Polymerase Proofreading-associated Polyposis Research Articles

Rare germline variants in POLE and POLD1 encoding the catalytic subunits of DNA polymerases ε and δ in glioma families

Pathogenic germline variants in the DNA polymerase genes POLE and POLD1 cause polymerase proofreading-associated polyposis, a dominantly inherited disorder with increased risk of colorectal carcinomas and other tumors. POLE/POLD1 variants may result in high somatic mutation and neoantigen loads that confer susceptibility to immune checkpoint inhibitors (ICIs). To explore the role of POLE/POLD1 germline variants in glioma predisposition, whole-exome sequencing was applied to leukocyte DNA of glioma patients from 61 tumor families with at least one glioma case each. Rare heterozygous POLE/POLD1 missense variants predicted to be deleterious were identified in glioma patients from 10 (16%) families, co-segregating with the tumor phenotype in families with available DNA from several tumor patients. Glioblastoma patients carrying rare POLE variants had a mean overall survival of 21 months. Additionally, germline variants in POLD1, located at 19q13.33, were detected in 2/34 (6%) patients with 1p/19q-codeleted oligodendrogliomas, while POLE variants were identified in 2/4 (50%) glioblastoma patients with a spinal metastasis. In 13/15 (87%) gliomas from patients carrying POLE/POLD1 variants, features of defective polymerase proofreading, e.g. hypermutation, POLE/POLD1-associated mutational signatures, multinucleated cells, and increased intratumoral T cell response, were observed. In a CRISPR/Cas9-derived POLE-deficient LN-229 glioblastoma cell clone, a mutator phenotype and delayed S phase progression were detected compared to wildtype POLE cells. Our data provide evidence that rare POLE/POLD1 germline variants predispose to gliomas that may be susceptible to ICIs. Data compiled here suggest that glioma patients carrying POLE/POLD1 variants may be recognized by cutaneous manifestations, e.g. café-au-lait macules, and benefit from surveillance colonoscopy.

Characterization of POLE c.1373A > T p.(Tyr458Phe), causing high cancer risk

The cancer syndrome polymerase proofreading-associated polyposis results from germline mutations in the POLE and POLD1 genes. Mutations in the exonuclease domain of these genes are associated with hyper- and ultra-mutated tumors with a predominance of base substitutions resulting from faulty proofreading during DNA replication. When a new variant is identified by gene testing of POLE and POLD1, it is important to verify whether the variant is associated with PPAP or not, to guide genetic counseling of mutation carriers. In 2015, we reported the likely pathogenic (class 4) germline POLE c.1373A > T p.(Tyr458Phe) variant and we have now characterized this variant to verify that it is a class 5 pathogenic variant. For this purpose, we investigated (1) mutator phenotype in tumors from two carriers, (2) mutation frequency in cell-based mutagenesis assays, and (3) structural consequences based on protein modeling. Whole-exome sequencing of two tumors identified an ultra-mutator phenotype with a predominance of base substitutions, the majority of which are C > T. A SupF mutagenesis assay revealed increased mutation frequency in cells overexpressing the variant of interest as well as in isogenic cells encoding the variant. Moreover, exonuclease repair yeast-based assay supported defect in proofreading activity. Lastly, we present a homology model of human POLE to demonstrate structural consequences leading to pathogenic impact of the p.(Tyr458Phe) mutation. The three lines of evidence, taken together with updated co-segregation and previously published data, allow the germline variant POLE c.1373A > T p.(Tyr458Phe) to be reclassified as a class 5 variant. That means the variant is associated with PPAP.

Teenage-Onset Colorectal Cancers in a Digenic Cancer Predisposition Syndrome Provide Clues for the Interaction between Mismatch Repair and Polymerase δ Proofreading Deficiency in Tumorigenesis.

