Women who had gestational diabetes mellitus had an increased risk for later type 2 diabetes if they carried certain genetic risk factors for the disease, according to a new analysis in BMJ Open Diabetes Research & Care of data from two independent populations.
A higher genetic risk score (GRS) had a modest association with developing type 2 diabetes, but a healthier diet may mitigate this risk, as Mengying Li, PhD, and her colleagues found for participants in the Nurses’ Health Study and members of the Danish National Birth Cohort who developed gestational diabetes mellitus (GDM).
Of 1,884 white women with a history of GDM in the Nurses’ Health Study II (NHSII), 446 (23.7%) went on to develop type 2 diabetes, and of the 550 women who had GDM in the Danish National Birth Cohort (DNBC), 155 (28.2%) developed the disease. The researchers calculated a GRS for type 2 diabetes for the full cohort. Genome-wide association studies completed in European populations were used to identify 59 single-nucleotide polymorphisms (SNPs) associated with the disease.
Dr. Li, an epidemiologist and postdoctoral researcher at the Eunice Kennedy Shriver National Institute of Child Health and Human Development in Bethesda, Md., and her coauthors found that women whose GRS was in the highest quartile had a relative risk of 1.19 for type 2 diabetes. The relative risks for the three lower quartiles were 1.25, 0.97, and 1.00, respectively (P value for trend = .02). For each increase of five risk alleles in the GRS, NHSII participants had a 7% increased risk for type 2 diabetes, and DNBC participants saw a 9% increased risk.
Comparing these findings with other studies looking at genetic risk and type 2 diabetes in the general population, Dr. Li and her coauthors noted that the increase in relative risk for type 2 disease with increase in GRS was actually slightly weaker in the GDM cohort they studied. “The smaller effect size among women with GDM likely reflects an already higher baseline genetic risk for [type 2 diabetes] than the general population, as we have demonstrated,” they explained.
Though 11 individual SNPs had a significant individual association with the risk for type 2 diabetes initially, that association disappeared after correction for a false-discovery rate. Dr. Li and her coinvestigators conducted a sensitivity analysis that included only 42 SNPs that were later definitively associated with type 2 disease and they saw essentially unchanged results.
The researchers also investigated how dietary quality affected the GRS–type 2 diabetes association by dichotomizing self-reported diet quality in both cohorts into healthier diet quality and less healthy diet quality. They found a tighter association between GRS and type 2 diabetes for women with diet quality below the median, whereas women with higher diet quality did not have such a strong association between GRS and type 2 disease. The researchers wrote that there was “suggestive evidence that a healthful diet might mitigate the excessive risk of T2D [type 2 diabetes] related to greater genetic susceptibility, which supports public health efforts of encouraging a healthful diet” for diabetes prevention in this high-risk population.
Patients in the NHSII were followed for a mean 21.3 years, and those in the DNBC were followed for a mean 12.7 years. Mean age at index pregnancy was 30.5 years for the NHSII cohort and 31.7 for the DNBC cohort. In the NHSII cohort, just 8.4% of participants reported smoking before pregnancy, compared with 26.4% of those in the DNBC cohort. The NHSII cohort participants, wrote Dr. Li and her coauthors, “were also less likely to have a family history of diabetes, less likely to smoke, and be leaner than women in the DNBC.”
Dr. Li and her coauthors noted that, “despite being the largest genetic study by far on [type 2 diabetes] among women with GDM, our study may not be sufficiently powered to examine the associations of individual T2D SNPs in relation to the risk of developing T2D.” Another limitation was that for the Danish cohort, information about diet was drawn from a one-time questionnaire administered between 9 and 16 years after the index pregnancy, so full data about dietary quality over time was not available. Also of note is that the study included only white participants, limiting generalizability to women of color. The authors called for expanding this research into more racially diverse populations.
The study was supported by the National Institutes of Health. The authors reported that they had no conflicts of interest.
SOURCE: Li M et al. BMJ Open Diab Res Care. 2020 Feb 13. doi: 10.1136/bmjdrc-2019-000850.