Thirty-one women whose first birth was SPTB (< 37 weeks) or who experienced multiple SPTBs (< 37 weeks) in seven multiplex pedigrees selected from UPDB for early SPTB were included in this study. All pedigrees contained at least three SPTB cases. Four of the seven pedigrees had ≥ 15 meioses between SPTB cases and were included in single-pedigree SGS analyses (Fig. 1, Panel A). Three pedigrees had fewer meiosis and were used in secondary analyses to identify regions shared in two pedigrees, using duo-pedigree SGS analysis (Fig. 1, Panel B).
In single-pedigree SGS analysis, we identified one genome-wide significant SGS region (P-value = 9.99×10− 8; µ = 0.028) at chromosome 8 (8q24.23) (Table 1, Fig. 2). The genome-wide significant 8q24.23 SGS region is a 1.75 Mb segment of 406 contiguous SNPs (chr8: 137,013,447–138,759,742 GRCh37 bp) shared by five out of six women with SPTB in Pedigree 1 (Fig. 1, Panel Ai). The 1.75 Mb region was inherited through 24 meioses to five women with SPTB. Sixteen single-pedigree SGS genome-wide suggestive regions were also identified across the four pedigrees analyzed (Table 1).
In duo-pedigree analysis, we required the joint P-value to be genome-wide significant, µ(duo)\(\:\le\:\)0.05, and each pedigree to have single-pedigree evidence at µ\(\:\le\:\)5.0 to avoid finding results that are simply driven by one of the two pedigrees. With these criteria only two regions reached genome-wide significance in duo-pedigree SGS analysis (Table 2). The same 8q24.23 region from single-pedigree SGS results was also identified as genome-wide significant in duo-pedigree SGS when Pedigree 1 was paired with Pedigree 2, yielding an order of magnitude smaller expected by chance rate than in Pedigree 1 alone, µ(duo)=0.0019. The overlapping region is 1.05 Mb at 8q24.23 (137,485,507–138,537,206 bp) and shared by 5 of 6 early SPTB cases in Pedigree 1 and 3 of 4 early SPTB cases in Pedigree 2. Figure 3A shows characteristics of the shared region, which contains two genes, FAM135B and KHDRBS3. A second genome-wide significant duo-pedigree SGS region was identified at 12q21.1-q21.2 for Pedigree 3 paired with Pedigree 1. This region was previously identified as suggestive in single-pedigree SGS analysis for Pedigree 3 (µ = 0.68; Table 1), and gained significance in the joint analysis, µ(duo) = 0.01. The very small 67,291 bp genome-wide significant segment (75,525,950–75,593,241 bp) is shared by all 3 cases in Pedigree 3 and all 6 cases in Pedigree 1. Figure 3B shows the region containing the intersecting shared segment and genes CAPS2 and KCNC2.
Comment
Principal findings
We identified two genome-wide significant SGS regions at 8q24.23 and 12q21.1-q21.2 using SGS analyses in large, multiplex SPTB pedigrees, providing evidence for segregating risk variants from a common founder. The 8q24.23 is a 1.75 Mb segment on chromosome 8 (contains FAM135B and KHDRBS3 genes), shared by 8 women with SPTB in two pedigrees. The 12q21.1-q21.2 a 67 kb segment on chromosome 12 (contains CAPS2 and KCNC2 genes), shared by 9 women in two pedigrees.