Simple SummaryColorectal cancer (CRC) in adolescents and young adults (AYA) is rare. Genetic causes include autosomal recessive and dominant monogenic disorders due to pathogenic variants (PVs) in genes involved in DNA repair. However, the genetic etiology of the majority of AYA-CRC remains unidentified. In two teenage siblings with CRC, we show to our knowledge for the first time that AYA-CRC cases can be caused by digenic inheritance of each a heterozygous pathogenic variant (PV) in the mismatch-repair (MMR) gene PMS2 and the proofreading polymerase (PP) Pol δ gene POLD1. With the aim to elucidate how the constitutional polymerase proofreading defect and the high propensity to MMR deficiency (MMRd) interact, we performed a comprehensive tumor analysis of the two siblings’ tumors. Results indicate that tumorigenesis is initiated by MMRd and the inherited POLD1 PV contributes to fast tumor progression reflected by an ultra-high tumor mutational burden (TMB) and specific mutational signatures.Colorectal cancer (CRC) in adolescents and young adults (AYA) is very rare. Known predisposition syndromes include Lynch syndrome (LS) due to highly penetrant MLH1 and MSH2 alleles, familial adenomatous polyposis (FAP), constitutional mismatch-repair deficiency (CMMRD), and polymerase proofreading-associated polyposis (PPAP). Yet, 60% of AYA-CRC cases remain unexplained. In two teenage siblings with multiple adenomas and CRC, we identified a maternally inherited heterozygous PMS2 exon 12 deletion, NM_000535.7:c.2007-786_2174+493del1447, and a paternally inherited POLD1 variant, NP_002682.2:p.Asp316Asn. Comprehensive molecular tumor analysis revealed ultra-mutation (>100 Mut/Mb) and a large contribution of COSMIC signature SBS20 in both siblings’ CRCs, confirming their predisposition to AYA-CRC results from a high propensity for somatic MMR deficiency (MMRd) compounded by a constitutional Pol δ proofreading defect. COSMIC signature SBS20 as well as SBS26 in the index patient’s CRC were associated with an early mutation burst, suggesting MMRd was an early event in tumorigenesis. The somatic second hits in PMS2 were through loss of heterozygosity (LOH) in both tumors, suggesting PPd-independent acquisition of MMRd. Taken together, these patients represent the first cases of cancer predisposition due to heterozygous variants in PMS2 and POLD1. Analysis of their CRCs supports that POLD1-mutated tumors acquire hypermutation only with concurrent MMRd.

Genomic landscape of colorectal carcinogenesis.

The molecular pathogenesis of solid tumour was first assessed in colorectal cancer (CRC). To date, ≤ 100 genes with somatic alterations have been found to inter-connectively promote neoplastic transformation through specific pathways. The process of colorectal carcinogenesis via genome landscape is reviewed on the basis of an adenoma-to-carcinoma sequence, as shown by serial histological and epidemiological observations. The relevant literatures from PubMed (1980-2021) have been reviewed for this article. The major routes of CRC development, chromosomal instability (CIN), microsatellite instability (MSI), and the serrated route either via CIN or MSI, proceed through the respective molecular pathway of colorectal carcinogenesis. Particular aspects of CRC carcinogenesis can also be determined by evaluating familial CRCs (FCRC) and genotype-phenotype correlations. Specific causative gene alterations still leave to be identified in several FCRCs. Otherwise, recently verified FCRC can be particularly notable, for example, EPCAM-associated Lynch syndrome, polymerase proofreading-associated polyposis, RNF43-associated polyposis syndrome or NTHL1 tumour syndrome, and hereditary mixed polyposis syndrome. The oncogenic landscape is described, including representative pathway genes, the three routes of carcinogenesis, familial CRCs, genotype-phenotype correlations, the identification of causative genes, and consensus molecular subtypes (CMS). Whole genome research using multi-gene panels (MGPs) has facilitated high through-put detection of previously unidentified genes involved in colorectal carcinogenesis. New approaches designed to identify rare variants are recommended to consider their alterations implicated in the molecular pathogenesis.

