LINE-1 Methylation sustains telomere length in pregnant women: effects on pregnancy failure

Pregnancy loss is one of the most common adverse events during the first weeks of gestation, and the incidence increases with maternal age and in presence of selected risk factors. Nonetheless, no risk factors have been identified in most cases, considering these cases unexplained. Fertility rate decreases as maternal age increases and epigenetic age-dependent conditions may favor miscarriage. DNA methylation and telomere length are informative of aging and cell senescence, and their assessment has been evaluated as predictors of successful pregnancy.

Telomere length (TL; T/S) and LINE-1 methylation (LINE-1; %) have been assessed in a cohort of 242 pregnant women by comparing spontaneous early miscarriage (EPL, n = 129) with voluntary interruption (VPI, n = 113). Telomere size and LINE-1 methylation rate drastically decreased as the age of women increased (P < 0.000001) with EPL group having lower values (T/S: 322.6 ± 142.0 versus 455.0 ± 290.6; P < 0.000001 and LINE-1 %: 81.66 ± 4.2 versus 86.01 ± 3.7; P < 0.000001) also characterized by stronger age-dependent lowering compared to VPI (P = 0.00035 and P < 0.000001, respectively). A global improvement in TL was observed as LINE-1 methylation rate increased, and it was more evident in EPL than in VPI (P < 0.000001). Focusing on the area below the 25th percentile of TL and LINE-1 % distribution, an overrepresentation of EPL cases was observed (P < 0.000001). On the contrary, VPI controls were dramatically overrepresented (P < 0.000001) in the area above the respective 75th percentiles. The mutual comparison of the number of EPL and VPI in these two extreme areas (EPL/VPI_(<25th) = 3.12 versus EPL/VPI_(>75th) = 0.32) yielded a significant risk of early pregnancy failure when women carried both risk conditions, low TL and LINE-1 methylation (OR = 9.70, 3.94-23.72; P < 0.0001). The intracase analyses ascribed to recurrent EPL cases even higher risks (OR = 10.5, 3.6-29.5; P < 0.0001) and a risk dosage effect stratification recognized to low methylation highest odds than that of short telomeres (OR = 4.44, 2.45-8.03; P < 0.0001 and OR = 2.26, 1.26-4.04; P = 0.005, respectively).

Overall, this suggests a combined effect of short telomeres and low methylation in phenotype worsening and a significant role of methylation in sustaining telomere size. These data support the hypothesis that different levels of DNA methylation may capture different biological mechanisms underlying telomere dynamics and dysfunctions and chromatin organization. Therefore, the concomitant assessment of telomere, methylation and their mutual interactions may be a novel strategy to translate the classical informative biomarkers of aging in the field of human reproduction.