MYO5B Deficiency-Associated Cholestasis and the Role of the Bile Salt Export Pump

Variants of the MYO5B gene, which encodes the molecular motor protein myosin-Vb, have gained prominence as a causative factor in familial intrahepatic cholestasis (FIC). Understanding the disease mechanism is pivotal for therapy development and clinical decision-making. The prevailing theory for the mechanism underlying MYO5B-associated cholestasis implicates faulty trafficking of the ABCB11-encoded bile salt export pump (BSEP) in hepatocytes due to dysfunctional myosin-Vb. This is supported by cell and mouse studies. However, while BSEP localization was abnormal in some patients' liver biopsies, BSEP appeared normally localized in others, raising questions with regard to the role of BSEP in MYO5B-associated FIC. We present a focused systematic narrative review of all cases of MYO5B variant-associated isolated FIC reported in the MEDLINE database. We assembled a comprehensive patient dataset and assessed clinical features of MYO5B-associated FIC, their relationship with MYO5B genotype, the clinical value and significance of BSEP abnormalities, and the relationship of MYO5B-associated FIC to ABCB11 variant-associated FIC. Our review revealed that aberrant BSEP localization correlated with the absence of one MYO5B allele carrying a truncating nonsense or frameshift variant. Notably, biochemical and clinical parameters including treatment outcome were indistinguishable between patients presenting with normal and aberrant BSEP localization. Further, myosin-Vb and BSEP deficiency-associated FIC patient cohorts showed distinct biochemical and clinical phenotypes, indicating different underlying mechanisms. This suggests that whether or not BSEP localization was abnormal depended on the MYO5B genotype without a predictable effect on clinical parameters and treatment response. Treatment decisions should be guided by clinical parameters rather than by genotype or immunohistochemistry findings.