Di‐genic inheritance of germline POLE and PMS2 pathogenic variants causes a unique condition associated with pediatric cancer predisposition

Polymerase proofreading-associated polyposis (PPAP) and Lynch syndrome, caused by mutated POLE and mismatch repair (MMR) genes, respectively, are associated with adult-onset cancer. PPAP and MMR-deficient tumors are both hypermutated, and each has a unique mutational signature. We describe a 4.5-year-old boy with multiple café au lait spots who presented with metastatic Sonic Hedgehog-activated medulloblastoma, with partial response to intensive chemotherapy and immunotherapy. The tumor showed microsatellite stability, loss of PMS2 nuclear expression, and an exceptionally high tumor mutational burden of 276 Mut/Mb. Germline molecular analysis revealed an inherited heterozygous pathogenic POLE variant and a de novo heterozygous PMS2 pathogenic variant. The tumor featured the MMR, POLE, and POLE+MMR mutational signatures. This is the first description of a di-genic condition, which we named "POL-LYNCH syndrome," manifested by an aggressive ultra-mutant pediatric medulloblastoma with a unique genomic signature.

Genotypic and Phenotypic Characteristics of Hereditary Colorectal Cancer.

The genomic causes and clinical manifestations of hereditary colorectal cancer (HCRC) might be stratified into 2 groups, namely, familial (FCRC) and a limited sense of HCRC, respectively. Otherwise, FCRC is canonically classified into 2 major categories; Lynch syndrome (LS) or associated spectra and inherited polyposis syndrome. By contrast, despite an increasing body of genotypic and phenotypic traits, some FCRC cannot be clearly differentiated as definitively single type, and the situation has become more complex as additional causative genes have been discovered. This review provides an overview of HCRC, including 6 LS or associated spectra and 8 inherited polyposis syndromes, according to molecular pathogenesis. Variants and newly-identified FCRC are particularly emphasized, including MUTYH (or MYH)-associated polyposis, Muir-Torre syndrome, constitutional mismatch repair deficiency, EPCAM-associated LS, polymerase proofreading-associated polyposis, RNF43- or NTHL1-associated serrated polyposis syndrome, PTEN hamartoma tumor syndrome, and hereditary mixed polyposis syndrome. We also comment on the clinical utility of multigene panel tests, focusing on comprehensive cancer panels that include HCRC. Finally, HCRC surveillance strategies are recommended, based on revised or notable concepts underpinned by competent validation and clinical implications, and favoring major guidelines. As hereditary syndromes are mainly attributable to genomic constitutions of distinctive ancestral groups, an integrative national HCRC registry and guideline is an urgent priority.

Constitutional POLE variants causing a phenotype reminiscent of constitutional mismatch repair deficiency.

Heterozygous POLE or POLD1 germline pathogenic variants (PVs) cause polymerase proofreading associated polyposis (PPAP), a constitutional polymerase proofreading deficiency that typically presents with colorectal adenomas and carcinomas in adulthood. Constitutional mismatch-repair deficiency (CMMRD), caused by germline bi-allelic PVs affecting one of four MMR genes, results in a high propensity for the hematological, brain, intestinal tract, and other malignancies in childhood. Nonmalignant clinical features, such as skin pigmentation alterations, are found in nearly all CMMRD patients and are important diagnostic markers. Here, we excluded CMMRD in three cancer patients with highly suspect clinical phenotypes but identified in each a constitutional heterozygous POLE PV. These, and two additional POLE PVs identified in published CMMRD-like patients, have not previously been reported as germline PVs despite all being well-known somatic mutations in hyper-mutated tumors. Together, these five cases show that specific POLE PVs may have a stronger "mutator" effect than known PPAP-associated POLE PVs and may cause a CMMRD-like phenotype distinct from PPAP. The common underlying mechanism, that is, a constitutional replication error repair defect, and a similar tumor spectrum provide a good rationale for monitoring these patients with a severe constitutional polymerase proofreading deficiency according to protocols proposed for CMMRD.

