Maternal and fetal genetic predispositions to insulin deficiency and resistance affect fetal growth through distinct pathways

We aimed to investigate whether maternal and fetal genetic predispositions to insulin deficiency and resistance affect offspring fetal growth through distinct pathways in multi-ethnic populations.

In 5065 multi-ethnic mother–infant pairs, we examined the conditional associations of maternal and fetal partitioned polygenic risk scores (pPRSs) for type 2 diabetes-related pathways with fetal growth outcomes, including birthweight, sum of skinfold thicknesses (SSF), large-for-gestational-age (LGA) births and small-for-gestational-age (SGA) births. Two-sample Mendelian randomisation (2SMR) in Europeans was performed for triangulation. Exposures were eight type 2 diabetes-related pathways (n=1,812,017), eight beta cell function indices (n=26,356) and two insulin sensitivity indices (n=53,657). Outcomes were maternal and fetal genetically determined birthweight (n=406,063). Mediation analysis was used to assess the mediation effects of maternal glucose levels and BMI on maternal genetic effects and of cord blood C-peptide on fetal genetic effects. Co-localisation analyses were performed to test for shared causal variants.

Fetal type 2 diabetes polygenic risk score (PRS) and pPRSs for lipodystrophy-related insulin resistance and impaired fasting glucose (IFG)-related insulin deficiency were associated with lower birthweight and SSF, while maternal type 2 diabetes PRS and pPRSs for IFG-related insulin deficiency and obesity-related insulin resistance were associated with higher offspring birthweight, SSF and LGA. These associations were consistent across five ethnic groups. Maternal post-load hyperglycaemia mediated 44.2% and 34.2% of the effects of type 2 diabetes PRS and IFG pPRS, respectively, while maternal BMI mediated 43.4% of the effect of Obesity pPRS. 2SMR found consistent results in Europeans and further revealed that fetal insulin sensitivity index and corrected insulin response were associated with higher birthweight. Some loci with shared causal variants acted through multiple pathways, including CDKAL1, TCF7L2, ADCY5 and MACF1.

Reduced fetal growth may be driven by lipodystrophy-related insulin resistance and IFG-related insulin deficiency pathways. Targeting pregnant women with high type 2 diabetes PRS/pPRS and prescribing interventions to reduce their post-load hyperglycaemia and BMI may help reduce offspring risk of LGA.

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