Msh6 protects mature B cells from lymphoma by preserving genomic stability

Epigenetic disorders: Lessons from the animals–animal models in chromatinopathies

Chromatinopathies are defined as genetic disorders caused by mutations in genes coding for protein involved in the chromatin state balance. So far 82 human conditions have been described belonging to this group of congenital disorders, sharing some molecular features and clinical signs. For almost all of these conditions, no specific treatment is available. For better understanding the molecular cascade caused by chromatin imbalance and for envisaging possible therapeutic strategies it is fundamental to combine clinical and basic research studies. To this end, animal modelling systems represent an invaluable tool to study chromatinopathies. In this review, we focused on available data in the literature of animal models mimicking the human genetic conditions. Importantly, affected organs and abnormalities are shared in the different animal models and most of these abnormalities are reported as clinical manifestation, underlying the parallelism between clinics and translational research.

Diffuse Large B-Cell Lymphoma, Epstein-Barr Virus -Positive Kappa Monotypic Plasma Cell Proliferation and Invasive Carcinoma, Developing in a Child With Defective Mismatch Repair

Constitutional mismatch repair deficiency (CMMRD) syndrome is characterized by biallelic mutations in a mismatch repair gene and is associated with development of childhood cancers and symptoms resembling neurofibromatosis type 1, like café-au-lait spots. We describe the extremely rare case of a 12-year-old male presenting with several light brown macular lesions on the skin, gastrointestinal diffuse large B-cell lymphoma, adenomatous polyposis throughout the gastrointestinal tract and an intra-abdominal invasive carcinoma derived from upper gastrointestinal system. All neoplasia, as well as normal tissues, showed loss of Msh6 expression with immunohistochemistry. Molecular studies showed pathogenic homozygous p.F1088Sfs*2 mutation in MSH6. Furthermore, signs consistent with immunodeficiency, namely decreased levels of IgG and IgA in the serum, nodular lymphoid hyperplasia and EBV-associated plasma cell proliferation with monotypic kappa light chain expression in the ileum, were also noted. Our case depicts the phenotypic diversity of CMMRD syndrome and emphasizes its association with immunodeficiency, raising awareness to a feature not widely recognized.

The MutSβ complex is a modulator of p53-driven tumorigenesis through its functions in both DNA double-strand break repair and mismatch repair

Loss of the DNA mismatch repair (MMR) protein MSH3 leads to the development of a variety of tumors in mice without significantly affecting survival rates, suggesting a modulating role for the MutSβ (MSH2-MSH3) complex in late-onset tumorigenesis. To better study the role of MSH3 in tumor progression, we crossed Msh3(-/-) mice onto a tumor predisposing p53-deficient background. Survival of Msh3/p53 mice was not reduced compared with p53 single mutant mice; however, the tumor spectrum changed significantly from lymphoma to sarcoma, indicating MSH3 as a potent modulator of p53-driven tumorigenesis. Interestingly, Msh3(-/-) mouse embryonic fibroblasts displayed increased chromatid breaks and persistence of γH2AX foci following ionizing radiation, indicating a defect in DNA double-strand break repair (DSBR). Msh3/p53 tumors showed increased loss of heterozygosity, elevated genome-wide copy-number variation and a moderate microsatellite instability phenotype compared with Msh2/p53 tumors, revealing that MSH2-MSH3 suppresses tumorigenesis by maintaining chromosomal stability. Our results show that the MSH2-MSH3 complex is important for the suppression of late-onset tumors due to its roles in DNA DSBR as well as in DNA MMR. Further, they demonstrate that MSH2-MSH3 suppresses chromosomal instability and modulates the tumor spectrum in p53-deficient tumorigenesis and possibly has a role in other chromosomally unstable tumors as well.

Extracolonic manifestations of lynch syndrome

Lynch syndrome has classically been defined by several predominant malignancies. Initial clinical criteria for diagnosis of Lynch syndrome would miss 40% of affected individuals. As time has passed, our understanding of Lynch syndrome has evolved and will continue to do so. The number of cancer types that are included in the Lynch phenotype is growing. This has allowed clinicians to redefine Lynch syndrome, at risk populations, screening needs, and diagnostic criteria. Inclusion of extracolonic malignancies and alternative genetic pathways gives new insight into the true prevalence and penetrance of Lynch syndrome.