Genetic testing for inherited colorectal cancer and polyposis, 2021 revision: a technical standard of the American College of Medical Genetics and Genomics (ACMG)

Colorectal cancer (CRC) is the fourth most frequently diagnosed cancer and 30% of all cases of CRC are believed to have a familial component and up to one-third of these (10%) are hereditary. Pathogenic germline variants in multiple genes have been associated with predisposition to hereditary CRC or polyposis. Lynch syndrome (LS) is the most common hereditary CRC syndrome, caused by variants in the mismatch repair (MMR) genes MLH1, MSH2, MSH6, and PMS2 and is inherited in a dominant manner. Heritable conditions associated with colonic polyposis include familial adenomatous polyposis (FAP) associated with APC pathogenic variants, MUTYH-associated polyposis (MAP) caused by biallelic MUTYH pathogenic variants, and polymerase proofreading-associated polyposis (PPAP) caused by POLE or POLD1 pathogenic variants. Given the overlapping phenotypes of the cancer syndromes along with the limited sensitivity of using clinical criteria alone, a multigene panel testing approach to diagnose these conditions using next-generation sequencing (NGS) is effective and efficient. This technical standard is not recommended for use in the clinic for patient evaluation. Please refer to National Comprehensive Cancer Network (NCCN) clinical practice guidelines to determine an appropriate testing strategy and guide medical screening and management. This 2021 edition of the American College of Medical Genetics and Genomics (ACMG) technical standard supersedes the 2013 edition on this topic.

The clinical features of polymerase proof-reading associated polyposis (PPAP) and recommendations for patient management

Pathogenic germline exonuclease domain (ED) variants of POLE and POLD1 cause the Mendelian dominant condition polymerase proof-reading associated polyposis (PPAP). We aimed to describe the clinical features of all PPAP patients with probably pathogenic variants. We identified patients with a variants mapping to the EDs of POLE or POLD1 from cancer genetics clinics, a colorectal cancer (CRC) clinical trial, and systematic review of the literature. We used multiple evidence sources to separate ED variants into those with strong evidence of pathogenicity and those of uncertain importance. We performed quantitative analysis of the risk of CRC, colorectal adenomas, endometrial cancer or any cancer in the former group. 132 individuals carried a probably pathogenic ED variant (105 POLE, 27 POLD1). The earliest malignancy was colorectal cancer at 14. The most common tumour types were colorectal, followed by endometrial in POLD1 heterozygotes and duodenal in POLE heterozygotes. POLD1-mutant cases were at a significantly higher risk of endometrial cancer than POLE heterozygotes. Five individuals with a POLE pathogenic variant, but none with a POLD1 pathogenic variant, developed ovarian cancer. Nine patients with POLE pathogenic variants and one with a POLD1 pathogenic variant developed brain tumours. Our data provide important evidence for PPAP management. Colonoscopic surveillance is recommended from age 14 and upper-gastrointestinal surveillance from age 25. The management of other tumour risks remains uncertain, but surveillance should be considered. In the absence of strong genotype–phenotype associations, these recommendations should apply to all PPAP patients.

A practical screening strategy for Lynch syndrome and Lynch syndrome mimics in colorectal cancer.

The objective of the study is to provide an efficient and practical screening strategy to distinguish a broader spectrum of Lynch syndrome (LS) and LS mimics-associated colorectal cancer (CRC), including Lynch-like syndrome (LLS), constitutional mismatch repair-deficiency, familial CRC type X (FCCTX), and polymerase proofreading-associated polyposis syndrome. 1294 cases of CRC samples were detected mismatch repair (MMR) status using immunohistochemistry (IHC) staining, in which the cases with MLH1-deficient CRC underwent BRAF mutation analysis by IHC. Following the personal and/or family history survey, next-generation sequencing (NGS) was used to detect gene variants. 1294 CRC patients were dichotomized into tumors caused by a deficient MMR (dMMR) system and a proficient MMR (pMMR) system after MMR status analysis. 45 patients with suspected sporadic dMMR CRC were then separated from MLH1-deficient CRC though BRAF mutation status analysis by IHC. Following the personal and/or family history survey for 1294 patients, as well as germline genetic testing by NGS, 34 patients were diagnosed as LS (8 cases), SLS (13 cases), LLS ( 6 cases), FCCTX (3 cases), and sporadic CRC (4 cases). Our screening strategy, which consists of clinical and molecular analyses, is expected to improve the screening efficiency and management for the LS and LS mimics.

