Objective
We conducted a genomewide linkage analysis to identify pelvic organ prolapse (POP) predisposition genes using a resource of high-risk POP pedigrees.
Study Design
Cases are defined as women who reported bothersome symptoms of POP based on standardized symptom questions (Pelvic Floor Distress Inventory, moderately or quite bothered), and/or received treatment for POP documented in medical records. Our complete pedigree resource contains 299 familial POP cases in 83 high-risk pedigrees. Genotype data were obtained from Illumina HumanHap550, 610Q, the Human1M-Duo, Human Omni1-Quad, or the Human Omni 2.5 platforms. A set of single nucleotide polymorphism markers common to all platforms was identified and markers in high linkage disequilibrium were removed. We performed a genomewide linkage analysis under general dominant and recessive models using a Markov chain, Monte Carlo linkage analysis method implemented in MCLINK (University of Utah) software. Because 70 individuals in 32 pedigrees were used in a previously published linkage analysis for a phenotype of POP requiring treatment/surgery, we also performed linkage only including the 225 newly recruited and genotyped cases in 61 pedigrees.
Results
Linkage analysis using our complete pedigree resource for the loosened criteria of bothersome POP showed evidence for significant genomewide linkage on chromosome 10q24-26 (recessive model, maximum heterogeneity logarithm of odds 3.4); suggestive evidence was identified on chromosomes 6 and 17, and an additional region on chromosome 10. In the subset of only the newly recruited familial POP cases, significant evidence for genomewide linkage was observed on chromosome 17q25 (recessive model, maximum heterogeneity logarithm of odds 3.3), and suggestive evidence for linkage was observed on chromosomes 10 and 11. Neither analysis duplicated the previously published linkage evidence for the POP requiring treatment/surgery phenotype observed on chromosome 9.
Conclusion
While the etiology of this common condition is unknown, this study provides evidence that loci on chromosomes 10q and 17q may contribute to POP etiology.
Pelvic organ prolapse (POP) is a common condition in adult women age >50 years; the estimated lifetime risk of surgery for POP is 12.6% by the age of 80 years. Over 200,000 surgeries are performed annually in the United States for POP, exceeding $1 billion in direct costs ; many more women manage their symptoms without surgery.
Established environmental risk factors for POP include age, race, body mass index, family history of POP, chronic intraabdominal pressure, and childbirth factors such as the number of full-term pregnancies, infant birthweight, and forceps delivery. However, the underlying molecular mechanism remains unknown. It is well recognized that POP clusters in families, giving credence to a hereditary contribution to the phenotype. Genetic contributions to POP are under investigation.
To better understand the genetics of POP, we have identified and recruited women with a family history of POP to be part of a genetic study of high-risk pedigrees. We previously performed an initial linkage analysis of this resource including primarily sibling pairs, both of whom had been surgically treated for POP. Significant linkage evidence for POP was identified on chromosome 9q21. This same resource was analyzed for a genomewide association study of POP; 6 single nucleotide polymorphisms (SNPs) significantly associated with POP were identified. We have continued to expand this Utah genetics resource and recruit women with symptoms and a diagnosis of POP in high-risk POP families, regardless of their surgical treatment status.
The purpose of this study was to assess evidence of genetic linkage for a POP phenotype that included bothersome symptoms of POP in Utah high-risk POP pedigrees. In contrast to our prior linkage publication where subjects were required to have had surgery to treat their POP, the POP case definition used for this analysis included women with bothersome symptoms of POP and/or women who had been treated for POP. This expansion of the POP case definition allows inclusion of younger women in pedigrees who are symptomatic but have not yet been surgically treated for POP, which may increase power to detect additional evidence for linkage. Because we previously published linkage evidence on a subset of the complete resource analyzed here, we repeated the linkage analysis using only cases and pedigrees not included in the original publication.
