Cas9 is an RNA-guided DNA cleavage enzyme being actively developed for genome editing and gene regulation. To be cleaved by Cas9, a double stranded DNA, or the protospacer, must be complementary to the guide region, typically 20-nucleotides in length, of the Cas9-bound guide RNA, and adjacent to a short Cas9-specific element called Protospacer Adjacent Motif (PAM). Understanding the correct juxtaposition of the protospacer- and PAM-interaction with Cas9 will enable development of versatile and safe Cas9-based technology. We report identification and biochemical characterization of Cas9 from Acidothermus cellulolyticus (AceCas9). AceCas9 depends on a 5′-NNNCC-3′ PAM and is more efficient in cleaving negative supercoils than relaxed DNA. Kinetic as well as in vivo activity assays reveal that AceCas9 achieves optimal activity when combined with a guide RNA containing a 24-nucleotide complementarity region. The cytosine-specific, DNA topology-sensitive, and extended guide-dependent properties of AceCas9 may be explored for specific genome editing applications.

723 Background: Active modification of surrounding stroma is a critical mechanism of tumor cell growth in pancreatic ductal adenocarcinoma (PDAC). There is some early translational evidence that losartan (L) and vitamin D (D) may affect the tumor microenvironment and supportive stroma in PDAC to help restore a physically and biologically tumor-suppressive stroma. These effects of potentially stroma-modifying drugs have not been established in the clinical management of metastatic PDAC (mPDAC), where there have been limited advances in treatment and prognosis remains poor. Methods: This is a single center, retrospective analysis of patients with mPDAC treated at Fox Chase Cancer Center from 2010 to 2019. All data including L and D use were extracted from the medical record. Patient characteristics were compared across subgroups using Chi-square tests, Fisher’s exact tests, and Wilcoxon rank sum tests. The primary outcome was overall survival (OS) from the time of metastatic diagnosis. Kaplan-Meier curves and log-rank tests were used to compare unadjusted OS across subgroups. Cox proportional hazards models were used to characterize OS adjusted for age at metastatic diagnosis, sex, initial stage at diagnosis, performance status, Charlson Comorbidity Index (CCI), hypertension, and renal disease. Results: 518 patients were included; a majority were male (54.1%), White (82.4%), and diagnosed with de novo mPDAC (54.6%). About half of patients had hypertension (53.5%) and few had renal dysfunction (5.6%). 153 (29.5%) patients were taking D and 41 (8%) patients were taking L. Women were more likely to take D than men (p < 0.001) and White patients were more likely to take L than other races/ethnicities (p = 0.006). Median OS since metastatic diagnosis for patients taking L was 11.4 months compared to 9.6 months for patients not taking L (p = 0.07). Median OS for patients taking D was 11.3 months compared to 8.7 months for those not taking D (p = 0.10). A Cox proportional hazards model adjusting for confounders, however, showed that both L (HR 0.66, p = 0.02) and D (HR 0.81, p = 0.04) were significantly associated with improved OS from time of metastatic diagnosis. Conclusions: This large retrospective study demonstrates potential clinical and survival benefit of L and D in conjunction with mPDAC treatment. L and D are safe, well tolerated, and widely used in general medical practice. These results support further investigation of the stroma-modifying potential of L and D, as well as conducting prospective trials with these agents to further validate these findings.

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