CHAPTER VI
CONCLUSIONS AND FUTURE DIRECTIONS
Figure 6-1. Hypothesized genetic mechanism for the regulation of cervical cytokine concentrations. Genetic associations with cytokine concentrations differs between EA (left panel) and AA (right panel). Genes and cytokines are represented as follows: blue – anti- inflammatory, pink – pro-inflammatory and green – downstream signaling molecules. Dashed lines indicate that there is an indirect connection between the two molecules, meaning that other factors lead to the inhibition or stimulation of the proceeding factor. Solid lines indicate a direct connection between the two factors, meaning that the factor shown is necessary and sufficient to stimulate the proceeding factor.
Secondly, the associations with cervical cytokine concentrations are involved with different signaling pathways between AA and EA (Figure 6-1). IL-6R, IL-10RA and TLR4, all associated with cervical cytokine concentrations in EA but not AA, act through the JAK/STAT signaling pathway to either inhibit (IL-10RA) or stimulate pro-
inflammatory cytokine production (IL-6 and TLR4). IL-1RAP and TNFR2, both associated with cervical cytokine concentrations in AA but not EA, signal through the MAPK/NF-kB signaling pathway to induce production and release of pro-inflammatory cytokines. This may be another plausible mechanism for the ethnic disparity observed in the prevalence of BV. Separate downstream signaling molecules may be contributing to
not only the differences in genetic associations with cervical cytokines but also differences in the correlation structure observed between EA and AA.
Lastly, the genetic associations with cervical cytokine concentration involve the cytokine receptor genes, not the genes themselves. Most studies of the cervical milieu focus on cytokine genes, not other cofactors and receptors. This may be an indication of why many genetic association studies examining vaginal and cervical cytokines fail to detect associations or fail to replicate. Genetic associations with cervical cytokine
concentrations appear to involve a complex mechanism of regulation that may contribute to the pathogenesis and possibly the progression of BV.
Genetic Associations with PTB
Four main pathways have been hypothesized to lead to PTB: 1) activation of maternal or fetal hypothalamic pituitary-adrenal axis, 2) inflammation/infection, 3) decidual hemorrhage, and 4) uterine distension (Lockwood and Kuczynski, 2001). These pathways converge on a final terminal pathway where uterotonins, proteases and
prostaglandins are released, which lead to contractions, rupture of the membranes, and eventually, PTB. Our studies have identified genes associated with PTB that are involved in the previously mentioned pathways (Figure 6-2).
Figure 6-2. Hypothesized mechanisms for PTB. Major pathways are highlighted in pink, secondary pathways in blue and terminal pathways in grey. Genes with significant (p < 5x10-3) associations from the MoBa study or significant pooled (MoBa and Cenn) p- values (p < 5x10-3) are listed above the secondary pathway. Genes in red text indicate maternal associations, blue text indicates fetal associations and green text indicates maternal and fetal associations. Genes with a red asterisk indicate maternal replication, blue asterisk indicates fetal replication.
Several studies have identified SNPs in the inflammation/infection pathway that are associated with PTB; however, these studies often fail to replicate (Velez et al., 2007;
Velez et al., 2008b). In this study we identified many SNPs in inflammation/infection related genes such as IL-1R2, IL-1RAP and IL-6R that consistently associated with PTB (Figure 6-2). The majority of these associations are in maternal samples only, indicating the importance of the maternal genome on infection related genetic associations with PTB. Studies in part B of chapter IV demonstrated that in a separate EA population, a SNP in IL-6R was significantly associated with cervical IL-6 concentrations. In an AA population SNPs in IL-1RAP were associated with IL-1β concentrations. Both of these associations were affected by the presence of BV. The SNPs associated with BV and
studied. Nevertheless, it is possible that there is a pleiotropic effect for these genes in influencing two related reproductive disorders. Further studies will need to be conducted to determine if one or multiple SNPs in these genes are affecting these disorders.
Another important pathway that is consistently associated with PTB is the decidual hemorrhage pathway that activates ECM degradation. Collagen genes play a central role in ECM degradation, and several SNPs from many of these genes were significant in maternal (COL1A1 and COL5A1) and fetal (COL1A2 and COL3A1) pooled samples with allelic or genotypic p < 5x10-3
Collagen plays an important role in all stages of cervical function during pregnancy (Word et al., 2007). During the dilation and ripening phases, collagen concentration decreases with increased collagen dispersal, solubility and degradation (Word et al., 2007). Collagen, in particular types I and types III, plays vital roles in the changes in tissue consistency associated with cervical ripening (Word et al., 2007).
Perturbations in these collagen genes could cause an increased weakening of the
membranes surrounding the fetus and result in rupture of the membranes, thereby leading to PTB. It is evident from significant results in two separate studies, that contributions from both the maternal and fetal genomes are involved in the associations of collagen genes with PTB.
. This suggests that collagen genes are particularly important in the pathogenesis of PTB. Additionally, collagen genes are critical in the uterine distension pathway, suggesting that through collagen genes multiple pathways are activated that lead to PTB.
This study identified multiple genes with significant or marginally significant associations with PTB in two separate studies. When these samples were combined,
multiple SNPs with strong allelic or genotypic p-values were identified, including SNPs involved in inflammation and infection, ECM degradation and decidual hemorrhage. This study has validated previously identified genetic associations with PTB and further studies can address the biological role these genes play in PTB.