Microsatellite instability in colorectal cancer and association with thymidylate synthase and dihydropyrimidine dehydrogenase expression

Background

Microsatellite instability (MSI) refers to mutations in short motifs of tandemly repeated nucleotides resulting from replication errors and deficient mismatch repair (MMR). Colorectal cancer with MSI has characteristic biology and chemosensitivity, however the molecular basis remains unclarified. The association of MSI and MMR status with outcome and with thymidylate synthase (TS) and dihydropyrimidine dehydrogenase (DPD) expression in colorectal cancer were evaluated.

Methods

MSI in five reference loci, MMR enzymes (hMSH2, hMSH6, hMLH1 and hPMS2), thymidylate synthase (TS) and dihydropyrimidine dehydrogenase (DPD) expression were assessed in paraffin embedded tumor specimens, and associated with outcome in 340 consecutive patients completely resected for colorectal cancer stages II-IV and subsequently receiving adjuvant 5-fluorouracil therapy.

Results

MSI was found in 43 (13.8%) tumors. Absence of repair protein expression was assessed in 52 (17.0%) tumors, which had primarily lost hMLH1 in 39 (12.7%), hMSH2 in 5 (1.6%), and hMSH6 in 8 (2.6%) tumors. In multivariate analysis MSI (instable) compared to MSS (stable) tumors were significantly associated with lower risk of recurrence (hazard ratio (HR) = 0.3; 95% CI: 0.2–0.7; P = 0.0007) and death (HR = 0.4; 95% CI: 0.2–0.9; P = 0.02) independently of the TS and DPD expressions. A direct relationship between MSI and TS intensity (P = 0.001) was found, while there was no significant association with DPD intensity (P = 0.1).

Conclusion

The favourable outcome of MSI colorectal carcinomas is ascribed mainly to the tumor biology and to a lesser extent to antitumor response to 5-fluorouracil therapy. There is no evidence that differential TS or DPD expression may account for these outcome characteristics.

Modern origin of numerous alternatively spliced human introns from tandem arrays

Despite the widespread occurrence of spliceosomal introns in the genomes of higher eukaryotes, their origin remains controversial. One model proposes that the duplication of small genomic portions could have provided the boundaries for new introns. If this mechanism has occurred recently, the 5' and 3' boundaries of each resulting intron should display distinctive sequence similarity. Here, we report that the human genome contains an excess of introns with perfect matching sequences at boundaries. One-third of these introns interrupt the protein-coding sequences of known genes. Introns with the best-matching boundaries are invariably found in tandem arrays of direct repeats. Sequence analysis of the arrays indicates that many intron-breeding repeats have disseminated in several genes at different times during human evolution. A comparison with orthologous regions in mouse and chimpanzee suggests a young age for the human introns with the most-similar boundaries. Finally, we show that these human introns are alternatively spliced with exceptionally high frequency. Our study indicates that genomic duplication has been an important mode of intron gain in mammals. The alternative splicing of transcripts containing these intron-breeding repeats may provide the plasticity required for the rapid evolution of new human proteins.

Identification of a novel VNTR polymorphism in C6orf37 and its association with colorectal cancer risk in Chinese population

BACKGROUND

C6orf37 was a gene up-regulated in colorectal adenoma in our previous study. A variable region of C6orf37 sequence was found when we blasted its full sequence with NCBI nucleotide database.

METHODS

RT-PCR and sequencing were conducted to identify the variable region of C6orf37 as VNTR. DHPLC was applied to detect the VNTR genotypes in 122 colorectal carcinoma patients and 166 healthy controls.

RESULTS

A novel VNTR sequence found in C6orf37 second exon was composed of 15 base pair consensus sequence encoding 5-amino-acid (G-G-D-F-G). The repeat timePOST alleles contain three repeats (a), 4 repeats (b) and 5 repeats (c), respectively, which produced 3 homozygotes (a/a, b/b and c/c) and 3 heterozygotes (a/b, a/c and b/c). a, b, c allele frequencies were 0.145, 0.304, 0.551, respectively in Chinese population. Heterozygosity (H) was 0.583. Polymorphism information content (PIC) was 0.510. The distribution of genotypes and allele frequencies of the VNTR reached no significant difference between patients and healthy controls and there was no correlation between VNTR polymorphism and colorectal cancer clincopathological features.

CONCLUSION

A novel VNTR polymorphism in C6orf37 exists in Chinese population and is not associated with colorectal cancer risk.