Materials and Methods
Subjects
We initially recruited women who underwent surgical repair of POP on the urogynecology service at the University of Utah from 1996 through 2008 and who had at least one sister who also received surgical treatment for POP and was willing to participate in our study. Documentation of surgical management was required for women who had surgery outside the University of Utah system. Our original published linkage analysis included 70 women surgically treated with POP, most of whom were part of sibships. Since that publication we have expanded our resource in a number of different ways. First, using the Utah Population Database (UPDB), a population-based computerized genealogical resource that has been linked to electronic medical records at the University of Utah, extended relationships between some previously recruited sibpairs were recognized and these sibships were combined into larger, extended pedigrees that have increased power to detect regions of chromosomal sharing. As siblings tend to share on average half their genomes, it can be difficult to differentiate chance sharing from excess sharing due to a shared disease phenotype. Larger pedigrees have the potential to identify chromosomal sharing between distant relatives. We have also identified additional related POP cases in some of the original pedigrees by using POP International Classification of Diseases, Ninth Revision diagnosis codes and Current Procedural Terminology procedure codes in the electronic medical records; all diagnoses have been verified through examination of the medical record, and eligible subjects have been invited to participate in the study. Finally, we have identified new high-risk pedigrees with a significant excess number of POP cases ( P < .05) among descendants of founders in the UPDB. For the new high-risk pedigrees, the number of observed POP cases among descendants within a pedigree was compared to expected numbers of POP cases calculated within UPDB cohorts based on a 5-year birth year, sex, and birth place (in or out of Utah). Recruitment efforts have been targeted to expand the high-risk pedigree resource with a primary focus of recruiting women with symptoms, diagnosis, or treatment of POP who also have a family history of POP.
Study subjects have completed a number of questionnaires including the standardized Pelvic Floor Distress Inventory. We defined POP cases for this study as a self-report of bothersome symptoms of POP on the Pelvic Floor Distress Inventory (moderately or quite bothered) and/or treatment for POP documented in medical records. Eligible families for the linkage analysis had to have at least 2 female relatives meeting the POP case definition. This study was approved by the University of Utah institutional review board, and informed consent was obtained from all study participants prior to participation in the study.
Genotype data
DNA was extracted from all eligible subjects and genomewide genotyping was performed using the Illumina HumanHap550, 610Q, the Human1M-Duo, Human Omni1-Quad, or the Human Omni 2.5 platforms. We identified a set of SNP markers common to all platforms and used this as the genomewide marker set. To avoid inflation of linkage statistics due to linkage disequilibrium and false-positive results, the marker set was pruned by eliminating SNPs in high linkage disequilibrium. The 25,436 markers selected had a minimum spacing of 0.1 cM, a minimum heterozygosity of 0.3, and a maximum [max] r 2 of 0.16 over a sliding 500,000 base pair window in the publically available HapMap CEPH/Utah data, and exceeded an individual call rate of 98% for genotyped subjects.
Linkage analysis
Linkage analysis was performed using a multipoint Markov chain, Monte Carlo linkage method implemented in MCLINK (University of Utah). MCLINK is capable of analyzing extended pedigrees with multilocus markers and was used previously for linkage analysis of POP. MCLINK calculates multipoint logarithm of odds (LOD) scores (ie, theta LOD [TLOD] and heterogeneity LOD [HLOD] ), which are robust to model misspecification. We performed an analysis of only those affected using general dominant and recessive models. Phenotypes of all nonaffected individuals were considered unknown for the analysis; some of the women were too young to be affected and males, although they cannot express the phenotype, may be carriers of a predisposition gene(s). We assumed a disease allele frequency of 0.001 for the dominant model and 0.01 for the recessive model. The penetrance estimates for carriers and noncarriers were 0.5 and 0.0005, respectively. We have used general dominant and recessive models for the linkage analysis; general models will be less powerful than the true underlying genetic model, but they will not have an increase in type I error rates. We report HLOD scores for all pedigrees combined as they allow multiple heterogeneous loci to contribute to a complex disease trait, like POP. Linkage significance was defined using the Lander and Kruglyak genomewide criteria, which assumes a dense genotype map. Suggestive linkage evidence was defined by HLOD scores ≥1.86 and significant genomewide linkage evidence was defined by HLOD scores ≥3.30. For regions where significant genomewide linkage evidence was observed, we report the number of pedigrees that obtained nominal pointwise significance (ie, pedigree-specific TLOD >0.5) on a by-pedigree basis at that specific locus.
