Lynch syndrome current screening criteria and how to improve it

Lynch syndrome (LS), formerly called hereditary non polyposis colorectal cancer (HNPCC) was first described by Warthin in 1913 in a large family manifesting colorectal cancers (CRC) without polyposis,alongside cases of gastric and uterine cancers. Epidemiological studies have identified LS as the most common hereditary CRC, accounting for almost 3% of all CRC cases. The molecular pathogenesis ofLS is based on defective mismatch repair (MMR) system, because of mutations in its responsible genes. These mutations were described to cause replication error leading to molecular change known asmicrosatellite instability (MSI), which is detectable by polymerase chain reaction (PCR). MSI was found to be present in LS tumours, which motivated scientists to design a guideline for CRC patients whoneeded MSI testing, this guideline was named Bethesda criteria. At present, direct sequencing of all patients’ MMR genes is not possible. Thus for current screening purposes, clinical features of LS incombination with MSI-high (MSI-H) results (Bethesda criteria) is used to suggest germline mutation testing as the diagnostic test for the patients. On the other hand, it has been reported that only around25% of MSI-H tumours are related to underlying MMR gene mutation. Hence, it can be said that MSI is sensitive but not specific for LS. As a result, some laboratories has suggested immunohistochemistry(IHC) assay as a complementary test for MSI testing. However, IHC has its own drawbacks, such as missing around 5% of LS cases which have full length but not functional MMR proteins and need of anexperienced pathologist to confirm the test result.MSI analysis, as well as being used for screening for Lynch syndrome, is also important for the prognosis of all CRC as these patients are resistant to thetreatment with 5-flurouracil chemotherapy regimen. Hence, its accurate detection is important. At present, multiplex PCR suffices for MSI analysis with good efficiency. But with progressing development, in a few years NGS will replace PCR for MSI analysis.A modified computational algorithm can be used for MSI analysis using NGS. The FASTQ file obtained after sequencing is not aligned to the reference human genome. Instead, the adaptor sequences used for enrichment of the MSI loci are used as a reference to group the reads into the respective locus. This reduces the burden of computational analysis to a massive extent and excludes the need for a highly trained bioinformatician.The cost of sequencing using an NGS platform has been reducing and this will confer a monetary benefit. Moreover, within 24 hours NGS analysis can deliver far more data and interpretation than a conventional MSI-PCR. Also, it will be useful for analysis using liquid biopsy. Circulating tumour DNA or exosomal DNA can be extracted from peripheral blood sample and used for DNA analysis. This excludes the need for a tumour biopsy which, as mentioned earlier, can be difficult to obtain on occasion and carries its own risks. However, NGS is known for its high error ratein sequencing repeats. But, if subjected to an ultra-deep sequencing, a very high concordance in the reads can be obtained which eliminates the chances of false negatives and false positives. The inclusion of specific MSI loci is also flexible. Also, apart from the size of the microsatellite, the variants in the CRC gene panel can also be detected in the same NGS analysis, which is not possible by PCR. This might have a prognostic value.NGS might replace the conventional screening processes in the near future. Clinical screening can be coupled with stratified medicine research using the NGS data which will lead to a systematic process for cancer diagnosis and treatment.LS has a different prognosis and hence it is very important to screen them out of all CRC cases. The present screening program through Bethesda criteria, no doubt efficient, has some flaws. Hence, screening of all CRC cases will be beneficial as all LS cases will thus be detected. This will affect the management and treatment positively and will lead to many lives being saved. We also propose to use NGS for the MSI analysis of CRC. Not only does NGS confer a lot of benefits over the conventional multiplex PCR, it will also be a foundation of a systematic LS screening. This will also be useful for future research in stratified medicine for CRC and other clinical trials, apart from providing a precise prognosis for CRC and LS cases.

Keywords:

Colorectal Cancer , Cancer Diagnosis , Gene and Cancer , Cell and Cancer , Cancer Treatment and Management