The BRCA1-Δ11q Alternative Splice Isoform Bypasses Germline Mutations and Promotes Therapeutic Resistance to PARP Inhibition and Cisplatin

Breast and ovarian cancer patients harboring BRCA1/2 germline mutations have clinically benefitted from therapy with PARP inhibitor (PARPi) or platinum compounds, but acquired resistance limits clinical impact. In this study, we investigated the impact of mutations on BRCA1 isoform expression and therapeutic response. Cancer cell lines and tumors harboring mutations in exon 11 of BRCA1 express a BRCA1-Δ11q splice variant lacking the majority of exon 11. The introduction of frameshift mutations to exon 11 resulted in nonsense-mediated mRNA decay of full-length, but not the BRCA1-Δ11q isoform. CRISPR/Cas9 gene editing as well as overexpression experiments revealed that the BRCA1-Δ11q protein was capable of promoting partial PARPi and cisplatin resistance relative to full-length BRCA1, both in vitro and in vivo Furthermore, spliceosome inhibitors reduced BRCA1-Δ11q levels and sensitized cells carrying exon 11 mutations to PARPi treatment. Taken together, our results provided evidence that cancer cells employ a strategy to remove deleterious germline BRCA1 mutations through alternative mRNA splicing, giving rise to isoforms that retain residual activity and contribute to therapeutic resistance. Cancer Res; 76(9); 2778-90. ©2016 AACR.

Abstract AP05: ULTRA–DEEP SEQUENCING DETECTS OVARIAN CANCER CELLS IN PERITONEAL FLUID AND REVEALS SOMATIC TP53 MUTATIONS IN NON–CANCEROUS TISSUES

High–grade serous ovarian carcinoma (HGSOC), the most common and most aggressive type of ovarian cancer, is characterized by near–universal prevalence of TP53 mutations, even at the early stages of the disease. Thus, the detection of tumor–specific TP53 mutations in clinical samples could potentially enable early cancer detection, monitoring, and screening. A main limitation, however, is the high error rate of conventional DNA sequencing technologies (estimated at 1 in 1000 nucleotides), which precludes the identification of low frequency tumor mutant molecules from technical error. Duplex Sequencing is a next–generation sequencing technology that uniquely tags each strand of a DNA molecule and quantifies mutations only when present in both strands of DNA. This reduces the error rate of sequencing to 1 in 10 million nucleotides, an unprecedented sensitivity that could enable the detection of extremely low frequency tumor mutant molecules by ultra–deep sequencing. The goal of this study was to provide proof–of–principle of the ability of Duplex Sequencing to detect extremely rare TP53 mutated cancer cells disseminated into the peritoneal cavity of women with HGSOC. We analyzed 17 peritoneal fluid samples from women with HGSOC and known TP53 tumor mutation (cases) and 20 peritoneal fluid samples from women without cancer (controls). The tumor–specific TP53 mutation was detected in matched peritoneal fluid from 94% of women with HGSOC (16/17), including 2 patients with occult tubal intraepithelial neoplasia, 7 patients with early stage HGSOC, and 8 with negative peritoneal fluid cytopathology. Tumor–specific alleles were detected as low as 1 mutant molecule per 24,737 normal genomes. Additionally, we detected extremely low–frequency TP53 mutations (median mutant fraction 1/13,139) in peritoneal fluid from nearly all patients with and without cancer (35/37). These mutations were mostly deleterious, clustered in hotspots, increased with age, and were more abundant in women with cancer than in controls. The total burden of TP53 mutations in peritoneal fluid distinguished cases from controls with 82% sensitivity (14/17) and 90% specificity (18/20). Age–associated, low frequency TP53 mutations were also found in 100% of matched peripheral blood samples from 15 women with and without ovarian cancer (none with hematologic cancer). Our results demonstrate the ability of Duplex Sequencing to detect very rare cancer cells and provide evidence of widespread, low frequency, age–associated somatic TP53 mutation in non–cancerous tissue. These mutations likely represent a premalignant mutational background that accumulates in cancer and aging.

Citation Format: Krimmel JD, Schmitt MW, Kennedy SR, Harrell MI, Agnew KJ, Loeb LA, Swisher EM, Risques RA. ULTRA–DEEP SEQUENCING DETECTS OVARIAN CANCER CELLS IN PERITONEAL FLUID AND REVEALS SOMATIC TP53 MUTATIONS IN NON–CANCEROUS TISSUES [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr AP05.

