The findings of this study were as follows:

the majority of path_MMR carriers did not develop any colorectal adenoma(s) during follow up;

colonoscopy appeared to prevent CRC only in path_PMS2 carriers; and.

there was no association between the incidences of adenomas and CRCs in any group of carriers.

In contrast to previous reports on carriers not subjected to colonoscopy [14], no CRC was diagnosed prospectively in path_PMS2 carriers below 75 years of age, to the combined conclusion that colonoscopy as undertaken had prevented CRC in path_PMS2 carriers in the current study.

The lower incidences of colorectal adenomas in path_PMS2 carriers compared to path_MLH1, path_MSH2 and path_MSH6 carriers, may suggest that some adenomas in the latter groups are attributable to the path_MMR genotypes.

Considered by the sojourn time paradigm, the PLSD findings are in keeping with a theory that colonoscopy blocks the adenoma-carcinoma pathway and that the CRCs observed had no or short sojourn times as adenomas. CRCs arising by the CTNNB1 pathway appear to have no sojourn time as adenomas [3], are not frequent in path_MMR carriers [31], and may be specific for path_MLH1 carriers [32,33,34].

Our findings pose the following question: if the adenoma-carcinoma pathway that is initiated by APC mutation [3, 6, 31] is blocked by colonoscopy and the carcinogenetic pathway has to include an early mutation causing an increased cell proliferation rate [3, 20], how have the observed CRCs emerged? There are many published pathophysiological, biochemical and DNA analyses of adenomas and CRCs. Based on these, we have previously suggested an additional pathway in path_MMR carriers in which dMMR crypts may trigger the adenoma-carcinoma pathway [6]. A recent mathematical analysis of the most frequently mutated genes in CRCs supported this theory [20].

Descriptions of tumours reflect different time-points in different carcinogenetic processes and, when tumours are removed for observation, subsequent events that might have occurred are blocked (“the very act of measurement or observation directly alters the phenomenon under investigation”) [35]. There is no way of fully characterising an adenoma or CRC without altering it. As no direct observations of the order of events in carcinogenetic processes are possible, the suggested linear models that are based on biological findings in adenomas and CRCs are assumed, not observed. In addition, mutations observed in adenomas and CRCs are influenced by selection (in path_MMR carriers including survival in the face of the host immune system’s continuous efforts to identify and remove the tumours) and genetic drift which may involve interacting stochastic processes.

When re-evaluating our previous proposal of three linear pathways to CRC in path_MMR carriers [6] and adding the findings reported in this paper, we conclude that the following model is probable: The main pathway to CRC in path_MMR carriers may be triggered by a second-hit in the wild-type MMR allele leading to a dMMR crypt and increasing the acquisition of mutations in driver genes [3, 20] in a continuous chaotic stochastic process which may lead to CRC. The probability of any dMMR clone leading to CRC seems very low. Adult path_MLH1 and path_MSH2 carriers may have thousands of dMMR crypts during life [36] but develop no or very few dMMR adenomas or MSI cancers. Information on dMMR crypts and dMMR adenomas incidences in path_MSH6 and path_PMS2 carriers is limited. While our previous mathematic modelling was consistent with the linear theory that dMMR crypts initiate the adenoma-carcinoma pathway, it did not indicate that there necessarily is an adenoma stage before an MSI CRC [20].

The findings of the current study indicate that CRCs developing from dMMR crypts may have no or only short sojourn times as adenomas (Fig. 4). This is in keeping with a chaotic stochastic pathway initiated by the dMMR/MSI crypts and is similar to ‘the Big Bang’ theory suggested for CRC in the general population [37]. Instead of an accelerated adenoma-carcinoma pathway, our current results may indicate an accelerated stochastic pathway accounting for most CRCs in path_MSH2, path_MLH1 and path_MSH6 carriers, but not path_PMS2 carriers (Fig. 5). A similar theory with mutations causing invasive growth and spread before clonal expansion has been suggested causing cancer in path_BRCA1/2 carriers leading to the same consequences of multiple subclones in one tumour, some of which having no or short sojourn time in adenoma stage [38]. Both these two theories (the one suggesting a Big Bang, the other considering different mutations in different parts of the tumours at different times), are based on recognizing high probabilities for stochastic mutations and may cause the same observed consequences. A theoretical discussion on complex probabilities as causative factors for events that are yet to happen is outside the scope of this report. For simplicity, we have used the well-known and easy-to-remember annotation ‘Big Bang’ in our title to indicate a causative chaotic stochastic probability where the outcome may not be predicted and for which the outcome that is observed does not elucidate exactly the events that led to that outcome.The theory we propose may explain our previously reported findings of similar or increased incidences of CRC in path_MMR carriers receiving regular colonoscopy compared to historical retrospective cohorts and the lack of any association between time since last colonoscopy and stage of CRC at diagnosis [9, 10, 39] as well as the current finding of no association between incidences of adenomas and CRCs. Noting that more than one mutation may occasionally occur simultaneously in a chaotic stochastic process [37], and that the host immune system may remove invasive cancer cells (as demonstrated by the success of immunotherapy for MSI CRCs in path_MMR carriers), all of our previously reported findings that were in conflict with the accelerated adenoma-carcinoma hypothesis may now be explained. Our current findings and theory are consistent with colonoscopy blocking the adenoma-carcinoma pathway but do not support the accelerated adenoma-carcinoma paradigm as the major cause of increased CRC incidence in path_MMR carriers.

