Abstract
Background
Mucins are excellent candidates for contributing to the presence of meconium ileus (MI) in cystic fibrosis (CF) due to their extensive genetic variation and known function in intestinal physiology. The length of variants in mucin central repetitive regions has not been explored as “risk” factors for MI in CF.
Methods
We investigated the length polymorphisms in the central repetitive regions of MUC1, MUC2, and MUC5AC by Southern blot and tested for association with MI in CF subjects.
Results
No significant associations were found for the allele sizes of any of the genes with respect to the prevalence of MI (p values = 0.33, 0.16, and 0.71 for MUC1, MUC2, and MUC5AC, respectively).
Conclusions
The genetic length variants in the central repetitive region of three MUC genes studied do not associate with MI in subjects with CF.
Keywords: meconium ileus, genetic modifier, MUC1, MUC2, MUC5AC
1. Introduction
Meconium ileus (MI) occurs in 15-20% of cystic fibrosis (CF) subjects with pancreatic insufficiency, which is defined as an intestinal obstruction with thick meconium in newborns [1, 2]. MI is thought to be under little influence of environmental factors, so it is likely to reflect genetic variation [1, 3-6]. Genetic variants in several genes and genomic loci have been associated with the MI phenotype in different CF populations, including ADIPOR2, MSRA, SLC4A4, SLC6A14, and SLC26A9, and also at loci on chr4q35.1, chr11q25, chr20p11.22, and chr21q22.3 [3-6]. Due to technical limitations, genetic length polymorphisms in repetitive regions and structural variations are not included in panels used for genome wide association studies (GWAS).
Mucins are key components of mucus, which is the front-line host defense system in the gastrointestinal tract. Enterocytes are covered by secreted and transmembrane mucins, such as MUC1, MUC2 and MUC5AC [7-8]. Additionally, MUC1 could modulate the expression of other mucins in the small intestine, as demonstrated in the CF/Muc1 -/- transgenic mice [9]. MUC1 and MUC5AC/Muc5ac have also been reported to play important roles in inflammatory bowel diseases and intestinal nematode infection [8, 10-12]. MUC2/Muc2 are known to contribute to severe colitis and colon cancer [13, 14]. The MUC1, MUC2 and MUC5AC length polymorphisms in the core region, described here as the “central repetitive region” (previously referred to as the “variable number tandem repeat (VNTR) region” [15-17]), have been demonstrated to show association between a specific MUC5AC allele and severity of CF lung disease [17]. Length variation of the central repetitive region may affect the properties and functions of the intestinal mucus, and therefore, we tested to see if these variants influence the susceptibility of MI in a large CF population.
2. Methods
This study was conducted in accordance with the approval of the Institutional Review Board from the University of North Carolina at Chapel Hill. CF subjects with known meconium ileus (MI) status confirmed through source documents at birth, or a compatible history of MI plus evidence of an abdominal scar (if primary records were unobtainable), were included [6]. CF subjects with good quality DNA samples (> 50 kb on average) were selected [17]. The study was limited to Caucasians to minimize population stratification; only Phe508del homozygous subjects were included in the no MI group to eliminate the impact of the CFTR genotype. The main characteristics are summarized in Table 1.
Table 1.
Clinical characteristics of CF patients with and without meconium ileus (MI).
| Gene |
MUC1
|
MUC2
|
MUC5AC
|
|||
|---|---|---|---|---|---|---|
| Phenotype | MI | noMI | MI | no MI | MI | no MI |
|
|
|
|
|
|||
| Subjects, n | 138 | 373 | 161 | 384 | 135 | 339 |
| Age range, yrs | 5.7 - 50.8 | 8.1 - 52.1 | 5.7 - 50.8 | 8.1 - 52.1 | 5.7 - 50.8 | 8.1 - 52.1 |
| Mean age, yrs ± SD | 20.2 ± 8.7 | 23.9 ± 10.0 | 20.6 ± 8.5 | 24.2 ± 10.1 | 20.0 ± 8.4 | 24.1 ± 10.2 |
| Male (%) | 74 (53.6) | 193 (51.7) | 86 (53.4) | 197 (51.3) | 76 (56.3) | 175 (51.6) |
|
CFTR homozygous Phe508del (%) |
117 (84.8) | 373 (100) | 133 (82.6) | 384 (100) | 109 (80.7) | 339 (100) |
To assess the allele sizes in the central repetitive region for MUC1, MUC2, and MUC5AC, Southern blotting of HinfI-digested genomic DNA was performed as described previously [15, 17]. Differences of the allele size distribution in the central repetitive region, between MI and no MI CF groups, was analyzed by the non-parametric Wilcoxon rank-sum test (Intercooled Stata 10; StataCorp, College Station, TX).
3. Results
3.1. Allele distributions and association with MI
MUC1 allele sizes ranged from 2.6 to 8.0 kb. The distribution was bimodal, with two common peaks around 3.6 and 5.6/5.7 kb. There was no significant association between the overall allele size and the MI phenotype (p value = 0.33), nor was there any obvious trend distinguished between the MI and no MI groups (Fig. 1A).
Fig. 1.
