Impact of Weight Loss on the Human Sperm Epitranscriptome

1. Background and present state of the art in the research field. Sperm non-coding RNA is an important vector for epigenetic inheritance of diet-induced obesity Obesity is a multifactorial pathology associated with a high heritability (50-75%). To date, multiple Genome Wide Association Studies (GWAS) have attempted and failed to identify the genetic factors that contribute to the etiopathogenesis of obesity. In fact, the variance explained by a high Body Mass Index (BMI) at these GWAS mutations is very low, estimated to be approximately 2%. Recent epidemiological and experimental studies strongly suggest that this high heritability of obesity might be associated with the transmission of obesity-induced epigenetic modifications.

Non-genetic inheritance of a newly acquired phenotype is a concept in biology whereby changes induced by a specific signal and/or environmental stress can be passed to the next generation in the absence of a genetic change. The biological relevance of this process is proven by its diversification in various organisms including plants, insects, nematodes, and mammals. However, evidence is now emerging that a wide range of dietary factors including fats and proteins lead to changes in gene expression that are maintained through mitotic and meiotic divisions. These changes can significantly impact the health of the offspring.

Very recently, the investigator has gained some insight into the molecular mechanisms involved in this process, in particular those that pertain to the epigenetic inheritance of newly acquired paternal pathologies. However, much remains to be uncovered about the functional and molecular characterization underlying the mode of action of the specific factors responsible.

Changes in DNA methylation, chromatin modification and expression levels of non-coding small RNA, including miRNA, piRNA and transfer RNA fragments occur in germ cells upon changes in environmental cues. However, it is not clear whether these changes would mediate epigenetic inheritance. While several studies have indicated that an altered DNA methylation signature of spermatozoa from HFD-obese individuals could be transmitted to progeny, a recent study indicated that the sperm methylome is shaped by genetic and epigenetic variations, but not diet, thus disproving the idea that DNA methylation is the vector of epigenetic inheritance in this HFD model.

Based on this first evidence showing that small non-coding RNAs (sncRNAs) act as trans-generational vectors of epigenetic information in mice, the investigative team and others extended this discovery by deep investigation of the role of sperm small RNAs as determinants of the inheritance in an acquired metabolic disorder. In this regard, experimental approaches include the micro-injection of one-cell embryos with well-defined sperm RNAs from individuals fed by high-fat (HFD) or control diet (CD). They found that mice derived from the microinjection of HFD sperm RNAs into naive one-cell embryos developed adult onset diet-induced pathologies, such as obesity or signs of type 2 diabetes even though they had been fed a control diet. Deep-sequencing analysis of small RNAs of testis from HFD-fed animals and control revealed that several small-RNAs are indeed deregulated upon HFD. Among those small RNAs, we found miRNAs, piRNAs and tRNA-derived small RNAs (tsRNA). Importantly, microinjection into naive one-cell embryos of one of the deregulated microRNAs - namely microRNA-19b, induced metabolic alterations that were similar to the diet-induced phenotypes. Likewise, other groups found that tsRNA may also contribute to intergenerational inheritance of metabolic disorders. Indeed, Chen et al. showed that micro-injecting sperm tsRNAs from HFD-fed individuals into naive one-cell embryos caused impaired glucose tolerance in the resulting offspring. However, in this model, synthetic microRNAs or synthetic tsRNAs did not induce metabolic disorders in the offspring, suggesting a role of post-transcriptional RNA modifications in the transfer of epigenetic information. This hypothesis is consistent with the elevated levels of m5C and m2G modifications of tsRNAs in sperm from the HFD group. Hence, sperm small RNAs represent a type of paternal epigenetic vector involved in intergenerational inheritance of diet-induced metabolic phenotypes.