| The physiological function of mCH remains poorly described in both mouse and human. Based on current profiling efforts, mCH is found in human ESCs (hESCs) [144], induced pluripotent stem cells (iPSCs) [145], brain neurons [146, 147], oocytes [148, 149], PGCs [150, 151], and select organs [152]. Interestingly, there is an unusually high abundance of mCH in the oocyte, enriched at neighboring genomic regions with high-density mCG levels, but its functional significance is presently unclear [148, 153]. In the context of development, mCH is more enriched in oocytes than in sperm cells [154], and both mCG and mCH are lost from the zygote upon fertilization [153]. Interestingly, mCH is highly enriched on repetitive elements, suggesting a potential repressive role that is similar to mCG [155, 156]. Indeed, mCH appears to contribute to retrotransposon silencing during spermatogenesis [157]. DNMT3L, which is catalytically inactive, plays a crucial adapter role in establishing 5mC in germ cells [158]. In a DNMT3L loss-of-function study, a mutation to its ADD domain (responsible for interaction with histone H3) led to the ectopic reactivation of IAPs and LINEs in male germ cells [157]. When compared to controls, the most significant form of 5mC that was lost was in the context of mCH where in total, and only 15% (mCHH) and 11% (mCHG) were retained in the mutant respectively, in contrast to mCG (84%) [157]. The authors further verified that mCH is indeed abundant on both LINEs and IAPs, which is lost in the ADD mutant, suggesting that mCH likely plays a role in suppressing the expression of IAPs and LINEs, or perhaps retrotransposons broadly. In support of this, hESCs also harbor elevated mCH levels at SINE repeats [159]. However, it is important to note that although mCH loss is extensive on both LINEs and IAPs, mCG levels are also partially depleted in the DNMT3L-ADD mutant. Thus, the exact contribution by mCH to retrotransposon repression requires further investigation [157]. Future targeted epigenetic engineering experiments are critical to clarify the function of mCH in TE regulation and its function in development. Additionally, given that mCH is also enriched in cell types lacking DNMT3L, further studies are required to understand how noncanonical methylation is established in those settings. |
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