Novel genetic and epigenetic alterations in ... - Ous-research.no

DiscussionClinical impact of molecular biologyEarly tumor markersDNA promoter hypermethylation has been shown to occur early in the colorectaltumorigenesis[163], making it possible to detect pre-neoplastic lesions. In contrast to genemutation analyses where multiple tests are required in order to correctly establish themutation status of the gene in question, promoter hypermethylation requires only one assayper gene, reducing the workload (the principle is illustrated Figure 14). APC and TP53 areexamples of genes in which several exons must be analyzed, while BRAF and KRAS havemutation hot-spots and require only one analysis. These four genes in addition to BAT26, amarker for microsatellite instability, are included in one of the few currently commerciallyavailable mutation tests for colorectal cancer. The sensitivity of this and comparable testshave been shown to fall within 52%-91% (reviewed in [164]). When combined with digitalmelting curves (a method which improves the level of mutation detection to 0.1%, far betterthan many conventional assays[165]) and/or DNA integrity assay (measures long DNAstretches, more abundant in patients with a tumor[166]), the overall sensitivity has beenshown to be around 90% in stool samples from CRC patients with known mutations[165].As mutations are highly tumor-specific, the specificity of mutation based test are superior toe.g. the detection of occult blood in stool samples.Expression of cancer-specific mRNA transcripts in colonocytes extracted from feces hasalso a been explored as a potential method for diagnostic tests[167]. When analyzing a panelof four genes (MMP7, MYBL2, PTGS2 and TP53) in fecal samples, the resulting sensitivityand the specificity were 58% and 88%, respectively. A challenge using this method is thatRNA is easily degraded in feces, and only intact colonocytes will provide suitable mRNA forfurther analyses. In the cited article, a success rate of 75% was obtained[167], a numberinferior to e.g. analyses of methylated promoters in stool.So far, only a handful of epigenetic markers with diagnostic potential have beenidentified[168-170]. Among the most promising ones is VIM, which is present in 73% offecal samples from individuals with CRC. When combined with a DNA integrity assay VIMhypermethylation provides a sensitivity and specificity in stool of 88% and 82%,54