Discordant DNA mismatch repair protein status between synchronous or metachronous gastrointestinal carcinomas: frequency, patterns, and molecular etiologies.

The widespread use of tumor DNA mismatch repair (MMR) protein immunohistochemistry in gastrointestinal tract (GIT) carcinomas has unveiled cases where the MMR protein status differs between synchronous/metachronous tumors from the same patients. This study aims at examining the frequency, patterns and molecular etiologies of such inter-tumoral MMR discordances. We analyzed a cohort of 2159 colorectal cancer (CRC) patients collected over a 5-year period and found that 1.3% of the patients (27/2159) had ≥ 2 primary CRCs, and 25.9% of the patients with ≥ 2 primary CRCs (7/27) exhibited inter-tumoral MMR discordance. We then combined the seven MMR-discordant CRC patients with three additional MMR-discordant GIT carcinoma patients and evaluated their discordant patterns and associated molecular abnormalities. The 10 patients consisted of 3 patients with Lynch syndrome (LS), 1 with polymerase proofreading-associated polyposis (PAPP), 1 with familial adenomatous polyposis (FAP), and 5 deemed to have no cancer disposing hereditary syndromes. Their MMR discordances were associated with the following etiologies: (1) PMS2-LS manifesting PMS2-deficient cancer at an old age when a co-incidental sporadic MMR-proficient cancer also occurred; (2) microsatellite instability-driven secondary somatic MSH6-inactivation occurring in only one-and not all-PMS2-LS associated MMR-deficient carcinomas; (3) "compound LS" with germline mutations in two MMR genes manifesting different tumors with deficiencies in different MMR proteins; (4) PAPP or FAP syndrome-associated MMR-proficient cancer co-occurring metachronously with a somatic MMR-deficient cancer; and (5) non-syndromic patients with sporadic MMR-proficient cancers co-occurring synchronously/metachronously with sporadic MMR-deficient cancers. Our study thus suggests that inter-tumoral MMR discordance is not uncommon among patients with multiple primary GIT carcinomas (25.9% in patients with ≥ 2 CRCs), and may be associated with widely varied molecular etiologies. Awareness of these patterns is essential in ensuring the most effective strategies in both LS detection and treatment decision-making. When selecting patients for immunotherapy, MMR testing should be performed on the tumor or tumors that are being treated.

An update on the CNS manifestations of brain tumor polyposis syndromes.

Cancer predisposition syndromes are associated with an increased risk of developing primary malignancies. Here we discuss those which are associated with an increased risk of tumors of the central nervous system (CNS) and gastrointestinal (GI) tract. These can be grouped into those in which the CNS tumors predominate versus those in which the GI cancers predominate. The former include constitutional mismatch repair deficiency (CMMRD) syndrome, Li-Fraumeni syndrome (LFS), and Cowden syndrome (CS) while the latter include familial adenomatosis polyposis 1 (FAP1), Lynch syndrome and polymerase proofreading-associated polyposis syndrome (PPAP). Tumor specificity does exist as medulloblastoma occur in FAP, LFS and CMMRD while glioma are most commonly seen in all replication repair-deficient genes and LFS. Choroid plexus carcinoma is strictly observed in LFS while Cowden syndrome patients develop Lhermitte Duclos disease or meningioma. In each syndrome, specific types of low-grade and high-grade gastrointestinal cancers can occur, but these will be discussed elsewhere. Underlying cancer predisposition syndromes are important to consider when faced with brain tumors, particularly in the pediatric and young adult age groups, as identification of an underlying germ line mutation may change the upfront management of the patient and has implications for future cancer surveillance for both the patient and potentially affected family members. Considerations of family history, presence of skin lesions and consanguinity provide valuable information in identifying patients at potential increased risk.

