MYO5B Gene Mutations: A Not Negligible Cause of Intrahepatic Cholestasis of Infancy with Normal Gamma-glutamyl Transferase Phenotype

Objectives: Progressive Familial Intrahepatic Cholestasis, is an expanding group of autosomal recessive intrahepatic cholestatic disorders. Recently, Next Generation Sequencing allowed identifying new genes responsible for new specific disorders. Two biochemical phenotypes have been identified according to gamma-glutamyltransferase (GGT) activity. Mutations of the myosin 5B gene (MYO5B) are known to cause Microvillus Inclusion Disease. Recently, different mutations in MYO5B gene have been reported in patients with low-GGT cholestasis. Methods: a multicenter retrospective and prospective study was conducted in 32 children with cryptogenic intrahepatic cholestasis. Clinical, biochemical, histological, and treatment data were analyzed in these patients. DNA from peripheral blood was extracted, and all patients were studied by Whole Exome Sequencing followed by Sanger sequencing. Results: six patients out of 32 had mutations in the MYO5B gene. Of these 6 patients, the median ageat disease onset was 0.8 years, and the median length of follow-up was 4.2 years. The most common signs were pruritus, poor growth, hepatomegaly, jaundice, and hypocholic stools. Two patients also showed intestinal involvement. Transaminases and conjugated bilirubin were moderately increased, serum bile acids elevated, and GGT persistently normal. At anti-Myo5B immunostaining, performed in liver biopsy of two patients, coarse granules were evident within the cytoplasm of hepatocytes while BSEP was normally expressed at the canalicular membrane. Six variants in homozygosity or compound heterozygosity in the MYO5B gene were identified, and three of them have never been described before. All nucleotide alterations were located on the myosin motor domain except one missense variant found in the IQ Calmodulin-binding motif. Conclusions: we identified causative mutations in MYO5B in 18.7% of a selected cohort of patients with intrahepatic cholestasis confirming a relevant role for the MYO5B gene in low-GGT cholestasis.