Here, we demonstrated that gene variants of SLC2A5 gene encoding the fructose transporter GLUT5 are not generally involved in the pathogenesis of acquired fructose malabsorption. Based on the number of subjects studied (n=35) we cannot exclude a rare role of such gen variants in every individual patient suffering from fructose malabsorption, but the rate of these cases having “pathogenic GLUT5 variants” among all these patients are likely to be < 3-5%.
A major strength of the study is the clinically based definition of the study cohort prior to molecular analysis. Related gastrointestinal disorders such as hereditary lactose-intolerance and acquired lactose malabsorption, both leading to pathological lactose breath test, were excluded. Hereditary ALDOB deficiency was practically ruled out by analyzing the three major mutations of ALDOB leading to this disease. Coeliac disease, also considered as chimera among gastrointestinal diseases, was kept out of the study group by two approaches. First, since secondary lactose malabsorption is a well-known leading symptom for coeliac disease, these patients were excluded by abnormal lactose breath test. Second, for 34 out of the 35 individuals, corresponding serological parameters (IgA/IgG tissue anti-transglutaminase or deamidated anti-gliadin antibodies) were found to be negative.
The symptoms reported by our patients are rather unspecific and in line with those reported in similar studies with patients (I) suffering from fructose malabsorption tested by breath test (29–31), or (II) classified as having irritable bowel disease (IBS) (32, 33). However it is notable that symptoms in our studies were assessed retrospectively only by analyzing patients’ medical records, and no structured interview or assessment of questionnaire in context to e.g. IBS-related Rome criteria (34) was performed.
In summary, the 35 subjects included in the molecular analysis of the SLC2A5/GLUT5 gene presented (I) clinically relevant symptoms that are consistent with acquired fructose-malabsorption, (II) demonstrated abnormal fructose-induced breath test and (III) relevant other related diagnoses (e.g. coeliac disease, hereditary fructose intolerance) were basically excluded. Taken together, we are confident that overall the great majority of these 35 subjects are patients suffering from acquired fructose malabsorption. It has been shown that the individual ability of metabolizing fructose for subjects without any side effects differs widely from 5 – 50 g (reviewed in 21) supporting the multi-factorial etiology of acquired fructose malabsorption.
The aim of the study, molecular analysis of SLC2A5/GLUT5 gene concerning gene variants associated to fructose malabsorption, was based on other studies showing the role of SNPs/mutations affecting the uptake/metabolism of related sugars. Variants including partial deficiency of sucrose- isomaltase were shown to be associated with IBS (35). Analysis of UK biobank data revealed that gene variants in human ketohexokinase gene are associated with loss of function and resulting in the rare benign condition of fructosuria (36). In vitro mutation analysis in rats between GLUT5 and its closest related transporter (GLUT7) revealed that single amino acids (e.g. p.Gln166Glu) are responsible for the specific transport of fructose, and mutation of this residue to p.166Glu results in the uptake of glucose, whereas other variants and chimera between GLUT5 and GLUT7 demonstrated strong reduction or even complete lack of fructose uptake (37).
The fact that the allele frequencies of the 10 gene variants between ours and those reported in database were very similar strongly implies that none of these variants have a relevant role for the clinical manifestation of acquired fructose malabsorption. Notably, 5 of the 10 gene variants were singular findings that do not allow final any general conclusion due to study size. But taken into account the very low frequencies reported in databases, the potential relevance for the very frequent fructose-absorption syndrome seems to be very limited.
While this study demonstrate that SLC2A5/GLUT5-related gene variants do not playing a relevant role in the pathogenesis of acquired fructose malabsorption, other pathogenic factors have been recently identified to be associated with this disorder. Trelis and co-workers identified a frequent association of the disease with the infection of parasites, in particular Giardia intestinalis (38). Several animal studies identified specific changes in the gut microbiome in context to genetic host factors (39) and the intake of fructose (39, 40) showing that Akkermansia spec. seems to play an important role in the prevention fructose-induced metabolic dysregulation. Over-expression of slc2a5 in slc2a5/glut5-knock out mice let to profound increase of fructose utilization and subsequent higher levels of Clostridium and Enterococcus spec. (41). Overall most related studies demonstrate that higher intestinal luminal levels of fructose caused by changes in fructose consumption or absorption will likely affect bacterial load and composition of the microbiome (reviewed in 42).
In humans, several studies highlighted the role of the transcription factor ChREBP encoded by the MLXIPL gene for the predisposition concerning fructose intolerance malabsorption (21, 43) and diarrhea- predominant IBS patients with impaired intestinal fructose transport (44). The association between ChREBP and fructose malabsorption was further supported by animal models (45, 46). Nuclear receptor LXR (lxralpha, NR1H3) is another transcriptional regulator of GLUT5 expression identified in mice and human that is thought to be a potential pharmaceutical target for selective modulation of GLUT5 expression in context to cancer and metabolic disease (47). Notably, authors identified a functional LXR responsive element in the human SLC2A5/GLUT5 promoter region located at position -385 based on transcriptional start site, but none of our 35 patients showed a variation at this position.
Overall, these different findings strengthen the hypothesis that fructose-related malabsorption syndrome associated with different pathological conditions has multi-factorial etiology. Different transcriptional regulatory patterns affecting the SLC2A5/GLUT5 gene expression contribute to the pathology, whereas gene variants of SLC2A5/GLUT5 including the promoter region, which was the focus of this study, do not play a relevant role.