ARID1A A Fertile Ground for Mutations in Ovarian Clear Cell Carcinoma

Although ovarian clear cell carcinoma does not respond
well to conventional platinum–taxane chemotherapy
for ovarian carcinoma, this remains
the adjuvant treatment of choice, because effective
alternatives have not been identified.

This sentence is a depressing reminder of the status of medical treatment of far too many tumor types. Present in roughly 12% of U.S. ovarian cancer cases, ovarian clear cell carcinoma (OCCC) is a dreadful diagnosis.

Two papers this week made a significant step forward in understanding the molecular basis -- and heterogeneity -- of this horror. Seemingly the finale of an old-fashioned race to publish, groups centered at the British Columbia Cancer Center (in New England Journal of Medicine) and Johns Hopkins University (in Science) published papers with the same headline finding: inactivating mutations in the chromatin regulating gene ARID1A (whose gene product is known as BAF250) are a key step in many -- but not all -- OCCC. I'll use the shorthand Vancouver and Baltimore to refer to the respective groups.

Both papers got here by the largest applications of second generation sequencing to cancer so far published. The Vancouver work relied on transcriptome sequencing (RNA-Seq) of a discovery cohort of 18 patients; the Baltimore group used hybridization targeted exome sequencing on just 8 patients. Both used Illumina paired-end sequencing for the discovery phase; Vancouver also used the same platform for validation on a larger cohort.

Whole genome sequencing is likely the future for cancer genomics. A non-cancer paper just published 20 genomes in one shot, underscoring how this is becoming routine with easy samples & a work which is apparently in press (I have no inside knowledge; it has been discussed at several public meetings) will have perhaps a dozen human genomes in it. But, there are still cost advantages to focusing on expressed genetic regions (and perhaps a bit more) and perhaps further information to be gleaned from actually looking a gene expression. These two papers give an opportunity, albeit a bit constrained, to compare the two approaches.

One interesting note comes straight out of the Vancouver data. After finding ARID1A mutations in 6/18 discovery samples, they re-screened those samples plus 211 additional samples. In total this set included 1 OCCC cell line, 119 OCCC, 33 endometrioid carcinomas and 76 high-grade serous carcinomas. The validation screen was by long-range PCR (mean product size 2067 bp) products sheared and sequenced on the Illumina. One exon proved troublesome and required further PCR and sequencing by Sanger. In any case, the key bit here is in the discovery cohort this approach found ARID1A mutations which had been missed by the original RNA-Seq. As the authors state, a likely culprit is nonsense mediated decay (NMD). It would be interesting to go into their dataset to see if these samples had a markedly lower expression of ARID1A, though I don't have easy access to it (it has been deposited, but with protections that should be the subject of a future post).

One interesting contrast between the two studies is the haul of genes. The Vancouver group found ARID1A as a recurrently mutated gene; the Hopkins group not only bagged ARID1A but also KRAS, PIK3CA and PPP2R1A. KRAS and PIK3CA are well-known oncogenes in multiple tumor types and had previously been implicated in OCCC, but PP2R1A is a novel find. The Vancouver group did specifically search for KRAS and PIK3CA mutants in their cohorts by PCR assays and found one patient sample and one cell line with KRAS mutations. Again, it would be interesting to review the RNA-Seq data to generate hypotheses as to why these were not found in the Vancouver set. On the other hand, the RNA-Seq data did identify one case of a rearranged ARID1A. While it is possible to use hybridization capture to identify gene fusions, this cannot be practically done in a hypothesis-free manner. In other words, without advance interest in ARID1A that approach would not work. In addition, CTTNB1 (beta catenin) mutations had been found previously in OCCC and were specifically checked (and found) by the Vancouver group, but none were reported by the Baltimore group. One final small discrepancy: both groups looked at cell line TOV21G for their mutations of interest and both found the same activating KRAS and PIK3CA alleles. However, Vancouver found one ARID1A allele but Baltimore found that one and a second one (actually, the two mutations I am calling the same [1645insC and 1650dupC] aren't described precisely the same, though I'm guessing it is a difference in an ambiguous alignment).

One other surprise is that TP53 (p53) and PTEN mutants had apparently been reported either for OCCC or endometriosis-associated tumors, yet neither group reported any.