An Update on Inherited Colon Cancer and Gastrointestinal Polyposis.

It is estimated that 5-10% of colorectal cancers arise due to a known genetic syndrome. Individuals with these cancer syndromes are also at risk of extracolonic cancers. Polyposis and nonpolyposis hereditary syndromes are generally recognized. Inclusion of next-generation sequencing technology, especially multiple-gene panel testing, in routine laboratory practice has made identifying the causes of these diseases significantly easier. To summarize current knowledge of the causes, clinical manifestations, diagnostic criteria, and recommendations for presymptomatic screening of individuals at risk of hereditary gastrointestinal polyposis and colorectal cancer syndromes. We dicuss currently defined syndromes detected by multiple-gene panel next-generation sequencing; these include constitutional mismatch repair deficiency (biallelic MLH1, MSH2, MSH6, PMS2 gene mutations), gastric adenocarcinoma and proximal polyposis of the stomach (APC gene), NTHL1-associated polyposis, polymerase proofreading-associated polyposis (POLD1, POLE genes), juvenile polyposis (SMAD4, BMPR1A genes), and serrated polyposis syndromes. Another aim is to summarize recent knowledge about well-known syndromes, including hereditary nonpolyposis colon cancer (Lynch syndrome), familial adenomatous polyposis, MUTYH-associated polyposis, and Peutz-Jeghers and Cowden/PTEN hamartoma tumor syndromes. Awareness of hereditary polyposis/colon cancer syndromes enables early diagnosis and prevention of cancer in affected individuals and their relatives. Genetic counseling, presymptomatic testing of at-risk individuals, and efficient screening may be beneficial for affected families. Thank to Lenka Foretová, M.D., PhD, (Masaryk Memorial Cancer Institute, Brno) for a critical review of the manuscript and valuable advices. The author declares she has no potential conflicts of interest concerning drugs, products, or services used in the study. The Editorial Board declares that the manuscript met the ICMJE recommendation for biomedical papers. Submitted: 1. 3. 2019 Accepted: 6. 6. 2019.

Development of an MSI-positive colon tumor with aberrant DNA methylation in a PPAP patient

Polymerase proofreading-associated polyposis (PPAP) is a disease caused by germline variations in the POLE and POLD1 genes that encode catalytic subunits of DNA polymerases. Studies of cancer genomes have identified somatic mutations in these genes, suggesting the importance of polymerase proofreading of DNA replication in suppressing tumorigenesis. Here, we identified a germline frameshift variation in the POLE gene (c.4191_4192delCT, p.Tyr1398*) in a case with multiple adenomatous polyps and three synchronous colon cancers. Interestingly, one of the colon cancers showed microsatellite instability-high (MSI-H) and another microsatellite stable. Immunohistochemical staining revealed that the MSI-H tumor cells lost the expression of MLH1 protein. Whole genome sequencing of the MSI-H tumor did not find pathogenic somatic mutations in mismatch repair genes but found frameshift mutations in the TET genes that catalyze 5-methylcytosine hydroxylation. Bisulfite sequencing of the tumor corroborated an increase in the number of hypermethylated regions including the MLH1 promoter. These data indicate that PPAP patients might develop MSI-positive tumors through epigenetic silencing of MLH1. These findings will contribute to comprehensive understanding of the molecular basis of tumors that involve deficiency of proofreading activity of DNA polymerases.

The role of inherited genetic variants in colorectal polyposis syndromes.

