Naturally occurring horse model of miscarriage reveals temporal relationship between chromosomal aberration type and point of lethality

Abstract

Chromosomal abnormalities are a common cause of human miscarriage but rarely reported in any other species. As a result, there are currently inadequate animal models available to study this condition. Horses present one potential model since mares receive intense gynecological care. This allowed us to investigate the prevalence of chromosomal copy number aberrations in 256 products of conception (POC) in a naturally occurring model of pregnancy loss (PL). Triploidy (three haploid sets of chromosomes) was the most common aberration, found in 42% of POCs following PL over the embryonic period. Over the same period, trisomies and monosomies were identified in 11.6% of POCs and subchromosomal aberrations in 4.2%. Whole and subchromosomal aberrations involved 17 autosomes, with chromosomes 3, 4, and 20 having the highest number of aberrations. Triploid fetuses had clear gross developmental anomalies of the brain. Collectively, data demonstrate that alterations in chromosome number contribute to PL similarly in women and mares, with triploidy the dominant ploidy type over the key period of organogenesis. These findings, along with highly conserved synteny between human and horse chromosomes, similar gestation lengths, and the shared single greatest risk for PL being advancing maternal age, provide strong evidence for the first animal model to truly recapitulate many key features of human miscarriage arising due to chromosomal aberrations, with shared benefits for humans and equids.

Keywords: chromosome; embryo; genetics; miscarriage; pregnancy.

Fig. 1.

High prevalence of triploidy and aneuploidy in equine POC. (A) Methodology to obtain POC (created in Biorender). (B) Proportion of equine POC with chromosomal deletions and duplications >10 Mb following pregnancy loss prior to 55 d (n = 95), between 56 and 110 d (n = 14), and 111 d to term (n = 147). Actual numbers shown in columns. (C) Number of aberrations by type and week of early gestation. (D) Number of aberrations by equine autosome with synteny with human chromosomes indicated. (E) Median stallion (Left) and mare (Right) age by aberration type. (F) BAF and log R ratio (LogRR) plots for subchromosomal aberrations in day 42 POC (chromosomes 10 and 28, left), day 26 POC (chromosome 3, middle), and day 44 (chromosome 12, right). Red line indicates location of the centromere. (G) BAF, logRR, and coverage on WGS for day 46 POC with monosomy 26 (Top) and day 62 POC with trisomy 20 (Bottom). (H) Matched allantochorion (ALC) and fetal DNA plots from day 63 POC with duplication of the p arm of chromosome 6 and day 66 POC with trisomy 25 (Right).

Fig. 2.

Triploid conceptuses are triallelic in both the fetal and placental compartments and most commonly include a fetus with developmental anomalies. (A) BAF and log R ratio (LogRR) plots for a XXY and XXX complex triploid POC (Left) and ratio of XXX, XXY, XYY triploid POC (Right). (B) Allele depth patterns using whole-genome sequencing show a unimodal pattern centered around 0.5 indicating a diploid heterozygote, and bimodal around 0.33 and 0.66 indicating a triploid heterozygote. (C) STR profiling of allantochorion (ALC) and fetus from a Day 45 triploid POC that is triallelic at STR loci ASB2, HMS1, and CA425. Day 37 POC that is biallelic at these loci and diploid on CMA. Triploids can present as anembryonic (image not shown), with fetal remnants (D) or embryonic (E). Anomalies of triploid conceptuses include pathology of the brain and cardiovascular systems reported elsewhere (15), as well as amniotic band syndrome (E).