Abstract 445: Ultra-deep sequencing detects ovarian cancer cells in peritoneal fluid and reveals somatic TP53 mutations in noncancerous tissues

The detection of subclonal tumor-specific somatic mutations in clinical samples could revolutionize cancer diagnostics, but is limited by insufficiently sensitive sequencing methods. Duplex Sequencing is a novel next-generation sequencing (NGS) technology that implements single-molecule barcoding of both strands of DNA to allow internal error correction. This modification reduces the error rate of NGS from 1 in 1000 to less than 1 in 10 million nucleotides, an unprecedented sensitivity that enables accurate ultra-deep sequencing for clinical applications. In this study we sought to determine whether Duplex Sequencing could detect extremely rare TP53 mutated cancer cells disseminated into the peritoneal cavity of women with high-grade serous ovarian carcinoma (HGSOC). HGSOC is the most common and most aggressive type of ovarian cancer, shows early transperitoneal dissemination, and is characterized by near-universal prevalence of driver TP53 mutations. We analyzed 17 peritoneal fluid samples from women with HGSOC and known TP53 tumor mutation (cases) and 20 peritoneal fluid samples from women without cancer (controls). The tumor-specific TP53 mutation was detected in matched peritoneal fluid from 94% of cases (16/17), including 2 patients with occult tubal intraepithelial neoplasia, 7 patients with early stage HGSOC, and 8 with negative peritoneal fluid cytopathology. Tumor-specific alleles were detected as low as <1/25,000. Surprisingly, we also detected additional extremely low-frequency somatic TP53 mutations in peritoneal fluid from nearly all (35/37) cases and controls. These mutations were more abundant in cases than in controls, and in the latter correlated with age. The total burden of somatic TP53 mutations (tumor specific and non-specific) in a peritoneal fluid sample could distinguish cases from controls with 82% sensitivity (14/17) and 90% specificity (18/20). Duplex Sequencing also revealed low-frequency, age-associated somatic TP53 mutations in 100% (15/15) of matched peripheral blood samples. Our results demonstrate the ability of Duplex Sequencing to detect very rare cancer cells, but also provide evidence of widespread, low frequency somatic TP53 mutation in non-cancerous tissue. This compromises the specificity of TP53 mutation tests for cancer detection in liquid biopsies.

Citation Format: Jeffrey D. Krimmel, Michael W. Schmitt, Scott R. Kennedy, Maribel I. Harrell, Kathy J. Agnew, Larry A. Loeb, Elizabeth M. Swisher, Rosa Ana Risques. Ultra-deep sequencing detects ovarian cancer cells in peritoneal fluid and reveals somatic TP53 mutations in noncancerous tissues. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 445.

Regulation of the Mycobacterium tuberculosis hypoxic response gene encoding alpha -crystallin

Unlike many pathogens that are overtly toxic to their hosts, the primary virulence determinant of Mycobacterium tuberculosis appears to be its ability to persist for years or decades within humans in a clinically latent state. Since early in the 20th century latency has been linked to hypoxic conditions within the host, but the response of M. tuberculosis to a hypoxic signal remains poorly characterized. The M. tuberculosis alpha-crystallin (acr) gene is powerfully and rapidly induced at reduced oxygen tensions, providing us with a means to identify regulators of the hypoxic response. Using a whole genome microarray, we identified >100 genes whose expression is rapidly altered by defined hypoxic conditions. Numerous genes involved in biosynthesis and aerobic metabolism are repressed, whereas a high proportion of the induced genes have no known function. Among the induced genes is an apparent operon that includes the putative two-component response regulator pair Rv3133c/Rv3132c. When we interrupted expression of this operon by targeted disruption of the upstream gene Rv3134c, the hypoxic regulation of acr was eliminated. These results suggest a possible role for Rv3132c/3133c/3134c in mycobacterial latency.

Peptidomimetic inhibitors of protein farnesyltransferase show potent antimalarial activity

Malaria continues to represent a very serious health problem in the tropics. The current methods of clinical treatment are showing deficiencies due to the increased incidence of resistance in the parasite. In the present paper we report the design, synthesis, and evaluation of potential antimalarial agents against a novel target, protein farnesyltransferase. We show that the most potent compounds are active against Plasmodium falciparum in vitro at submicromolar concentrations.

Transcriptional mapping and RNA processing of the Plasmodium falciparum mitochondrial mRNAs

The mitochondrial genome of Plasmodium falciparum encodes three protein coding genes and highly fragmented rRNAs. The genome is polycistronically transcribed and, since gene-size transcripts are much more abundant than the polycistronic transcripts, the latter are presumably cleaved to produce the smaller, mature mRNAs and rRNAs. Mapping the transcripts of the P. falciparum mitochondrial protein coding genes shows that the 3' end of each gene directly abuts the 5' end of the gene located immediately downstream. The 5' ends of the protein coding genes are also closely apposed to adjacent genes, with one directly abutting a gene on the same DNA strand and two others separated by just 13 nt from an rDNA fragment encoded on the opposite strand. These mapping data are consistent with production of the mRNAs by cleavage from a polycistronic precursor transcript. Further processing of the mRNAs comes from addition of oligo(A) tails. Unexpectedly, the presence and length of such tails varies in a gene-specific fashion. In this regard, polyadenylation of the P. falciparum mitochondrial mRNAs is more similar to that seen for the P. falciparum mitochondrial rRNAs than that of mitochondrial mRNAs in other organisms.