Colorectal carcinoma (CRC) is the third most common cancer in men and the second most common cancer in women across the world. Most CRCs occur sporadically, but in 15-35% of cases, hereditary factors are important. Some patients with an inherited predisposition to CRC will be diagnosed with a "genetic polyposis syndrome" such as familial adenomatous polyposis (FAP), MUTYH-associated polyposis (MAP), polymerase proofreading associated polyposis (PPAP), NTHL1-associated polyposis, MSH3-associated polyposis or a hamartomatous polyposis syndrome. Individuals with ≥10 colorectal polyps have traditionally been referred for genetic diagnostic testing to identify APC and MUTYH mutations which cause FAP and MAP respectively. Mutations are found in most patients with >100 adenomas but in only a minority of those with 10-100 adenomas. The reasons that diagnostic laboratories are not identifying pathogenic variants include mutations occurring outside of the open reading frames of genes, individuals exhibiting generalized mosaicism and the involvement of additional genes. It is important to identify patients with an inherited polyposis syndrome, and to define the mutations causing their polyposis, so that the individuals and their relatives can be managed appropriately.

Current clinical topics of Lynch syndrome.

Lynch syndrome (LS) is one of the most common genetic cancer syndromes, occurring at a rate of 1 per 250-1000 in the general population. This autosomal dominant disease is caused by a germline variant in one of the four mismatch repair genes, MSH2, MLH1, MSH6, PMS2, or the EPCAM gene. LS develops at early ages in colorectal cancer (CRC), endometrial cancer, and various other associated tumors. Accurate diagnosis of LS and utilization of various risk-reduction strategies such as surveillance, prophylactic surgery, and chemoprevention could improve clinical outcomes. The efficacy of surveillance has only been proven for CRC; however, specialists have proposed surveillance for other LS associated tumors. Universal screening for tumor tissue using microsatellite instability testing or the mismatch repair protein immunochemistry in all CRC or endometrial cancers is recommended not only as a diagnostic tool for LS, but also as a predictive, prognostic, and therapeutic marker. Next-generation sequencing methods have revealed several conditions with phenotypes similar to LS, such as Lynch-like syndrome, constitutional mismatch repair deficiency syndrome, and polymerase proofreading-associated polyposis. Distinguishing LS from these similar conditions is clinically important, since clinical management for patients differs according to the conditions. Recently, immune checkpoint inhibitors have been shown to be a promising treatment against mismatch repair-deficient (dMMR) solid tumors. The efficacy of immune-checkpoint inhibitors in LS-associated tumors has been shown to be similar to that in sporadic dMMR tumors. This review discusses current clinical topics related to LS screening, diagnosis, surveillance, and therapy.

Hereditary colorectal cancer : An update on genetics and entities in terms of differential diagnosis

The pathologist can contribute to recognizing hereditary causes of colorectal cancer via morphology. By identifying so-called index patients, it is possible to take preventive measures in affected families. The precise definition of the clinical presentation and the histopathological phenotype help to narrow the spectrum of expected genetic alterations. Novelties within Lynch syndrome include the recognition of EPCAM as afifth gene locus, as well as the newly defined Lynch-like syndrome with evidence of somatic mismatch repair (MMR) mutations. With regard to polyposis-associated syndromes, the spectrum of polyps, whether serrated, hamartomatous or classic adenoma, is of crucial importance. The resulting differential diagnosis includes (attenuated) familial adenomatous polyposis ([a]FAP), MUTYH-associated polyposis (MAP), polymerase proofreading-associated polyposis (PPAP), phosphatase and tensin homolog (PTEN) hamartoma tumor syndrome (PHTS), Peutz-Jeghers syndrome and juvenile polyposis, each with aspecific genetic background.

Abstract IA08: Polyposis beyond FAP (and APC)

