Regulation and Impact of Cytoplasmic ARID1A in Ovarian Cancer

Ovarian cancer results in more deaths than any other gynecologic cancer in the United States. There has been little progress in improving ovary cancer therapies. Currently, physicians treat most ovarian cancer patients with similar therapies regardless of the specific type of ovarian cancer. This 'one size fits all' patient management occurs despite the fact that ovarian cancer is really several related but distinct diseases. Understanding these differences is vital to individualizing treatment and developing more effective, less toxic therapies. Serous ovarian cancer is the most common type of ovarian cancer and the focus of most research. The second and third most common types, namely ovarian clear cell carcinoma (OCCC) and endometrioid ovarian carcinoma (EOC), are much less studied. Whereas most patients with serous ovarian cancer may initially respond to current chemotherapies, those with EOC and OCCC are much less likely to do so. It is critical that we focus attention on these common subtypes of ovarian cancer to truly treat this disease in a personalized manner.

Tumor suppressor genes provide blueprints for proteins that are responsible for preventing normal cells from becoming cancerous. Scientists, through recent advances in gene sequencing can now identify key mutations in tumor suppressor genes that render them inactive and unable to prevent the transformation of normal cells to cancer cells. ARID1A is one such tumor suppressor that is mutated and rendered dysfunctional in up to 50% of OCCC and EOC cancer patients. The impact on survival is the subject of intense investigation. This work involves pathologists who specifically stain for ARID1A in patient tumor tissues (collected at the time of the patient's primary surgery). The pathologist then 'scores' tumors that stain brown (e.g., a color indicating that ARID1A is present) differently than those tumors that do not stain brown for ARID1A. Statisticians then correlate these scores in hundreds to thousands of patients with outcomes such as progression-free and overall survival. To date, these data have revealed no strong association between mutational loss of ARID1A and ovarian cancer patient outcome. Although this result may seem surprising, the tumor suppressor may be required to prevent cancer from forming but have lesser impact on making cancers that have already formed more aggressive and/or deadly. These studies, however, focused exclusively on ARID1A that was located in the nucleus, a specialized compartment (organelle) in the cell that contains the DNA that make up genes. This nuclear focus was logical as ARID1A is involved in regulating DNA structure and function.

Our group recently identified a unique ARID1A staining pattern in tissues from 239 ovarian cancer patients. Upon careful review, we identified a subset of these patient tissues that have no nuclear ARID1A, but have very strong staining for ARID1A in the cytoplasm -- the cytoplasm is outside of the nucleus and contains many signaling proteins and other organelles. We then scored tissues for nuclear presence or absence and cytoplasmic presence or absence and asked whether an association with survival existed. Categorization of ovarian cancers by both nuclear and cytoplasmic ARID1A staining revealed a dramatic and statistically significant difference in overall survival between the four groups. Women with loss of nuclear ARID1A and prevalent cytoplasmic ARID1A only survived a median of 7 months in stark contrast to women with loss of nuclear ARID1A without prevalent cytoplasmic ARID1A who survived over 6 years. We then used molecular and cellular biology techniques in ovarian cancer cell lines to force ARID1A into the cytoplasm and demonstrated that these cells were more aggressive and grew significantly faster than without cytoplasmic ARID1A. These data suggest that although ARID1A is a tumor suppressor when it is in the nucleus, it may be oncogenic, or cancer promoting, when in the cytoplasm. Cytoplasmic ARID1A may make cancers more aggressive by driving cancer cell proliferation, propensity to metastasize, or drug resistance. Metaphorically, in the worst prognosis group, the mutated cell's brakes are out and the accelerator is stuck.

Our preliminary data reveal that women with this cytoplasmic ARID1A expression die on average 5 years sooner than expected. This grant details our strategy to understand how ARID1A comes to reside in the cytoplasm and why this cytoplasmic localization is so deadly. Ultimately, this knowledge will enable stratification of these women and development of novel and personalized methods to care for them.