Distribution of allele size in the central repetitive region of MUC1, MUC2, and MUC5AC by the MI phenotype. The number of alleles in the MI or no MI groups is given followed by the Wilcoxon rank-sum test p value. The proportion of alleles for MI and no MI were not significant for (A) MUC1 (p value = 0.33), (B) MUC2 (p value = 0.16), nor (C) MUC5AC (p value = 0.71). For easier viewing, the MUC1 alleles 2.6 - 2.9 kb were binned together and shown as a bar < 3.0 and 6.1 - 8.0 kb were binned as > 6.0 (A). Similarly, MUC2 alleles 3.7 - 5.8 kb were binned as < 5.9 and 7.3 - 10.7 kb were grouped as > 7.2 (B). Finally, MUC5AC alleles 5.6 - 6.1 kb were binned as < 6.2 and 7.1 - 8.5 kb were binned as > 7.0 (C).
MUC2 allele sizes ranged from 3.7 to 10.7 kb; however, the majority of alleles were between 5.9 to 7.2 kb, centering at the 6.7 kb allele. Although alleles > 6.7 kb tended to be more common in the MI group, there was no significant difference between the two groups (p value = 0.16) (Fig. 1B).
MUC5AC alleles ranged from 5.6 to 8.5 kb, with a major peak at 6.3/6.4 kb, and a minor one at 6.9 kb. There was no association of the allele distribution between two groups (p value = 0.71) (Fig. 1C).
3.2. Additional MUC2 association analysis
Additional tests were performed for MUC2 to search for any insight into the trend in association. Neither males nor females showed a significant association (p value = 0.18 and 0.48, respectively; data not shown) with MI. For 216 CF subjects with severe lung disease [17], a trend for larger alleles was noted to be more common in the MI group than those in the no MI group (p value = 0.08; data not shown). For the 317 CF subjects with mild lung disease, there was no association (p value= 0.93; data not shown).
Finally, we tested MUC2 association with MI by CFTR genotype, but noted no association for Caucasian homozygous Phe508del CF subjects for MI versus no MI (p value = 0.13; data not shown).
4. Discussion
The central repetitive region of the major intestinal mucins is a key factor in determining the molecular weight and glycosylation status of the mucins [7, 15, 16, 18]; thus, we hypothesized that length polymorphisms may associate with MI in CF subjects. In a CF Phe508del homozygous mouse model, the small intestinal mucus layer is denser and less penetrable, compared to normal mice [19]. MUC2 is the most abundant mucin of the intestinal mucus, and one study using a mouse model showed that Muc2 (via its O-glycosylation) was vital in determining the Salmonella infection status and the function of the intestinal epithelial barrier [13]. MUC1 is up-regulated and poorly glycosylated in colonic tissue from children with inflammatory bowel disease [10]. Several studies have also reported that MUC5AC plays an important role in inflammatory bowel disease [8, 11].
Our work made use of previous protocols [15, 17] that had several key features: 1) relatively large numbers of CF subjects in both MI and no MI groups; 2) CFTR genetic homogeneity of subjects; 3) gender balance for defining length polymorphisms; and 4) well-optimized Southern blot techniques with maximal resolution and minimal gel-to-gel variation [17]. This study did not demonstrate any significant association among the length variants in the central repetitive region of MUC1, MUC2, and MUC5AC with the presence of MI in CF subjects, and our negative MUC2 association is consistent with the ulcerative colitis disease report [20]. Our data demonstrate that the studied genetic variants do not influence the formation of MI directly in CF subjects, even though these variants could possibly affect properties of intestinal mucus. However, we cannot exclude that other variations in these mucin genes, or other mucin genes, could influence the formation of MI in CF.
Highlights.
Central repetitive region allele sizes are highly polymorphic in MUC1/2/5AC. (77)
Length variants of MUC1, MUC2 and MUC5AC central region are not linked to MI in CF. (84)
MUC2 variants gender analysis showed no association with the formation of MI in CF. (84)
Analysis of MUC2 alleles restricted by CFTR genotype showed no linkage to MI. (78)
Acknowledgements
The authors thank Dallas Swallow, Ann Harris, and Maimoona Zariwala for technical support and thoughtful discussion and Clayton Commander, Ethan Lange, Anthony Dang, Yunfei Wang, Michael Patrone, Evan Hawbaker, and John Yeatts for statistical analysis. We are grateful to Sally Wood and Sarah Norris for data collection, and Elizabeth Godwin and Syanne Olson for administrative and editorial assistance. We are indebted to Airong Xu and Hemant Kelkar for bioinformatics support. We acknowledge all CF patients, the efforts of the Gene Modifier Study Group, and all collaborators and participant sites. Supported by grants from the Cystic Fibrosis Foundation: STONE08G0 (J.R.S.); R026-CR02 (W.K.O.); KNOWLES00A0 (M.R.K.); and the National Institutes of Health: CTRC RR00046 and HL68890 (M.R.K.). None of the funding sources were involved in study design, the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication.
Footnotes
Competing Interests: The authors have declared that no competing interests exist.
Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Prior publication disclosure: There are no related manuscripts in press or submitted, although abstracts were presented in 2007 (Guo XL et al. Assessing mucin gene polymorphisms to determine their role as modifiers of different CF phenotypes. Pediatr Pulm 2007; 42(S30):265-266) and in 2008 (Guo XL et al. Assessing mucin gene VNTR polymorphisms as modifiers of lung disease severity and MI in CF. Pediatr Pulm 2008; 43(S31):275) reporting a subset of subjects.
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