We tested the association between GWG and tissue specific GPGE using five relevant maternal tissues and the fetal placenta. Many nominally significant associations demonstrated strong biologic plausibility. In maternal analyses, genes involved in immune system were among the top associations for early GWG; while top associations for late GWG included genes with function in cell aggregation, adhesion, and proliferation. There was also significant enrichment of several biologic pathways, such as metabolic processes, secretion, and intracellular transport, among nominally significant genes from the maternal analyses. The enriched biological pathways varied by pregnancy stage (e.g., early vs late GWG). In fetal analyses, placental expression of a gene encoding a pregnancy-specific protein was the leading association with total GWG. In addition to the top genes’ involvement in relevant biological processes, several of these genes have been previously tied to relevant phenotypes, including BMI and waist to hip ratio. After correcting for multiple testing, however, we did not find statistically significant associations between maternal and fetal tissue-specific gene expression and early, late, or total of GWG.
Previous research has found a moderate genetic contribution to weight gain. The only GWAS for GWG to date, from which summary statistics for our study were obtained, concluded that roughly 20% of the variability in GWG can be explained by common maternal variants, with variants from the fetal genome contributing a much smaller amount12. Other genetic studies, investigating only candidate variants or genes have also been performed. Previous research has found associations with variants tied to obesity and diabetes, including SNPs in KCNQ1, PPARG, and GNB312–19. A recent study found risk variants in TMEM18 and GNB3 are more frequent in females with increased GWG, a variant in GNPDA2 was more frequent in those with adequate GWG, and a variant in LEPR was more common in individuals with decreased GWG17. Unfortunately, there are currently no gene expression prediction models for KCNQ1 and PPARG in the maternal tissues. GNB3, TMEM18, GNPDA2, and LEPR was not significant in any tissue in the maternal analyses. KCNQ1, PPARG, GNB3, GNPDA2, and LEPR were also not significant in any fetal analyses. TMEM18 was nominally significant in only the early GWG fetal analysis (β = 0.04, p-value = 0.03).
The Warrington et al. study found a single variant in offspring genome that was significantly associated with total gestational weight gain: PSG512. Interestingly, the top nominally significant association in the fetal placenta analysis for total gestational weight gain was another pregnancy-specific beta-1 glycoprotein: PSG9. PSGs are produced by placental trophoblasts and secreted into the maternal bloodstream in increasing amounts throughout pregnancy20. The function of these proteins is not fully known, but they have several hypothesized roles, including modulation of the maternal immune system to avoid rejection of the fetus. Previous studies reveal an association between these proteins and preeclampsia and fetal growth restriction21,22. Our findings are consistent with previous work on PSGs and fetal growth restriction indicating that increased expression of PSG9 is associated with decreased total GWG.
Overexpression of JAZF1, the top nominally significant association with early GWG in the placenta, has been shown to reduce body weight gain and regulate lipid metabolism in mice models23. In contrast, our results demonstrate increased placental expression was associated with increased GWG in early pregnancy. JAZF1 has been previously implicated in GWAS for type 2 diabetes, body fat distribution, and body mass index24–27. The top suggestive associations in early (POU5F1-whole blood), late (CADM1-breast), and total (TLCD2-subcutaneous adipose) GWG using maternal genotypes have also been tied to related traits, including BMI, BMI-adjusted hip circumference, type 2 diabetes, and waist to hip ratio26,28–32.
We observed additional associations that had strong biological plausibility in our analyses stratified by time of the GWG measurement. For example, there were many maternal immune system genes (HCG27, HLA-C, C2, etc.) nominally associated with early GWG. There is a strong biological basis for this association, as the maternal immune system must adapt early in pregnancy to ensure the fetus is not rejected. In the GWAS Catalog, previous associations for these genes notably include obesity traits like waist to hip ratio, in addition to other immune and blood traits and autoimmune disorders such as lupus and psoriasis. POU5F1, which plays a key role in embryonic development and stem cell pluripotency, had the most significant association with early GWG in maternal analyses. In contrast, expression of genes involved in cell aggregation, adhesion, and proliferation, as well as splicing and membrane rigidity (CADM2, ZNF300P1, TLCD2, GEMIN7) were the leading associations with late GWG in maternal analyses. In the fetal placenta, genetically predicted expression of genes involved in transcription and cell proliferation (JAZF1, ALX4, TLC1A) were linked to early GWG. Enriched biologic pathways and functions also differed based on period of GWG, though purine nucleotide binding was enriched in both early and late GWG of maternal analyses and fetal analyses did not display any enrichment in processes or functions.
There are several considerations related to the data utilized in this study. First, our threshold for statistical significance accounts for the number of gene-tissue pairs tested. This threshold could be overly stringent as it does not take into account the correlation structures between coordinated expression both within tissues and for the same genes across tissues. Significant results reported in this analysis could be overly conservative and not detect additional true effects that did not reach this threshold. GTEx and the corresponding tissue-specific prediction models for maternal analyses utilize samples of non-diseased tissues from both female and male donors with a variety of races and ages. Investigation using sex-specific models and/or reproductive females only could yield additional insight. Sample sizes varied based on tissue and ranged from 70 to over 400, which could also impact our power to detect associations. We utilized publicly available data from a GWAS of over 10,000 females and 7,000 offspring12. All study participants were of European ancestry. Future research should include more diverse populations. Additionally, GPGE models are only available for the fetal portion of the placenta. Creation of models using the maternal side of the placenta would be beneficial for direct comparison.
In conclusion, many of the genes whose expression was nominally significant possess functions that are likely relevant to weight gain in pregnancy. Associations varied by time of weight gain, broadly mirroring the events during pregnancy. For example, immune genes were top associations in early GWG, a time when maternal immune systems must quickly adapt to ensure the fetus is not rejected. Several of these genes have also been previously implicated in GWAS of related traits. These results further support that diverse biological pathways impact GWG, inferring that their influence is likely to vary based on individual and tissue, as well as over the course of pregnancy.