Fig. 4

Possible pathways to CRC in LS modified from [6]. Carcinogenesis in LS follows a stochastic process where two major selection pressures apply: selection through conferred growth advantage and counter-selection through the immune system. MMR-deficient crypts may undergo immune elimination (arrows at the bottom central part of the graphic pointing to an empty space left by the eliminated crypt and now filled with immune cells), and MMR-deficient adenomas and cancers might be eliminated by the immune system, too (not shown by arrows). However, cancer may still arise via at least three pathways depicted here. Cancers arising through Pathway 1 may have a longer sojourn time as adenomas, as they spend part of their progression as MMR-proficient adenomas and only later acquire MMR deficiency. At this stage, Pathway 1-cancers can be prevented by colonoscopy. If not prevented, further mutations could accumulate eventually leading to cancer. Cancers arising through Pathway 2 may have shorter sojourn time as adenomas compared to Pathway 1, as they develop from initially MMR-deficient crypts acquiring further mutations to develop into MMR-deficient adenomas, thus not spending time as MMR-proficient lesions. However, the sojourn time as adenoma in Pathway 2 is longer compared to cancers arising through Pathway 3. The latter cancers likely have no sojourn time as adenomas and arise upon a single somatic hit (cnLOH) activating beta-catenin and inactivating MLH1. According to the current knowledge, this pathway is therefore specific for MLH1 carriers. Red arrows indicate regression due to counterselection by the immune system, black arrows indicate progression due to gained growth advantage, dotted black arrows indicate hypothetical progression if removal by colonoscopy did not take place

Attribution: Hourglass element is an image by freepik

Fig. 5

Simplified diagram of possible pathways to MSI CRC in path_MMR carriers, modified from [6, 20], compliant with Fig. 4 and specifying the accelerated chaotic stochastic process Big Bang theory [37] as discussed in the text

APC inactivation occurs prior to loss of the wild-type MMR allele resulting in adenoma initiation. Subsequent loss of the second MMR allele in the adenoma generates a dMMR clone with increased risk of progression to cancer

The initial event is loss of the second MMR allele resulting in a dMMR crypt causing an accelerated stochastic chaotic probability of mutations (that is greater for path_MLH1 and path_MSH2 compared to path_MSH6 and only marginally raised in Path_PMS2). Many different driver genes may be mutated and in different orders [38], causing no or only a short sojourn time of tumours as adenomas. A dMMR cell may also become a dMMR adenoma

In path_MLH1 carriers, a single LOH event at chromosome 3p22 may inactivate the wild-type alleles of the co-located CTNNB1 and MLH1 genes (in a cell that has already sustained a single CTNNB1 mutation, usually at codon 41 or 45 in Exon3). This specific initiating event may occur in up to 40% of CRC in Path_MLH1 carriers [32]

Regular colonoscopy may block the adenoma-carcinoma pathway. dMMR/MSI crypts, adenomas and cancer might be removed by the host immune system. Mutated genes in black. pMMR adenoma: MMR proficient adenoma. dMMR adenoma: MMR deficient adenoma

Colonoscopy is subject to time-trends. Our results are based on surveillance including colonoscopy that was carried out over several decades in the collaborating centres and they may be considered to represent largely historical observations. More recent and sophisticated colonoscopy techniques may detect smaller adenomas and/or adenomas with different morphological patterns. Staining/chromoendscopy might visualise dMMR crypts and guided or unguided machine learning (artificial intelligence) may provide new ways of interpreting digitalized colonoscopy images [40, 41]. The effects of these advances may be a longer mean sojourn time which could increase the probability of colonoscopy preventing CRC. One study using frequent colonoscopies with advanced techniques reported a high incidence of small adenomas and a low incidence of CRC, indicating that improved colonoscopy may reduce CRC incidences [42]. None of this is in conflict with our suggested theory of stochastic processes being the main driver for the CRCs we observed.

