Characterizing Pregnancy Outcomes in a Humanized Mouse Model of Sickle Cell Disease

Background: Sickle cell disease (SCD) is a group of hereditary disorders commonly associated with the distortion of red blood cells (RBC) at low oxygen levels. It is characterized by complications such as vaso-occlusion, anemia, and hemolysis. These complications are lifelong and become extremely relevant during periods of increased physiological stress such as pregnancy.

Pregnancy in sickle cell disease (SCD) is associated with an increased risk of complications including, pre-term delivery, deep vein thrombosis, intra-uterine growth restriction, pre-eclampsia, and intra-uterine death. Recent meta-analyses have shown that women with SCD are at 26x greater risk for maternal mortality than their healthy counterparts; a figure which, despite the adoption of improved care plans, has shown little to no significant improvement over the last two decades. While impaired vascular maintenance underlies multiple complications of pregnancy outside of sickle cell disease, it is especially relevant to, and characteristic of sickle cell-related pathologies at baseline and is highly likely to be exacerbated as pregnancy proceeds. Through this study we aimed to characterize the effects of SCD-related vasculopathy on pregnancy outcomes and overall placental health using the Townes humanized mouse model of SCD.

Methods: SS (sickle hemoglobin) and AA (normal hemoglobin) mice underwent timed breeding to produce pregnant dams. These dams were housed under normal conditions until 17.5 days post conception (dpc), at which point they began undergoing procedures for analysis including 24-hour urine collection, high-resolution ultrasound analysis (18.5dpc), and post-mortem analysis of pregnancy outcomes and tissue collection (18.5dpc). Included in this report are gross pregnancy outcomes, histological assessments of placental tissue, and reports from ultrasound analysis including 3-D volume measures, placental vascularization renders, and laser-Doppler assessment of uterine and umbilical arteries. Continuing research efforts will further utilize tissues from this study to evaluate the biochemical interactions influencing poor pregnancy outcomes related to SCD.

Results: These studies show that the SCD mice recapitulate many similarly poor outcomes as women. We observed: reduced litter sizes (AA 6.9 ± 1.5 embryos vs SS 5.2 ± 1.2 embryos**), fetal weight (AA 0.49 ± 0.14g vs SS 0.38 ± 0.16g*), viability of embryos (AA 100.0% IQR 0.0 vs SS 20.0% IQR 33.3****), and maternal mortality (AA 0.0% vs SS 8.7%, OR = 4.3 ns). In assessing placental histology, we identified evidence of vasculopathy common to end organ damage in SCD such as increased presence of avascular areas, reduced relative labyrinth area (AA 55.6 ± 8.6% vs SS 44.7 ± 6.0%*) and reduced vascular density (AA 38.9 ± 6.2% vs SS 34.4 ± 7.0%*) which are likely responsible for worsening outcomes. Additional pathologies include increased occurrence of placental ectasia, calcification of the labyrinth, vascular thrombi, and perivascular fibrosis when compared to controls. Further, ultrasound analysis revealed several measures of placental insufficiency amongst SS mice that were significantly impaired including uterine blood flow and an overall reduction in placental vascularization and oxygenation.

Conclusions: SCD mice demonstrate many similar pregnancy outcomes as humans, allowing for increased translational application. We've shown these outcomes to be severe and largely driven by vascular impairment specifically within the placenta, the main maternal-fetal interface. This study serves as the groundwork for future studies aimed at investigating the underlying mechanisms of these outcomes while also establishing baseline measures and novel methods of approach towards the broader goal of improving obstetric disparities faced by women with SCD.