Very little research has been done to characterise CRCs arising in Bangladeshi patients, despite the reportedly low frequency of CRC and suggestions of a high proportion of early-onset CRCs in this population . Here we confirmed that BAN patients have a relatively early age of onset of CRC compared to CAU patients presenting at the same hospital and that the age of onset remains low even after potential LS cases have been omitted. This raises the possibility that BAN CRCs have arisen as a result of environmental influences and/or a genetic predisposition that can lead to somatic molecular changes and clinicopathological features dissimilar to those found in tumours from the wider UK population. This study aimed to uncover any clinical or genetic features specific to BAN CRCs.
We observed no major differences in the clinical features between early- and late-onset BAN MSS, and most findings were consistent with CAU MSS cancers reported previously . Survival of the early-onset and the late-onset BAN MSS CRC was similar, even though the early-onset cases were all left-sided unlike the late-onset (62.5% left-sided). We cannot, however, rule out a relationship between age of onset and survival as our cohort was relatively small. It also remains to be seen if BAN CRC characteristics change over time and between generations, particularly as environmental factors such as diet change can impact on the population.
There were, however, an unexpectedly large proportion of mucinous tumours in the BAN sample set. This observation may be linked to the reduced frequency of KRAS mutations in BAN MSS compared with CAU CRCs and CRCs in other populations. Specifically, KRAS is mutated at frequencies between 30%-44% in African American, Chinese and Japanese CRCs [23–25]. It is difficult to ascertain if the high proportion of early-onset cases causes the reduction in the number of KRAS mutations in BAN, as previous studies suggested only marginal differences between early-onset and late-onset CAU CRCs with regard to KRAS mutations (<50 years, 29%; >50 Years, 33%) . These frequencies also concur with those we reported previously for CAU CRCs . On the other hand, there is evidence that KRAS mutation frequency is lower in early-onset CRC in India (≤50 years, 24%; ≥60 Years, 47%) . Further investigation in BAN CRCs will help to explain the aetiology behind the low KRAS mutations and high mucin percentage.
We found no BRAF mutations in BAN MSS CRC, although the absence of MSI cancers in the cohort may contribute to this observation . Overall, it will be interesting to confirm these results in a larger cohort of BAN patients and establish how this may affect clinical decision, particularly in view of the increased use of targeted therapy in CRC.
We utilised high-density SNP bead arrays to detect any genomic alterations that might be specific to BAN patients. The BAN cohort contained both diploid and polyploid cancers, and the copy number changes detected were largely comparable to those seen in the CAU CRCs. We also identified small, heterozygous deletions on chromosome 16p13.2 in a substantial number of BAN and CAU MSS CRCs. Deletion of this part of the genome has been previously described in a number of studies on genomic alterations in CAU and Japanese CRCs [29–33].
In our sample set, the deletion was more frequent in BAN CRCs compared to CAU cancers, although there was no difference in occurrence between early- and late-onset BAN CRCs. This region of the genome encodes RBFOX1, a highly conserved RNA-binding protein that regulates tissue-specific alternative splicing indicating important basic functions in development and differentiation. When we examined RBFOX1 in a large number of CRCs from The Cancer Genome Atlas Network sample set, we found that the regulatory regions and 5’ exons of the gene is often deleted, which may lead to aberrant RBFOX1 expression or isoform distribution. Interestingly, in a recent study the 5’ untranslated region of RBFOX1 was also rearranged in 4/25 melanoma , pointing to the possible importance of this part of the gene.
Very little is known about the expression or role of RBFOX1 in the intestine. Using immunohistochemistry, we confirmed that RBFOX1 is expressed at low levels in normal gut tissues and that expression is often lost in CRC. We also identified a small number of novel somatic mutations in CRC. Therefore, RBFOX1 appears to be targeted by various mechanisms in CRC. Functionally, loss of RBFOX1 activity may lead to aberrations in the splicing of a significant number of genes, generating diverse functional products that vary from those found in normal tissue . Alternative splicing is a key feature of cancer [36–38] including CRC [35, 39] and the identification of RBFOX1 targets in CRC will help to determine if RBFOX1 deletion is a critical feature of dysfunctional splicing in CRC.
To our knowledge, this is the first report of genomic characterisation of CRC in British Bangladeshi populations. We concluded that there were no genetic copy number alterations unique to BAN MSS CRCs, but that mutations of RAS signalling oncogenes were comparatively rare. The functional role of RBFOX1 mutations in BAN and CAU MSS CRC highlights aberrant alternative splicing in CRC as an important mechanism for further study.