Abstract Familial adenomatous polyposis (FAP) is the most common high-penetrant Mendelian syndrome predisposing to adenomatous polyposis. FAP is caused by monoallelic germline mutations in the gene APC, and shows autosomal dominant inheritance. A somatic mutation in the remaining wild type APC allele results in loss of APC activity, which is one of the initiating events of adenoma development. The high proliferation rate in the colorectum increases the chance that an intestinal epithelium cell obtains this somatic second hit, which explains why FAP patients usually develop hundreds to thousands of adenomatous polyps and, if untreated, inevitably develop colorectal cancer (CRC). Nevertheless, the majority of polyposis patients and families are not explained by mutations in APC. Since the discovery of FAP in 1991, several additional polyposis syndromes have been discovered, usually associated with lower number of polyps and somewhat less elevated risk of CRC. Many of the genes underlying these syndromes play a role in DNA repair and have a wider tumor spectrum. As becomes apparent from the somatic mutation profiles in tumors of these polyposis patients, these DNA repair deficiencies cause increased somatic mutation rates that affect proliferating tissues like the colon, and accelerate the formation of polyps and carcinomas. As an example, polymerase proofreading associated polyposis (PPAP) is a recently discovered autosomal dominant syndrome caused by mutations in the exonuclease domains of the polymerase genes POLE and POLD1. In addition, mutations in the base excision repair genes MUTYH and NTHL1, and the mismatch repair gene MSH3 underlie autosomal recessive polyposis syndromes. Notably, the incidences of these syndromes, particularly those recently discovered, are low. The question therefore arises how many more genes in which mutations cause high-penetrant polyposis still await their discovery. Here, I will present the latest findings on NTHL1-associated polyposis, it's expected incidence and its phenotypic characteristics. Furthermore, I will discuss the possible role of other DNA glycosylase genes of the base excision repair pathway in the development of polyposis and colorectal cancer. Citation Format: Roland P. Kuiper. Polyposis beyond FAP (and APC). [abstract]. In: Proceedings of the AACR Special Conference on Colorectal Cancer: From Initiation to Outcomes; 2016 Sep 17-20; Tampa, FL. Philadelphia (PA): AACR; Cancer Res 2017;77(3 Suppl):Abstract nr IA08.

Colorectal Adenomatous Polyposis: Heterogeneity of Susceptibility Gene Mutations and Phenotypes in a Cohort of Italian Patients.

Colorectal adenomatous polyposis entailing cancer predisposition is caused by constitutional mutations in different genes. APC is associated with the familial adenomatous polyposis (FAP/AFAP) and MUTYH with the MUTYH-associated polyposis (MAP), while POLE and POLD1 mutations cause the polymerase proofreading-associated polyposis (PPAP). We screened for mutations in patients with multiple adenomas/FAP: 121 patients were analyzed for APC and MUTYH mutations, and 36 patients were also evaluated for POLE and POLD1 gene mutations. We found 20 FAP/AFAP, 15 MAP, and no PPAP subjects: pathogenic mutations proved to be heterogeneous, and included 5 APC and 1 MUTYH novel mutations. The mutation detection rate was significantly different between patients with 5-100 polyps and those with >100 polyps (p = 8.154 × 10-7), with APC mutations being associated with an aggressive phenotype (p = 1.279 × 10-9). Mean age at diagnosis was lower in FAP/AFAP compared to MAP (p = 3.055 × 10-4). Mutation-negative probands showed a mean age at diagnosis that was significantly higher than FAP/AFAP (p = 3.46986 × 10-7) and included 45.3% of patients with <30 polyps and 70.9% of patients with no family history. This study enlarges the APC and MUTYH mutational spectra, and also evaluated variants of uncertain significance, including the MUTYH p.Gln338His mutation. Moreover this study underscores the phenotypic heterogeneity and genotype-phenotype correlations in a cohort of Italian patients.

A novel germline POLE mutation causes an early onset cancer prone syndrome mimicking constitutional mismatch repair deficiency

In a 14-year-old boy with polyposis and rectosigmoid carcinoma, we identified a novel POLE germline mutation, p.(Val411Leu), previously found as recurrent somatic mutation in ‘ultramutated’ sporadic cancers. This is the youngest reported cancer patient with polymerase proofreading-associated polyposis indicating that POLE mutation p.(Val411Leu) may confer a more severe phenotype than previously reported POLE and POLD1 germline mutations. The patient had multiple café-au-lait macules and a pilomatricoma mimicking the clinical phenotype of constitutional mismatch repair deficiency. We hypothesize that these skin features may be common to different types of constitutional DNA repair defects associated with polyposis and early-onset cancer.