Although much is known about the growth and differentiation of the reproductive tract, the molecular pathways involved in the development and maintenance of the USL are not well understood. The present study provides insight on the role of HOXA11 in the embryological development of the USLs, the key supporting ligaments of the uterus and vagina. To our knowledge, this is the first gene identified as necessary for USL development in any species.
The pathogenesis leading to attenuation of the USL in patients with POP is unclear. Cadaveric studies have shown that USLs are incredibly strong structures for their size, capable of supporting more than 17 kg of weight before failure (22
). However, the resiliency of USLs in women with POP has previously been shown to be reduced by more than 4-fold compared with women with normal support, suggesting intrinsic deficiencies in the quality of the USLs in these women (23
The mechanical strength of the supportive pelvic structures is dependent on the quantity and quality of the collagen fibers. Tissue remodeling is a continuous process, and a delicate balance between matrix synthesis and breakdown is necessary to maintain the integrity of the tissue. Degradation of collagens I and III is dependent on the activity of the interstitial MMPs, which ultimately receive input from MMP2 and MMP9 (24
Previous studies have examined the relationship regarding the quality and quantity of collagen in USLs in women with POP. Several immunohistochemical studies have shown a higher percentage of immunohistochemically positive areas along with increased density of collagen and decreased cellularity in patients with POP (10
). These findings are in contrast to our results which showed decreased procollagen in our subjects with POP. This may be explained by the findings of Suzme et al. and Lang et al., who describe increased diameter of collagen fibrils in patients with POP and decreased hydroxyproline, which is responsible for the tightly bound crosslink between fibrils (14
). These findings, coupled with our present results, suggest that fewer fibers of increased diameter size are present in USLs of patients with POP.
The decrease in collagen expression may also reflect the end result of sustained increases in MMP2 activity. We show here that proMMP2 mRNA increased 2-fold in our subjects with POP. This is consistent with findings from other authors who reported that pro- and activated MMP2 proteins are significantly increased in USLs of women with POP (12
). MMP2 is capable of digesting fibrillar collagen to completion and is a potent activator of MMP9 (24
). Our present results suggest that MMP2 plays a central role in collagen reorganization and may be a critical factor in USL attenuation associated with POP.
We found significant decreases in HOXA11 and collagen expression, accompanied by increased expression of MMP2, in the USLs of subjects with POP. Our in vitro data clarified this association and offer insight into what we believe to be a previously unknown pathway of ECM metabolism in these support structures. Expression of Hoxa11 increased procollagen type III expression and decreased MMP2 expression in murine fibroblasts, favoring collagen synthesis over catabolism.
Due to the cross-sectional design of this study, it is not possible to determine whether women with POP have weakened pelvic connective tissue as a result of changes in a signaling pathway involving HOXA11 regulation of collagen type III and MMP2 or if these findings are the result of POP. However, our in vivo findings suggest that POP is associated with decreased HOXA11 and collagen type III expression and increased MMP2 expression, and our in vitro findings elucidate the regulation of collagen type III and MMP2 by HOXA11.
These observations extend our understanding of molecular mechanisms that regulate the maintenance of a critical ligament of the pelvic support system, the USL. Deficient HOXA11 signaling may limit functional development or repair of the USL after trauma in susceptible women and contribute to alterations in the biomechanical strength of the USL, leading to uterovaginal prolapse. Additionally, alterations in HOX genes would be expected to produce a variety of defects in the expression of downstream target genes. Understanding both the regulation of HOXA11 and its regulation of downstream signaling pathways involved in the homeostasis of the USL may offer new therapeutic strategies for women with POP and improve the quality of life for those affected by this condition.