Analysis of microsatellite instability in gastric mucosa-associated lymphoid tissue lymphoma

In Helicobacter pylori gastritis, constant antigenic stimulation triggers a sustained B-cell proliferation. Errors made during this continuous DNA replication are supposed to be corrected by the DNA mismatch repair mechanism. Failure of this mismatch repair mechanism has been described in hereditary non-polyposis colorectal cancer (HNPCC) and results in a replication error phenotype. Inherent to their instability during replication, microsatellites are the best markers of this replication error phenotype. We aimed to evaluate the role of defects in the DNA mismatch repair (MMR) mechanism and microsatellite instability (MSI) in relation to the most frequent genetic anomaly, translocation t(11;18)(q21;q21), in gastric mucosa-associated lymphoid tissue (MALT) lymphoma. Therefore, we examined 10 microsatellite loci (BAT25, BAT26, D5S346, D17S250, D2S123, TGFB, BAT40, D18S58, D17S787 and D18S69) for instability in 28 patients with MALT lymphomas. In addition, these tumors were also immunostained for MLH1, MSH2, MSH6 and PMS2, as well as screened for the presence of t(11;18)(q21;q21) by real-time polymerase chain reaction (RT-PCR). We found MSI in 5/28 (18%) lymphomas, with MSI occurring in both t(11;18)(q21;q21)-positive and -negative tumors. One tumor displayed high levels of instability, and, remarkably, this was the only case displaying features of a diffuse large B-cell lymphoma. All microsatellite unstable lymphomas showed a loss of MSH6 expression. In conclusion, our data suggest that a MMR-defect may be involved in the development of gastric MALT lymphomas, and that a defect of MSH6 might be associated with those MSI-driven gastric lymphomas.

KAP1 regulates gene networks controlling mouse B-lymphoid cell differentiation and function

Chromatin remodeling is fundamental for B-cell differentiation. In the present study, we explored the role of KAP1, the cofactor of KRAB-ZFP transcriptional repressors, in this process. B-lymphoid-specific Kap1-KO mice displayed reduced numbers of mature B cells, lower steady-state levels of Abs, and accelerated rates of decay of neutralizing Abs after viral immunization. Transcriptome analyses of Kap1-deleted B splenocytes revealed an up-regulation of PTEN, the enzymatic counteractor of PIK3 signaling, and of genes encoding DNA-damage response factors, cell-cycle regulators, and chemokine receptors. ChIP/seq studies established that KAP1 bound at or close to several of these genes and controlled chromatin status at their promoters. Genome wide, KAP1 binding sites lacked active B cell-specific enhancers and were enriched in repressive histone marks, further supporting a role for this molecule in gene silencing in vivo. Likely responsible for tethering KAP1 to at least some of these targets, a discrete subset of KRAB-ZFPs is enriched in B lymphocytes. Our results therefore reveal the role of KRAB/KAP1-mediated epigenetic regulation in B-cell development and homeostasis.

Acquisition of Genetic Aberrations by Activation-Induced Cytidine Deaminase (AID) during Inflammation-Associated Carcinogenesis

Genetic abnormalities such as nucleotide alterations and chromosomal disorders that accumulate in various tumor-related genes have an important role in cancer development. The precise mechanism of the acquisition of genetic aberrations, however, remains unclear. Activation-induced cytidine deaminase (AID), a nucleotide editing enzyme, is essential for the diversification of antibody production. AID is expressed only in activated B lymphocytes under physiologic conditions and induces somatic hypermutation and class switch recombination in immunoglobulin genes. Inflammation leads to aberrant AID expression in various gastrointestinal organs and increased AID expression contributes to cancer development by inducing genetic alterations in epithelial cells. Studies of how AID induces genetic disorders are expected to elucidate the mechanism of inflammation-associated carcinogenesis.

Role of activation-induced cytidine deaminase in inflammation-associated cancer development

Human cancer is a genetic disease resulting from the stepwise accumulation of genetic alterations in various tumor-related genes. Normal mutation rates, however, cannot account for the abundant genetic changes accumulated in tumor cells, suggesting that certain molecular mechanisms underlie such a large number of genetic alterations. Activation-induced cytidine deaminase (AID), a nucleotide-editing enzyme that triggers DNA alterations and double-strand DNA breaks in the immunoglobulin gene, has been identified in activated B lymphocytes. Recent studies revealed that AID-mediated genotoxic effects target not only immunoglobulin genes but also a variety of other genes in both B lymphocytes and non-lymphoid cells. Consistent with the finding that several transcription factors including nuclear factor-κB (NF-κB) mediate AID expression in B cells, proinflammatory cytokine stimulation of several types of gastrointestinal epithelial cells, such as gastric, colonic, hepatic, and biliary epithelium, induces aberrant AID expression through the NF-κB signaling pathway. In vivo studies revealed that constitutive AID expression promotes the tumorigenic pathway by enhancing the susceptibility to mutagenesis in a variety of epithelial organs. The activity of AID as a genome mutator provides a new avenue for studies aimed at understanding mutagenesis mechanisms during carcinogenesis.