The frequency of CC to TT tandem mutations in mismatch repair-deficient cells is increased in a cytosine run

Sensitized photochemistry of di(4-tetrazolouracil) dinucleoside monophosphate as a route to dicytosine cyclobutane photoproduct precursors

The DNA cis-syn cyclobutane photoproduct formed between two adjacent cytosine residues is highly mutagenic and responsible for the tandem CC to TT transition. However, its instability has prevented its in vitro study, so far. With a view to prepare oligodeoxynucleotides containing the CC cyclobutane lesion, we have synthesized in good yield a ditetrazolouracil cyclobutane dinucleotide photoproduct as a stable precursor of this photoproduct. Our approach also overcomes the low photochemical reactivity of the cytosine-cytosine deoxydinucleoside monophosphate.

An analysis of substitution, deletion and insertion mutations in cancer genes

Cancer-associated mutations in cancer genes constitute a diverse set of mutations associated with the disease. To gain insight into features of the set, substitution, deletion and insertion mutations were analysed at the nucleotide level, from the COSMIC database. The most frequent substitutions were c→t, g→a, g→t, and the most frequent codon changes were to termination codons. Deletions more than insertions, FS (frameshift) indels more than I-F (in-frame) ones, and single-nucleotide indels, were frequent. FS indels cause loss of significant fractions of proteins. The 5′-cut in FS deletions, and 5′-ligation in FS insertions, often occur between pairs of identical bases. Interestingly, the cut-site and 3′-ligation in insertions, and 3′-cut and join-pair in deletions, were each found to be the same significantly often (p < 0.001). It is suggested that these features aid the incorporation of indel mutations. Tumor suppressors undergo larger numbers of mutations, especially disruptive ones, over the entire protein length, to inactivate two alleles. Proto-oncogenes undergo fewer, less-disruptive mutations, in selected protein regions, to activate a single allele. Finally, catalogues, in ranked order, of genes mutated in each cancer, and cancers in which each gene is mutated, were created. The study highlights the nucleotide level preferences and disruptive nature of cancer mutations.