L-Alanosine

l-Alanosine is a diazeniumdiolate (N-nitrosohydroxylamine) antibiotic that inhibits MTAP-deficient tumor cells by blocking de novo adenine biosynthesis. Previous work revealed the early steps in the biosynthesis of l-alanosine. In the present study, we used genome mining to discover two new l-alanosine-producing strains that lack the aspartate-nitrosuccinate pathway genes found in the original l-alanosine producer. Instead, nitrate is reduced with a unique set of nitrate-nitrite reductases. These enzymes are typically used as part of the nitrogen cycle for denitrification or assimilation, and our report here shows how enzymes from the nitrogen cycle can be repurposed for the biosynthesis of specialized metabolites. The widespread distribution of nitric-oxide-producing reductases also indicates a potential for the discovery of new nitric-oxide-derived natural products.

Homozygous deletion of MTAP upregulates de novo synthesis of purine (DNSP) and increases the proliferation of neoplastic cells. This increases the sensitivity of breast cancer cells to DNSP inhibitors such as methotrexate, L-alanosine and pemetrexed.

7,301 cases of MBC underwent hybrid-capture based comprehensive genomic profiling (CGP). Tumor mutational burden (TMB) was determined on up to 1.1 Mb of sequenced DNA and microsatellite instability (MSI) was determined on 114 loci. Tumor cell PD-L1 expression was determined by IHC (Dako 22C3).

208 (2.84%) of MBC featured MTAP loss. MTAP loss patients were younger (p = 0.002) and were more frequently ER- (30% vs. 50%; p < 0.0001), triple negative (TNBC) (47% vs. 27%; p < 0.0001) and less frequently HER2+ (2% vs. 8%; p = 0.0001) than MTAP intact MBC. Lobular histology and CDH1 mutations were more frequent in MTAP intact (14%) than MTAP loss MBC (p < 0.0001). CDKN2A (100%) and CDKN2B (97%) loss (9p21 co-deletion) were significantly associated with MTAP loss (p < 0.0001). Likely associated with the increased TNBC cases, BRCA1 mutation was also more frequent in MTAP loss MBC (10% vs. 4%; p < 0.0001). As for immune checkpoint inhibitors biomarkers, higher TMB >20 mut/Mb levels in the MTAP intact MBC (p < 0.0001) and higher PD-L1 low expression (1-49% TPS) in the MTAP loss MTAP (p = 0.002) were observed.

MTAP loss in MBC has distinct clinical features with genomic alterations (GA) affecting both targeted and immunotherapies. Further efforts are necessary to identify alternative means of targeting PRMT5 and MTA2 in MTAP-ve cancers to benefit from the high-MTA environment of MTAP-deficient cancers.