{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nMethyl‐CpG binding domain protein 4 (MBD4) is a multifunctional DNA repair enzyme that plays a pivotal role in maintaining genomic integrity. Biochemical and in vivo studies have demonstrated that MBD4 recognizes aberrant T:G or U:G mismatches produced by the spontaneous deamination of cytosine and 5‑methylcytosine at CpG sites, and then excises the mismatched base via its glycosylase activity. This repair function minimizes point mutations (especially C → T transitions) and thereby helps prevent oncogenic events."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "3"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nStructural analyses reveal that MBD4 harbors two key domains: an N‑terminal methyl‐CpG binding domain (MBD) that preferentially recognizes methylated DNA or specific mismatches in a CpG context, and a C‑terminal glycosylase domain responsible for excising the errant base. In addition, MBD4 physically interacts with mismatch repair proteins—most notably MLH1—and is functionally linked to other repair factors including thymine DNA glycosylase and components of de novo methylation pathways. These features underline the enzyme’s role as an integrator of DNA repair and epigenetic regulation."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "4", "end_ref": "7"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nBeyond its basal repair activity, MBD4 contributes to the cellular response to DNA damage. Experimental models have shown that deficiency of MBD4 leads to an increased occurrence of C → T transitions at CpG sites, accelerated tumor formation (for instance, in the gastrointestinal tract), and defective apoptotic responses upon exposure to DNA‐damaging agents. These observations underscore the enzyme’s tumor‐suppressive properties and its importance in orchestrating appropriate checkpoint and apoptosis processes following genotoxic stress."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "8", "end_ref": "10"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nWithin the adaptive immune system, roles for MBD4 have emerged in the context of immunoglobulin class switch recombination (CSR). Activation‐induced cytidine deaminase (AID)‐initiated deamination events create lesions that serve as substrates for several DNA repair enzymes, including MBD4. Although experimental inactivation of Mbd4 does not uniformly abolish CSR, studies indicate that MBD4 participates in processing DNA ends during CSR, and alternative transcripts may compensate for its loss."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "11", "end_ref": "13"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nIn addition to its canonical repair functions, MBD4 appears to influence transcriptional regulation and developmental processes. Some studies have linked MBD4 with the modulation of retinoid‐regulated transcription and suggested interactions with transcriptional coactivators, thereby hinting at roles that extend into gene expression regulation and differentiation."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "14"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nFinally, altered expression or regulation of MBD4 has been associated with heightened susceptibility to inflammatory tissue injury and inflammation‐driven cancers, as well as with autoimmune and arthritis phenotypes. For example, studies have observed that reduced MBD4 function can exacerbate tissue damage in the colon upon inflammatory challenge, and differential MBD4 expression has been noted in inflammatory disease models."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "16"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nNote: Although the original query mentioned TMEM101, the provided abstracts uniformly address the structure, repair functions, and biological roles of MBD4. Therefore, the summary focuses on MBD4 as derived from the cited literature.\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "B Hendrich, U Hardeland, H H Ng, et al. "}, {"type": "b", "children": [{"type": "t", "text": "The thymine glycosylase MBD4 can bind to the product of deamination at methylated CpG sites."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Nature (1999)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1038/45843"}], "href": "https://doi.org/10.1038/45843"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "10499592"}], "href": "https://pubmed.ncbi.nlm.nih.gov/10499592"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Catherine B Millar, Jacky Guy, Owen J Sansom, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Enhanced CpG mutability and tumorigenesis in MBD4-deficient mice."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Science (2002)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1126/science.1073354"}], "href": "https://doi.org/10.1126/science.1073354"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "12130785"}], "href": "https://pubmed.ncbi.nlm.nih.gov/12130785"}]}, {"type": "r", "ref": 3, "children": [{"type": "t", "text": "Edmund Wong, Kan Yang, Mari Kuraguchi, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Mbd4 inactivation increases Cright-arrowT transition mutations and promotes gastrointestinal tumor formation."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Proc Natl Acad Sci U S A (2002)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1073/pnas.232579299"}], "href": "https://doi.org/10.1073/pnas.232579299"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "12417741"}], "href": "https://pubmed.ncbi.nlm.nih.gov/12417741"}]}, {"type": "r", "ref": 4, "children": [{"type": "t", "text": "Peiying Wu, Chen Qiu, Anjum Sohail, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Mismatch repair in methylated DNA. Structure and activity of the mismatch-specific thymine glycosylase domain of methyl-CpG-binding protein MBD4."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Biol Chem (2003)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1074/jbc.M210884200"}], "href": "https://doi.org/10.1074/jbc.M210884200"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "12456671"}], "href": "https://pubmed.ncbi.nlm.nih.gov/12456671"}]}, {"type": "r", "ref": 5, "children": [{"type": "t", "text": "Owen J Sansom, Stefan M Bishop, Adrian Bird, et al. "}, {"type": "b", "children": [{"type": "t", "text": "MBD4 deficiency does not increase mutation or accelerate tumorigenesis in mice lacking MMR."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Oncogene (2004)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1038/sj.onc.1207767"}], "href": "https://doi.org/10.1038/sj.onc.1207767"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "15184874"}], "href": "https://pubmed.ncbi.nlm.nih.gov/15184874"}]}, {"type": "r", "ref": 6, "children": [{"type": "t", "text": "Michael J Boland, Judith K Christman "}, {"type": "b", "children": [{"type": "t", "text": "Characterization of Dnmt3b:thymine-DNA glycosylase interaction and stimulation of thymine glycosylase-mediated repair by DNA methyltransferase(s) and RNA."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Mol Biol (2008)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.jmb.2008.02.049"}], "href": "https://doi.org/10.1016/j.jmb.2008.02.049"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "18452947"}], "href": "https://pubmed.ncbi.nlm.nih.gov/18452947"}]}, {"type": "r", "ref": 7, "children": [{"type": "t", "text": "Junji Otani, Kyohei Arita, Tsuyoshi Kato, et al. 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