Results in the context of what is known in existing literature
Maternally inherited genes acting in the fetus are largely thought to be responsible for birth timing,20, 21 but only a few GWAS loci in candidate maternal genes (e.g., ZBTB38, WNT4, EBF1, HAND2, TET3, and KCNAB1) for SPTB have been identified5, 6 and replicated in recent GWAS meta-analyses.22 Preliminary investigations suggest rare, deleterious genetic variants could be enriched in both mothers and the preterm birth infant,23 but efforts to characterize rare inherited variants are lacking. Family-based studies can identify segregating regions and have the advantage of localizing regions allowing specific regulatory regions to be considered. To date, only one family-based linkage study of SPTB has been conducted.24 The study suggested novel fetal SPTB susceptibility genes, which included common SNPs near the IGF1R gene in 15q26.3 and 8q24.3, the latter in proximity to the genomic segment reported here. Previously there have not been any studies in large, multiplex SPTB pedigrees as we present here. Well-characterized large pedigrees are more likely to be homogeneous and genetically enriched, and thus provide the opportunity to discover segregating shared regions that harbor genetic variants for disease.12
In the present study, we used the UPDB to identify large pedigrees containing multiple women with early SPTB. We ascertained and studied the genetics of women within these pedigrees who experienced SPTB in their first or multiple pregnancies to identify maternally heritable effects on gestational age and on SPTB.15
Research and clinical implications
The genome-wide significant 8q24.23 segment localized a 1.05 Mb region containing KHDRBS3 and FAM135B, suggesting shared variants may regulate one of these two target genes. KHDRBS3 is a member of signal transduction and the activation of RNA family and regulates gene expression by RNA splicing.25, 26 Variation in this gene is associated with shorter telomere length or dysregulated telomerase activity, which have previously been associated with pregnancy complications.27 In particular, in a recent study, maternal blood samples from SPTB cases had shorter telomeres, compared with maternal blood samples from uncomplicated pregnancies.28 These findings suggest KHDRBS3 may contribute to SPTB by regulating telomere length and cell senescence. The other candidate target gene, FAM135B, promotes inflammation by inducing granulin (a protein-coding gene located on chromosome 17q21.32) precursor secretion and activating the PI3K/AKT/mTOR signaling pathway.29 Excessive activation of inflammatory molecules within the placenta has been associated with preterm labor.30 However, there were no previous links noted between FAM135B and SPTB.
The 8q24 region contains regulatory elements, including enhancers, that are thought to regulate nearby MYC oncogenes.31 Variants in 8q24 are associated with many cancers, including ovarian cancer.32, 33 Among women diagnosed with gynecologic cancer, the prevalence of preterm birth was higher compared with women without cancer diagnoses.34 Although there were no reported links between MYC proto-oncogenes and SPTB, novel pathways of inflammation, which are regulated by MYC proto-oncogenes,35 are associated with preterm birth and preterm pre-labor rupture of membranes.36 Furthermore, given the possible damage to oocytes and prolonged immunosuppression, further investigation into the 8q24 genomic region may lead to novel biomarker and therapeutic development for preterm birth.
The genome-wide significant 12q21.1-q21.2 segment localized a 67 kb region containing CAPS2 (Calcyphosine 2) and KCNC2 (potassium voltage-gated channel subfamily C member 2), genes which have both been well described in several disease mechanisms.37 CAPS2 is a calcium ion binding gene involved in the secretion of brain-derived neurotrophic factor and neurotrophin-338 and is implicated in autism in mice knock-out experiments.39, 40 Using probabilistic search model for identification of disease-causing variants, we previously identified CAPS2 among over-represented genes in key processes that may lead to SPTB.41 Epigenetic changes in calcium ion genes have also been detected using GO enrichment analyses among African American women with preterm birth history.42 Furthermore, epigenetic changes in CAPS2 in placenta and cord blood, respectively, have been associated with increased maternal blood pressure during pregnancy,43 a risk factor for preterm birth, and physical development in children.39 The potassium channel gene KCNC1 is linked with a group of rare, inherited disorders such as progressive myoclonus epilepsies.44 Potassium channel dysfunction has been associated with vascular-related complications of pregnancy, and interestingly, the ATP-sensitive potassium channel agonist nicorandil is currently being assessed for the prevention of preterm labor.45 During gestation, potassium channels are known to contribute to a normal gestational period and initiating labor contractions at term.46 In concert, these prior findings and our current SGS results suggest that further investigation into CAPS2 and KCNC2 genes for preterm birth therapeutic development is warranted.
Strengths and limitations
Our study has some limitations. We do not have data on non-genetic factors associated with SPTB (e.g., tobacco smoking, nutrition, and socioeconomic status). Therefore, we cannot estimate the interactive effects of genes with environmental risk factors. In addition, our data is based on European-ancestry study participants. Population-based family studies of SPTB with individuals from diverse racial/ethnic backgrounds and studies that account for environmental risk factors may be impactful, informing prevention and intervention strategies. Also, the large pedigree design is powerful for identification of dominantly inherited variants. Other genetic risk mechanisms acting through inherited recessive or de novo variants, as suggested by a previous study,47 cannot be identified or replicated with the SGS approach.23 However, the SGS approach is complementary and provides an additional advantage because genetic heterogeneity is reduced within families.48