263 PATERNAL ISODISOMY FOR CHROMOSOME 2 AS CAUSE OF BILE SALT EXPORT PUMP (BSEP) DEFICIENCY

Abstract

Background: Bile salt export pump (BSEP) deficiency, a defect in a canalicular bile acid excretion, falls into the progressive familial intrahepatic cholestasis. It is an autosomal recessive disorder with several mutations being reported in the ABCB11 gene on chromosome 2q24−31. We describe a case of BSEP deficiency with a new mutation, where the patient received both mutated alleles from the father, through the rare phenomenon of uniparental disomy (UPD). Methods: A 4-year-old infant, underwent to orthotopic liver transplantation (OLT) at the age of 9 months because of persistent cholestasis and liver failure. Immunohistochemistry was performed with BSEP antibody. Genomic DNA was extracted from the baby and his parents. Molecular studies of ABCB11 were performed using denaturing high-pressure liquid chromatography (DHPLC) and direct sequencing. Gene copy number determination, segregation of the mutation and reconstruction of the flanking haplotype employed quantitative real time PCR (qPCR), single nucleotide polymorphisms (SNPs) genotyping, and polymorphic microsatellites. Results: Liver histology showed cirrhosis. Immunohistochemistal stain using anti-BSEP antibodies revealed wholly absent marking at canalicular margin of hepatocytes, consistent with the diagnosis of BSEP deficiency. DHPLC and sequencing analysis showed a novel, homozygous c.2620C>T transition in exon 20 of the ABCB11 gene. The mutation replaces arginine for cysteine at residue 833 (p. R833C). This change was absent in a large panel of normal chromosomes, and it was heterozygous in peripheral blood from the father, but not in several tissues obtained from the child’s mother. qPCR analyses did not disclose abnormal gene copy number in both the patient and his mother ruling out a microdeletion of ABCB11. Segregation analysis of seven intragenic SNPs and six flanking microsatellite markers showed data suggestive of UID of the region on chromosome 2q24−31 inherited from the father. Conclusion: In severe forms, infants with BSEP deficiency developed cholestasis and fibrosis, and end-stage liver disease requires OLT. Molecular studies are needed to offer measures as to avoid recurrence of the disease. The present case highlights the importance of full gene analyses to offer a more accurate genetic counselling.