Results
The complete Utah high-risk pedigree resource contains 299 familial cases eligible for linkage analysis; this includes 225 newly recruited subjects who fit pedigree criteria (ie, at least 2 related women who meet POP case definition), 70 subjects who are mostly sibpairs used in the previous linkage analysis, and 4 additional, newly recruited subjects who are part of previously recruited sibship pedigrees but do not constitute a pedigree on their own. The complete POP pedigree resource contains 83 pedigrees that range in size from 2-22 genotyped and affected cases; 19 of the pedigrees contain ≥5 cases ( Table 1 ). The majority of all Utah POP pedigree subjects (78.6%) have been treated for POP ( Table 2 ).
| Families | Linkage analysis using newly recruited subjects | Linkage analysis using complete resource a |
|---|---|---|
| 2 affected females | 26 | 37 |
| 3 affected females | 16 | 21 |
| 4 affected females | 3 | 6 |
| 5 affected females | 7 | 10 |
| >5 affected females | 9 | 9 |
| Total | 61 | 83 |
a Includes 225 newly recruited subjects, 70 individuals analyzed previously for linkage evidence, and 4 additional, newly recruited individuals who are part of existing sibship pedigrees, but do not constitute pedigree on their own.
| Pelvic floor disorder | Linkage analysis using newly recruited subjects, n = 225 (POP cases) | Linkage analysis using complete dataset, n = 299 (POP cases) |
|---|---|---|
| POP | ||
| Bothersome symptoms | 62 (27.6%) | 64 (21.4%) |
| Primary treatment | 116 (51.6%) | 167 (55.9%) |
| Recurrent treatment | 47 (20.9%) | 68 (22.7%) |
| Total | 225 (100%) | 299 (100%) |
| Stress urinary incontinence | ||
| Bothersome symptoms | 43 (19.1%) | 53 (17.7%) |
| Primary treatment | 85 (37.8%) | 121 (40.5%) |
| Recurrent treatment | 14 (6.2%) | 23 (7.7%) |
| Total | 142 (63.1%) | 197 (65.9%) |
| Urge urinary incontinence | ||
| Bothersome symptoms | 49 (21.8%) | 68 (22.7%) |
| Treatment with anticholinergic agents | 43 (19.1%) | 65 (21.7%) |
| Treatment with onabotulinumtoxin A or neuromodulation | 3 (1.3%) | 3 (1.0%) |
| Total | 95 (42.2%) | 136 (45.5%) |
| Hernia | ||
| Bothersome symptoms | 10 (4.4%) | 10 (3.3%) |
| Surgical treatment | 19 (8.4%) | 23 (7.7%) |
| Recurrent treatment | 3 (1.3%) | 3 (1.0%) |
| Total | 32 (14.2%) | 36 (12.0%) |
The subset of newly recruited POP cases not used in the previously published linkage analysis includes 225 affected subjects in 61 pedigrees. The pedigrees range in size from 2–19 genotyped and affected individuals; 16 of the pedigrees had ≥5 genotyped cases ( Table 1 ). The majority of newly recruited subjects (72.4%) had received treatment for POP ( Table 2 ).
Table 2 summarizes the presence and severity of other pelvic floor disorders among affected POP cases including stress urinary incontinence (SUI), urge urinary incontinence (UUI), and hernia. The majority of POP subjects had >2 pelvic floor disorders; 63.1% of newly recruited subjects and 65.9% of subjects in the complete pedigree resource also had bothersome symptoms of SUI or had been treated for SUI. UUI was slightly less common among familial POP cases; 42.2% of newly recruited subjects and 45.5% of subjects in the complete resource had bothersome symptoms or had received treatment for UUI. Approximately 10% of POP cases had hernias.
Linkage results for the complete Utah POP pedigree resource are displayed in Figure 1 . Significant genomewide linkage evidence was observed on chromosome 10q24-26 under a recessive model (max HLOD 3.4). Thirty of the 83 total pedigrees (36.1%) attained nominal linkage evidence (TLOD >0.5) in this region. Two of the pedigrees had TLOD scores that individually exceeded 2.0 in this region. Suggestive evidence was observed on chromosome 6q21-22 (max HLOD 2.0, dominant model), 10q21 (max HLOD 2.0, dominant model), 10q23-25 (max HLOD 2.1, dominant model), 17q24 (max HLOD 2.0, recessive model), and 17q25 (max HLOD 2.0, recessive model).
