Gastrointestinal dysmotility in mitochondrial neurogastrointestinal encephalomyopathy is caused by mitochondrial DNA depletion

Chronic intestinal pseudo-obstruction is a life-threatening condition of unknown pathogenic mechanisms. Chronic intestinal pseudo-obstruction can be a feature of mitochondrial disorders, such as mitochondrial neurogastrointestinal encephalomyopathy (MNGIE), a rare autosomal-recessive syndrome, resulting from mutations in the thymidine phosphorylase gene. MNGIE patients show elevated circulating levels of thymidine and deoxyuridine, and accumulate somatic mitochondrial DNA (mtDNA) defects. The present study aimed to clarify the molecular basis of chronic intestinal pseudo-obstruction in MNGIE. Using laser capture microdissection, we correlated the histopathological features with mtDNA defects in different tissues from the gastrointestinal wall of five MNGIE and ten control patients. We found mtDNA depletion, mitochondrial proliferation, and smooth cell atrophy in the external layer of the muscularis propria, in the stomach and in the small intestine of MNGIE patients. In controls, the lowest amounts of mtDNA were present at the same sites, as compared with other layers of the gastrointestinal wall. We also observed mitochondrial proliferation and mtDNA depletion in small vessel endothelial and smooth muscle cells. Thus, visceral mitochondrial myopathy likely causes gastrointestinal dysmotility in MNGIE patients. The low baseline abundance of mtDNA molecules may predispose smooth muscle cells of the muscularis propria external layer to the toxic effects of thymidine and deoxyuridine, and exposure to high circulating levels of nucleosides may account for the mtDNA depletion observed in the small vessel wall. --------------------------------------------------------------------------------Chronic intestinal pseudo-obstruction (CIPO) is a highly morbid and often life-threatening condition characterized by marked dysfunction of gut propulsive motility, which results in a clinical picture mimicking mechanical obstruction.1,2,3 Patients with CIPO usually complain of severe symptoms including abdominal pain and distension, early satiety, bloating, and vomiting, as well as constipation and/or diarrhea. CIPO is an important cause of chronic intestinal failure, because affected individuals become unable to maintain normal nutrition and body weight. Concerning etiologies, CIPO may be primary or secondary to a variety of systemic diseases.2,3 Primary CIPO may be due to abnormalities of smooth muscle cells of muscularis propria (ie, visceral myopathy) and/or enteric neuronal supplies of gastrointestinal (GI) wall (ie, visceral neuropathy).3 In addition, abnormalities of the GI pacemaker cells, the interstitial cells of Cajal have been reported.4 CIPO is an increasingly recognized clinical feature of mitochondrial encephalomyopathies.5 This heterogeneous group of genetic disorders is caused by dysfunction of the mitochondrial respiratory chain that usually affects highly energy dependent tissues such as brain and muscle.6 Among mitochondrial encephalomyopathies, one most frequently associated with GI dysmotility and CIPO is mitochondrial neurogastrointestinal encephalomyopathy (MNGIE), an autosomal recessive syndrome due to mutations in the thymidine phosphorylase gene TYMP.7 MNGIE is defined clinically by severe GI dysmotility, cachexia, ptosis, ophthalmoparesis, peripheral neuropathy, white matter changes in brain magnetic resonance imaging, and mitochondrial abnormalities.7 GI dysmotility leads to progressive weight loss and cachexia of MNGIE patients, and diverticulosis of small intestine complicated by inflammation and perforation often causes their death in early adulthood. Biochemical abnormalities in MNGIE include drastically reduced thymidine phosphorylase activity leading to accumulation of thymidine (dThd) and deoxyuridine (dUrd) in blood and tissues.8,9 Toxic levels of dThd and dUrd induce nucleotide pool imbalances that in turn lead to mtDNA abnormalities (point mutations, multiple deletions, and depletion).8,10-12 The pathogenic mechanisms causing GI dysmotility in MNGIE are still unclear. We recently showed atrophy, mitochondrial proliferation, and mtDNA depletion in muscularis propria of small intestine in one patient.13 In the present study, we provide a detailed morphological and molecular investigation of the entire GI tract in five MNGIE patients, hence establishing a link between marked mtDNA depletion and myopathic changes of the external layer of muscularis propria in this syndrome.