A TMEM16F point mutation causes an absence of canine platelet TMEM16F and ineffective activation and death-induced phospholipid scrambling.

BACKGROUND: TMEM16F is an ion channel and calcium-dependent lipid scramblase that mediates phosphatidylserine (PS) exposure on the plasma membrane. Two disparate disease phenotypes are associated with TMEM16F loss-of-function mutations: a rare bleeding disorder (Scott syndrome) and skeletal malformations due to aberrant bone mineralization in a TMEM16F knockout mouse. We therefore undertook comparative studies of TMEM16F expression in canine Scott syndrome (CSS), an autosomal recessive platelet defect. OBJECTIVES: To define anoctamin proteins and scramblase response of CSS platelets and to determine whether TMEM16F is the CSS disease gene. METHODS: CSS TMEM16F cDNA and gene were sequenced and mutation detection was performed in CSS pedigrees. Platelet fractions from CSS dogs were isolated for proteomic and immunologic characterization of TMEM16F. Annexin V was used as a flow cytometric marker of induced platelet PS externalization. RESULTS: A TMEM16F splice site mutation segregated with the CSS trait and TMEM16F protein was undetectable in CSS platelet membranes; however, a second anoctamin, TMEM16K, was found. Proteomic analyses revealed a network of 32 proteins that differentially cosegregated with platelet plasma membrane TMEM16F. CSS platelets had profoundly impaired scramblase response to pharmacologic and physiologic agents that increase intraplatelet calcium and conditions that induce apoptotic and necrotic cell death. CONCLUSIONS: CSS platelets represent a TMEM16F-null mutant model that demonstrates a central role for TMEM16F in mediating platelet PS externalization in response to activating and death signals. Platelet TMEM16F may prove a novel drug target for modulating platelet procoagulant activity and extending platelet life span.

Abnormal gallbladder nuclear ejection fraction predicts success of cholecystectomy in patients with biliary dyskinesia.

The management of patients with symptoms consistent with biliary tract disease who do not have gallstones is difficult. We retrospectively reviewed the charts of 18 patients who underwent cholecystokinin cholescintigraphy at our institution to determine if this procedure was reliable in identifying patients who would benefit from cholecystectomy. All patients underwent biliary screening, and a gallbladder ejection fraction of less than or equal to 35% was considered abnormal. None of the patients had evidence of gallstones by ultrasound. There were 11 patients with abnormal ejection fractions. All 11 patients (100%) had "classic" biliary colic and underwent cholecystectomy. The pathologic diagnosis was chronic cholecystitis in every patient. All patients had complete relief of their symptoms postoperatively with a mean follow-up of 10 months. There were six patients with normal ejection fractions. Only one patient in this group had "classic" biliary colic. This patient had a gallbladder ejection fraction of 38% and endoscopic evidence of gastritis. This patient remains symptomatic despite H2 blockade. The remaining five patients had nonspecific right upper quadrant or epigastric pain. These patients had endoscopic evidence of gastritis, and symptoms were relieved with H2 blockade. The remaining patient had an indeterminate scan due to radioactivity in the duodenum overlying the gallbladder and was excluded from this analysis. Cholecystokinin cholescintigraphy is a useful test in identifying those patients with biliary dyskinesia or acalculous cholecystitis who will benefit from cholecystectomy.