Review for 10.1186/s12876-022-02395-7 (2022)

Title: Diagnostic accuracy of DNA-based SDC2 methylation test in colorectal cancer screening:
a meta-analysis
Version: 1 Date: 21 Nov 2021
Reviewer's report:
Syndecan-2 (SDC2) is a promising marker of early stage colorectal cancer (CRC), being
expressed in a high number of early stage CRC and high-grade adenomas. A number of
retrospective studies have been performed that demonstrate acceptable test characteristics, with
DNA derived from either stool or blood, and indicating that prospective, population based
trials are warranted. This is notwithstanding that detection of methylated stool DNA would
be challenging for diagnostic laboratories due to the very small amount of exfoliated DNA in
stool relative to background genomic DNA. This report, ‘Diagnostic accuracy of DNA-based
SDC2 methylation test in colorectal cancer screening: a meta-analysis’ is a systematic review
using meta-analysis of these studies. Perhaps the best known is Han, YD.  Clin Epigenet 11, 51
(NCT03146520). All the studies use clinical specimens from cases and controls appropriately
diagnosed with colonoscopy (In Han et al, the controls were recruited prospectively), allowing
the investigators to draw a diagnostic cut off (PCR cycle number or CT value) corresponding
to a certain LoD (the lowest amount of analyte in a sample that can be detected with the stated
probability). For example, in the study of Han et al, a CT of 40 detected methylated SDC-2
(equivalent to 34.5 pg DNA) in 90.2% of their CRC cases and in 24 of 245 control subjects,
a specificity of 90.2%.
The authors of the present study state ‘The objective of this study was to analyze the
screening performance of SDC2 methylation based on different sources for different stages of
colorectal cancer and adenoma, providing reliable scientific conclusions,’ but this objective is
ill-conceived, as the diagnostic thresholds for the included studies were drawn retrospectively,
as above.
Some minor points are:
P6L9: Why is gene methylation a good biomarker - how does this relate to adenomas (and
serrated polyps)?
P6L12: What is the actual sensitivity?
P6L18: what is the actual range of sensitivity in these studies?
P7L5: This seems a limited search strategy and the search syntax should be more accurately
written. Statement is needed to explain the research question and how the search strategy was
developed from this. For example, a quick run of the search phrase "FOBT AND SDC2"
identified 4 highly relevant papers (PMID 33393623; 33391430; 33765723 and 33858326), 3 of
which are not identified in this report, and 1/3 of these were published before the study search
was conducted.
P7L10: Sorry to nitpick, but the statement ' All relevant literature were also checked for possible
studies' cannot be correct!
P9L12: Diagnostic Accuracy Studies 2 (QUADAS-2) tool (13) was used; in the section
'Statistical Analysis' the authors state "RevMan 5.4 was used": since RevMan contains the
QUADAS-2 tool, these two statements need to be reconciled.
For QUADAS-2, the systematic review question should be reported, and review tailoring is
performed by inserting or removing signalling questions: these need to be stated (this is item
11 of the PRISMA 2020 checklist). At least 2 independent reviewers should be performing
this analysis and resolving differences (ie not just acting independently for data extraction). An
example of a simple description of this approach is found in Am J Infect Control. 2021;49:21.
P10L14: The authors have included  a study that used 'LTE-qMSP to detect SDC2; (reference
17). The methodology is an acronym devised by the authors of that study and used only by those
authors currently., to describe qMSP (40 PCR cycles) following a first step of target enrichment
of SDC-2 (35 PCR cycles). Conventional qMSP typically uses sequence-specific capture of
unmethylated DNA with beads or similar prior to PCRC (50 cycles for qMSP).