There is a need for diverse molecular libraries for phenotype-selective and high-throughput screening. To make marine natural products (MNPs) more amenable to newer screening paradigms and shorten discovery time lines, we have created an MNP library characterized online using MS. To test the potential of the library, we screened a subset of the library in a phenotype-selective screen to identify compounds that inhibited the growth of BRCA2-deficient cells.

The nature of mutations occurring in two colorectal carcinoma cell lines deficient in mismatch repair and displaying mutator phenotypes was determined. One of the lines (HCT116) exhibited a higher level of microsatellite instability than the second (DLD-1), although the rate of mutation at the selectable locus encoding the purine salvage enzyme hypoxanthine guanine phosphoribosyl transferase (HPRT) was equally elevated (about 350-450-fold relative to mismatch repair proficient cell lines). Transitions were the major class of mutations in the two mutator lines. In DLD-1 these mutations recurred at several sites that appeared to be hotspots. Frameshifts at a run of six guanine residues in the coding sequence for HPRT constituted 35% of mutations in HCT116. These frameshifts were highly unstable and reverted to wild type at high frequency. Larger deletions were also detected in HCT116. Although these deletions constituted a small proportion of mutations compared with the other types, our data suggest that the rate of deletion is elevated relative to mismatch repair proficient (hMLH1+) cell lines. These observations suggest that the gene(s) altered in DLD-1 may preferentially affect the repair of base mismatches while the alteration(s) in HCT116 may affect the repair of both mismatches and frameshifts.

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Two human tumor cell lines that are deficient in the mismatch repair protein hMSH2 show little or no increase in mutation rate relative to that of a mismatch repair-proficient cell line when the cells are maintained in culture conditions allowing rapid growth. However, mutations accumulate at a high rate in these cells when they are maintained at high density. Thus the mutator phenotype of some mismatch repair-deficient cell lines is conditional and strongly depends on growth conditions. These observations have implications for tumor development because they suggest that mutations may accumulate in tumor cells when growth is limited.