identified as highly expressed in lung cancer. The researchers then used an FAM38A antibody to look at the expression of the protein in human breast tissues, and found that, while normal tissue didn’t produce the FAM38A, it was highly expressed in malignant tissue. Notably, the protein was also expressed in every breast cancer cell line they looked at and was particularly elevated in those that were more resistant to EGFR-TKI treatment. “Indeed, breast cancer patients exhibiting high levels of FAM83A expression had significantly lower survival than did patients with low levels of FAM83A,” note lead author Mina J. Bissell, Ph.D., et al. And when breast cancer cells were treated with an shRNA that blocked FAM38A expression, the cells became less proliferative and more sensitive to EGFR-TKI treatment. Conversely, FAM83A overexpression led to elevated invasiveness.
Mechanistically, FAM83A was shown to interact with and cause phosphorylation of CRAF and PI3K, upstream of MAPK and downstream of EGFR. This finding correlates well with the mechanism reported for FAM38B by the Case Western team, the Berkeley researchers point out. “Their and our studies have identified a family of breast cancer–associated genes or a possible family of oncogenes and support the contention that FAM83A and FAM83B (and possibly other family members) are involved in therapeutic resistance in breast cancer and other cancer types,” Dr. Bissell, et al., state. “Our findings suggest the importance of FAM83 family members as potential drug targets for therapy as well as for sensitization to EGFR-TKIs.”