There are numerous reports on adenomas in the Lynch syndromes but few reports describing the relationships between adenomas and CRCs in carriers subjected to colonoscopy and none that report cumulative incidences of CRC or include sufficient path_MSH6 and path_PMS2 carriers to arrive at meaningful conclusions. Our findings were similar to those in a study of 112 carriers from Cleveland, Ohio among whom CRC was diagnosed only in path_MLH1 and path_MSH2 carriers [43]. A further recent report from Toronto on 429 carriers found that more than half of CRCs in path_MMR carriers occurred in patients without adenomas [44]. The methods used did not allow direct comparison with our results. The authors concluded that fewer CRCs were diagnosed in carriers with adenomas when intervals between colonoscopies were shorter, but they did not report findings for short intervals between colonoscopies for the majority of carriers who did not have adenomas. Furthermore, they scored advanced adenomas as CRC and censored observation time when advanced adenomas were found. They did not report cumulative incidences of CRC. The reduced CRC incidence in the minor fraction of carriers who had multiple adenomas and who received more frequent colonoscopy is not in conflict with the minor fraction of carriers with multiple adenomas in our study (Table 2; Figs. 1 and 2) who were too few in number to have an impact on the averages we report or to calculate separate cumulative CRC incidences by age.

A study on 136 carriers from Houston, reported on associations between the presence of adenomas, their stages and CRC [45]. As in the Toronto study, advanced adenomas and CRCs were grouped together and CRC cumulative incidences in carriers with and without adenomas were not reported. Corresponding with our results, they observed no difference in the likelihood of advanced adenomas or CRCs for any of the measured covariates.

In North-America there has been a discussion on prophylactic colectomy in path_MMR carriers [46]. But none of the above reports that grouped advanced adenomas and CRC together indicated whether they did so because colectomy was undertaken when advanced adenomas were found, and none is in conflict with our suggested theory of stochastic processes being the main driver for the CRCs we observed.

The strengths of our study include the large number of carriers and follow-up years available and that no assumption on the mechanisms underlying CRC was included in the ascertainment criteria. Determination of the results of stochastic processes in time requires sufficient numbers of observations in all strata of interest and sufficient observation time for all strata to reach endpoints. The PLSD was designed to investigate why the observed effects of colonoscopy could not be explained by the accelerated adenoma-carcinoma paradigm alone. Its aim of considering empirical observations in a sufficiently large number of carriers who were subjected to colonoscopy in relation to theories regarding carcinogenetic pathways has been achieved in the current study. CRC incidences in all groups of carriers at all ages have been similar from the first to the most recent PLSD reports, while the numbers of carriers and observation years had tripled, indicating that the CRC incidences in carriers receiving colonoscopy surveillance that are reported by the PLSD are valid. We find no reason to doubt the cumulative incidences of CRCs in carriers with and without adenomas in the current report.

The main weakness of our study is that, to our knowledge, it is the only study so far to compare cumulative incidences of adenomas with cumulative incidences of CRCs in all age groups of carriers, thus our results have not been validated in an independent replication cohort. We therefore plan to continue this study and to collect data on adenomas from those PLSD contributors who have so far only reported cancers to PLSD. All the information required is likely recorded in medical history of subjected patients but retrieving and contributing these data will require time, ethical approval and funding, resources that may not be available in all contributing centres.

The aim of this report was to gain knowledge of adenoma and CRC incidences in path_MMR carriers subjected to regular colonoscopy. We hope the findings may contribute to the refinement of clinical guidelines for the prevention of CRC in path_MMR carriers. Our previous empirical observations and the theories we have suggested to interpret them have been used as arguments for current clinical guidelines [47, 48], but in light of our recent and current findings, some current recommendations may need to be reconsidered.

1st, since colonoscopy prevents CRC in young path_PMS2 carriers the advice to postpone colonoscopy in young adult path_PMS2 carriers until 35 years of age should be questioned.

2nd, since undertaking colonoscopy more frequently than every three years may not reduce CRC incidences in path_MLH1 and path_MSH2 carriers, the rationale for more frequent colonoscopies should be questioned.

3rd, the very minor fraction of path_MMR carriers who have multiple adenomas will be identified at colonoscopy and may be followed more intensively. The driving forces causing their multiple adenomas remain unclear, and empirical knowledge on effects of interventions in this group are limited.