Assessing the Clinical Significance of the MCM6 c.-14011C/T Polymorphism in Lactose Intolerance: Insights from a Case Series

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Lactose intolerance, lactase nonpersistence, or simply adult-type hypolactasia is a common trait affecting approximately 70% of the world population. The condition is autosomal recessive and associated with an ancestral C in position c.-13910 in the regulatory minichromosome maintenance protein 6 (MCM6) gene. Several rare polymorphisms exist, and some are known to modulate the lactose intolerance phenotype, whereas others have a yet unknown function, including the MCM6 c.-14011C/T (rs4988233) polymorphism studied here (1). Two index cases present gastrointestinal symptoms on ingesting milk products. Case 1 is a 34-year-old woman of Filipino ancestry, and case 5 is a 52-year-old woman of Northern African ancestry. The cases’ general practitioners requested genetic testing, and both cases had the classical lactose intolerance genotype, MCM6 c.-13910CC (rs4988235), and were heterozygous for the c.-14011C/T polymorphism. To assess the function of the c.-14011C/T polymorphism, the relatives were invited for genetic and phenotypic testing and to fill out a symptom questionnaire (Table 1). Results from genetic testing, phenotype testing, and symptom questionnaire. aInterview: + indicates the presence and − indicates the absence of a symptom. bTime-point for measurement of the most significant blood-glucose increase. Each case was instructed to fast for 8 hours and avoid physical activity for 2 hours before examination in a hospital setting. First, a venous blood sample for genetic testing was drawn. Genetic testing was performed using a protocol for melting curve analysis on the Roche Cobas 4800 module z480 allowing for identification of the MCM6 c.-13907C/G, c.-13908C/T, c.-13909C/A, c.-13910 C/T, c.-13913T/C, c.-13914G/A, c.-13915T/G, c.-14009T/G, c.-14010G/C, c.-14010G/A, and c.-14011C/T polymorphisms (2). Second, the baseline blood-glucose level was determined. Following oral administration of a 50 g lactose solution, the blood-glucose levels were monitored at 15, 30, and 60 minutes using the Roche Accu-Chek Inform II. The most significant blood-glucose increase was registered, and an increase >1.4 mmol/L suggested lactase persistence according to the laboratory protocol adhering to the national guideline. None of the cases seemed to suffer from gastroparesis as maximum glucose levels occurred within 15–30 minutes (Table 1). During the procedure, the cases were interviewed about symptom presentation. After the lactose tolerance test, the cases answered a questionnaire rating the presence of 5 relevant symptoms on a scale from 1 (absent) to 10 (severe) (3). The index cases 1 and 5 and case 2 all had a rise in blood glucose <1.4 mmol/L during the phenotype testing, and they reported symptoms compatible with lactose intolerance in the interview and questionnaire. These results indicate that the MCM6 c.-14011CT genotype does not influence the lactose intolerance phenotype caused by the MCM6 c.-13910CC genotype. Neither case 3 nor 6 reported any severe symptoms, and their rise in blood glucose was >1.4 mmol/L, which agrees with lactose tolerance and the presence of the MCM6 c.-13910CT genotype, known to cause lactase persistence (1). Case 4 also had a rise in blood glucose >1.4 mmol/L consistent with the MCM6 c.-13910CT genotype but reported symptoms of lactose intolerance in the questionnaire. However, diarrhea and abdominal cramping were not reported during the interview. Cases diagnosed with lactose intolerance received dietary advice and information about lactase substitution. All participants provided written informed consent to publish anonymized results at inclusion. The study followed national legislation on quality assurance projects, adhered to good clinical practice, and was approved by the Danish Data Protection Agency, Region of Southern Denmark (24/511). Two prior studies reported the presence of the c.-14011C/T polymorphism in conjunction with the c.-13910CC genotype. A case from Uruguay experienced at least one symptom associated with dairy intake (4). Two cases from Ethiopia underwent a hydrogen breath test and were classified as lactose digester and nondigester, respectively (5). These data broadly support this study, and the single conflicting observation might be caused by hydrogen nonexcretion, which is a common problem in patients examined with the hydrogen breath test. Since 2020, Lillebaelt Hospital has examined patients with the MCM6 c.-13910CC genotype for 10 rare MCM6 polymorphisms using melting curve analyses. In 3 years, 2477 patients with the c.-13910CC genotype were identified, and 12 patients harbored a heterozygous c.-14011C/T polymorphism (0.5%). This makes the c.-14011C/T polymorphism almost as frequent as the neighboring c.-14010G/C (rs145946881) polymorphism, which was found in 14 patients (0.6%) and is known to be associated with lactase persistence (1). The c.-14011C/T polymorphism appears to be widespread as this study found it in Danish individuals descending from the Philippines and Northern Africa, and other studies found it in Uruguayan and Ethiopian individuals. Global mobility increases the occurrence of rare MCM6 polymorphisms in our hospital facility, and knowledge of the polymorphisms’ impact on lactose intolerance is therefore warranted. By elucidating the significance of the rare MCM6 polymorphisms with unclear functionality, the interpretation of genetic testing results can be improved in the future. Author Contributions: The corresponding author takes full responsibility that all authors on this publication have met the following required criteria of eligibility for authorship: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; (c) final approval of the published article; and (d) agreement to be accountable for all aspects of the article thus ensuring that questions related to the accuracy or integrity of any part of the article are appropriately investigated and resolved. Nobody who qualifies for authorship has been omitted from the list. Stine Bruun (Data curation-Equal, Investigation-Equal, Project administration-Equal, Resources-Equal, Writing—original draft-Lead, Writing—review & editing-Equal), Jonna Madsen (Conceptualization-Equal, Funding acquisition-Lead, Methodology-Equal, Resources-Lead, Supervision-Equal, Writing—review & editing-Equal), and Pernille Bøttger (Conceptualization-Equal, Data curation-Equal, Formal analysis-Lead, Investigation-Equal, Methodology-Equal, Resources-Equal, Software-Lead, Supervision-Equal, Validation-Lead, Writing—review & editing-Equal) Authors’ Disclosures or Potential Conflicts of Interest: Upon manuscript submission, all authors completed the author disclosure form. Research Funding: This work was supported by the Region of Southern Denmark. Disclosures: None declared. Acknowledgments: We thank the laboratory team at the Department of Biochemistry and Immunology, Lillebaelt Hospital, University Hospital of Southern Denmark, for undertaking the practical collection and analysis of patient samples.