A 34-year-old woman with mucolipidosis type IV predisposing to a neuroendocrine tumor in the gastric fundus

Abstract Introduction/Objective Mucolipidosis type IV (MLIV) is a lysosomal storage disorder with a gastric phenotype. Patients develop achlorhydia and compensatory hypergastrinemia, with pathology showing vacuolated parietal cells with lysosomal inclusions. MLIV is caused by mutation of TRPML1, a lysosomal membrane calcium channel required for trafficking proton pumps to the apical surface in parietal cells. We report the first case of MLIV predisposing to a neuroendocrine tumor (NET). This novel finding could impact management of patients with this disease, who may require frequent monitoring for NETs. We also show that immunohistochemistry for TRPML1 and H+/K+ ATPase can be used in conjunction to identify gastric MLIV. We hope that our updated immunohistochemical tools can aid in diagnosis of this disease. Methods/Case Report 34-year-old woman with MLIV requiring endoscopic resection for an incidentally discovered gastric polyp. Pathology showed Grade 2 NET invasion into submucosa (pT1). Background mucosa showed markedly vacuolated parietal cells, ranging from unilocular to multilocular with up to 20 vacuoles per cell. We stained the patient’s and a control stomach with antibodies against the TRPML1 and H+/K+ ATPase proteins. The H+/K+ ATPase antibody highlighted healthy parietal cells in the control stomach with strong cytoplasmic signal, but strikingly outlined lysosomal inclusions in MLIV parietal cells. TRPML-1 antibody showed cytoplasmic staining in parietal cells in the control. However, TRPML-1 staining showed markedly decreased signal in the patient and nuclear staining, possibly representing mislocalization. Results (if a Case Study enter NA) NA Conclusion We present the first case of MLIV predisposing to a NET in the stomach. NETs have potential for metastatic spread with subsequent mortality risk. Clinicians should maintain suspicion for NET development in this population, and protocols for frequent surveillance should be considered. This is also the first report using immunohistochemistry for TRPML1 and H+/K+ ATPase in combination to identify affected parietal cells in MLIV. Possibly, these tools can be used in the future by pathologists to diagnose MLIV.