An analysis that is not explicitly found in either paper but I feel is valuable is to look at the co-occurrence of these mutations. If we look only at patient samples, then the big take-home is that neither group saw co-occurrence of KRAS and ARID1A (the TOV21G cell line is at odds with this conclusion). Mutually-exclusive mutations have been seen in many tumors. For example, KRAS mutations are generally mutually-exclusive with other mutations in the RTK-RAS-RAF-MAPK pathway. In contrast, ARID1A mutations are found in conjunction with mutations in CTTNB1, PIK3CA and PPP2R1A -- one patient sample in the Baltimore data was even triple mutant for ARID1A, PIK3CA and PPP2R1A. About 30-40% of sample are mutated for none of these genes as far as this data can tell; the hunt for further causes will continue. Will they be epigenetic? Mutations in regulatory elements?

Another interesting comparison is simply the number of mutations per sample. The Hopkins exome data typically has very small numbers of mutations (after filtering out germ line variants); as few as 13 in a sample and as many as 125 -- and the high number was from a tumor which had previously been treated with DNA-damaging agents (all of the other tumors in the Hopkins study were treatment naive). In contrast, the Vancouver data often found more than 1000 non-synonymous variants per tumor. Unfortunately, no clinical history information is available for the Vancouver cohort, so we don't know if this is from DNA-damaging therapeutics or differences in the sequencing or variant filtering. In an ideal world, we could filter each data set with the other group's filtering scheme to see how much of an effect that would have.

The Vancouver group went beyond sequencing to examine samples by immunohistochemistry (IHC) for expression of the ARID1A gene product, BAF250. There is a strong, but imperfect, negative correlation between mutations and BAF250 expression. Some mutated but BAF250-expressing samples may be explained by the target of the antibody; the truncated forms may still express the correct epitope. Alternatively, ovarian cells may be very sensitive to the dosage of this gene product (in some samples both wt and mutant alleles were clearly found in the RNA-Seq data). Also of interest will be samples lacking expression but unmutated; these may be the places to identify further mechanisms for tumors to eliminate BAF250 expression.

The Vancouver study illustrates one additional bonus from RNA-Seq data: a list (in the supplemental data) of genes differentially expressed between ARID1A mutant and ARID1A wild-type cells.

Another interesting bit from the Vancouver paper is looking at two cases in which the tumor was adjacent to endometrial tissue. In one of these, the same truncating mutation was found in the adjacent lesion and tumor -- but not in a distant endrometriosis. Hence, the mutation was not driving the endometriosis but occurred afterwards.

I'm sure I'm short-shrifting further details from the paper; there's a lot of data packed in these two reports. But, what will it all mean for ovarian cancer patients? Alas, none of the genes save PIK3CA are obvious druggable targets. PIK3CA encodes the alpha isoform of PI3 kinase, a target many companies are working on. But that wasn't novel to these papers. PP2R1A is a regulatory subunit of a protein phosphatase and the mutations are concentrated on a single amino acid, suggesting these are activating mutations (as seen in ARID1A, inactivating mutations can sprawl all over a gene). Phosphatases have not been a productive source of drugs in the past, but perhaps that can be changed in the future. Chromatin regulation is a hot topic, but ARID1A is deficient here, not active. Given that tumors can apparently live with two mutated copies, the idea of further inactivating complexes with ARID1A mutations is probably not a profitable one. But, perhaps there is a ying-yang relationship with another chromatin regulator which can be leveraged. In other words, perhaps inhibiting an opposing complex could restore balance to the cell's chromatin regulation and inhibit the tumor. That's the sort of work which can build off of the foundation these two cancer genomics papers have provided.




Kimberly C. Wiegand, Sohrab P. Shah, Osama M. Al-Agha, Yongjun Zhao, Kane Tse, Thomas Zeng, Janine Senz, Melissa K. McConechy, Michael S. Anglesio, Steve E. Kalloger, Winnie Yang, Alireza Heravi-Moussavi, Ryan Giuliany,Christine Chow, John Fee, Abdalnas (2010). ARID1A Mutations in Endometriosis-Associated Ovarian Carcinomas New England Journal of Medicine : 10.1056/NEJMoa1008433

Jones S, Wang TL, Shih IM, Mao TL, Nakayama K, Roden R, Glas R, Slamon D, Diaz LA Jr, Vogelstein B, Kinzler KW, Velculescu VE, & Papadopoulos N (2010). Frequent Mutations of Chromatin Remodeling Gene ARID1A in Ovarian Clear Cell Carcinoma. Science (New York, N.Y.) PMID: 20826764