UCL EGA Institute for Women's Health, London WC1E 6AU, UK.
German Cancer Research Center (DKFZ), 69120 Heidelberg, Germany.
Hum Reprod Update. 2020 Nov 1;26(6):841-873. doi: 10.1093/humupd/dmaa025.
Studies in non-human mammals suggest that environmental factors can influence spermatozoal DNA methylation, and some research suggests that spermatozoal DNA methylation is also implicated in conditions such as subfertility and imprinting disorders in the offspring. Together with an increased availability of cost-effective methods of interrogating DNA methylation, this premise has led to an increasing number of studies investigating the DNA methylation landscape of human spermatozoa. However, how the human spermatozoal DNA methylome is influenced by environmental factors is still unclear, as is the role of human spermatozoal DNA methylation in subfertility and in influencing offspring health.
The aim of this systematic review was to critically appraise the quality of the current body of literature on DNA methylation in human spermatozoa, summarize current knowledge and generate recommendations for future research.
A comprehensive literature search of the PubMed, Web of Science and Cochrane Library databases was conducted using the search terms 'semen' OR 'sperm' AND 'DNA methylation'. Publications from 1 January 2003 to 2 March 2020 that studied human sperm and were written in English were included. Studies that used sperm DNA methylation to develop methodologies or forensically identify semen were excluded, as were reviews, commentaries, meta-analyses or editorial texts. The Grading of Recommendations, Assessment, Development and Evaluations (GRADE) criteria were used to objectively evaluate quality of evidence in each included publication.
The search identified 446 records, of which 135 were included in the systematic review. These 135 studies were divided into three groups according to area of research; 56 studies investigated the influence of spermatozoal DNA methylation on male fertility and abnormal semen parameters, 20 studies investigated spermatozoal DNA methylation in pregnancy outcomes including offspring health and 59 studies assessed the influence of environmental factors on spermatozoal DNA methylation. Findings from studies that scored as 'high' and 'moderate' quality of evidence according to GRADE criteria were summarized. We found that male subfertility and abnormal semen parameters, in particular oligozoospermia, appear to be associated with abnormal spermatozoal DNA methylation of imprinted regions. However, no specific DNA methylation signature of either subfertility or abnormal semen parameters has been convincingly replicated in genome-scale, unbiased analyses. Furthermore, although findings require independent replication, current evidence suggests that the spermatozoal DNA methylome is influenced by cigarette smoking, advanced age and environmental pollutants. Importantly however, from a clinical point of view, there is no convincing evidence that changes in spermatozoal DNA methylation influence pregnancy outcomes or offspring health.
Although it appears that the human sperm DNA methylome can be influenced by certain environmental and physiological traits, no findings have been robustly replicated between studies. We have generated a set of recommendations that would enhance the reliability and robustness of findings of future analyses of the human sperm methylome. Such studies will likely require multicentre collaborations to reach appropriate sample sizes, and should incorporate phenotype data in more complex statistical models.
非人类哺乳动物的研究表明，环境因素会影响精子 DNA 甲基化，一些研究表明，精子 DNA 甲基化也与生育能力低下和后代印迹障碍等情况有关。随着更经济有效的 DNA 甲基化检测方法的出现，越来越多的研究开始调查人类精子的 DNA 甲基化图谱。然而，环境因素如何影响人类精子的 DNA 甲基化组，以及人类精子 DNA 甲基化在生育能力低下和影响后代健康方面的作用仍不清楚。
本系统评价的目的是批判性地评估当前关于人类精子 DNA 甲基化的文献质量，总结现有知识，并为未来的研究提出建议。
使用“semen”或“sperm”和“DNA methylation”等搜索词，对 PubMed、Web of Science 和 Cochrane Library 数据库进行全面文献检索。纳入了 2003 年 1 月 1 日至 2020 年 3 月 2 日发表的研究人类精子且用英文撰写的文献。排除了使用精子 DNA 甲基化来开发方法或法医鉴定精液的研究，以及综述、评论、荟萃分析或社论文章。使用推荐、评估、发展和评估（GRADE）标准客观评估每个纳入文献的证据质量。
检索共确定了 446 条记录，其中 135 条被纳入系统评价。这 135 项研究根据研究领域分为三组；56 项研究调查了精子 DNA 甲基化对男性生育能力和异常精液参数的影响，20 项研究调查了精子 DNA 甲基化与妊娠结局（包括后代健康）的关系，59 项研究评估了环境因素对精子 DNA 甲基化的影响。根据 GRADE 标准评为“高”和“中”质量证据的研究结果进行了总结。我们发现，男性生育能力低下和异常精液参数，特别是少精症，似乎与印迹区域的精子 DNA 甲基化异常有关。然而，在全基因组、无偏分析中，尚未令人信服地复制出特定的精子生育能力低下或异常精液参数的 DNA 甲基化特征。此外，尽管研究结果需要独立重复，但目前的证据表明，吸烟、年龄增长和环境污染物会影响精子的 DNA 甲基化。然而，从临床角度来看，没有令人信服的证据表明精子 DNA 甲基化的变化会影响妊娠结局或后代健康。
尽管人类精子 DNA 甲基组似乎可以受到某些环境和生理特征的影响，但研究之间没有发现任何一致的结果。我们提出了一系列建议，这些建议将增强未来对人类精子甲基组分析的可靠性和稳健性。这样的研究可能需要多中心合作来达到适当的样本量，并且应该在更复杂的统计模型中纳入表型数据。
