Mutations identified in the human multidrug resistance P-glycoprotein 3 (ABCB4) gene in patients with primary hepatolithiasis

Abstract

Primary hepatolithiasis (HL), highly prevalent in the Far East, including Japan, is characterized clinically by chronic proliferative cholangitis with frequent recurrences. In HL patients, hepatic hyposecretion of phospholipid due to decreased multidrug resistance P-glycoprotein 3 (MDR3; now referred to as ABCB4) expression levels (Hepatology 2001;33:1194-1205) may contribute to the formation of aggressive ductular lesions through a decreased formation of mixed micelles. However, specified factors underlying the decreased expression levels of MDR3 have not been well defined. To determine whether the decreased MDR3 expression level is associated with the gene mutations, mutation analysis of cDNA of the MDR3 gene with focus on the coding region was performed using liver specimens. Heterozygous mutations were detected in only two of 16 HL patients. By sequence analysis of the gene, a 77-bp deletion at nucleotides 537-613 in exon 7 in transmembrane domain (TM) 3, which results in a frameshift at codon 179 and an early stop codon predicting a truncated protein, was found as a heterozygous mutation in two of the 16 patients. A 1-bp deletion at nucleotide 1015 in exon 10 in TM 6 was found as a heterozygous mutation in one of those two patients, and a 242-bp deletion at nucleotides 2683-2924 in exons 22-23 in TM 11 was found as a heterozygous mutation in the same patient. No other mutations were found in the other 14 patients. In real-time polymerase chain reaction (PCR), no significant difference was found between the mRNA levels of MDR3 in the two HL patients with mutations nor in the other 14 patients without mutations. Immunostaining of MDR3 protein was found in the bile canaliculi of liver sections from the two patients with mutations. The results suggest that in primary HL the decreased transcription levels of MDR3 in the liver are not due to the mutations detected in the coding region of the gene.