Homozygous Loss-of-Function Mutations in AP1B1, Encoding Beta-1 Subunit of Adaptor-Related Protein Complex 1, Cause MEDNIK-like Syndrome

Abstract

MEDNIK syndrome (mental retardation, enteropathy, deafness, peripheral neuropathy, ichthyosis, and keratoderma) is an autosomal-recessive disorder caused by bi-allelic mutations in AP1S1, encoding the small σ subunit of the AP-1 complex. Central to the pathogenesis of MEDNIK syndrome is abnormal AP-1-mediated trafficking of copper transporters; this abnormal trafficking results in a hybrid phenotype combining the copper-deficiency-related characteristics of Menkes disease and the copper-toxicity-related characteristics of Wilson disease. We describe three individuals from two unrelated families in whom a MEDNIK-like phenotype segregates with two homozygous null variants in AP1B1, encoding the large β subunit of the AP-1 complex. Similar to individuals with MEDNIK syndrome, the affected individuals we report display abnormal copper metabolism, evidenced by low plasma copper and ceruloplasmin, but lack evidence of copper toxicity in the liver. Functional characterization of fibroblasts derived from affected individuals closely resembles the abnormal ATP7A trafficking described in MEDNIK syndrome both at baseline and in response to copper treatment. Taken together, our results expand the list of inborn errors of copper metabolism.

Keywords: AP-1; ATP7A; Menkes; clathrin coated vesicles (CCV); copper; trafficking.

Figure 1

Identification of Two Families Whose Affected Members Have a MEDNIK-like Phenotype (A) Pedigree of family 1. (B1–B4) Clinical images of family 1_II-2. (C1 and C2) Clinical images of family 1_II-1. Note that the skin and hair phenotype is strikingly similar to that seen in family 2. (D) Pedigree of family 2. (E1–E4) Clinical images of family 2_II-2, showing hairline recession, generalized fine small scales on erythematous background over the body, and plantar keratoderma; these hair and skin features are similar to those seen in family 1.

Figure 2

Identification of Homozygous AP1B1 Loss-of-Function Variants in MEDNIK-like Syndrome (A) Cartoon of AP1B1 showing the deletion that removed exons 1 and 2 and introns 2 and 3 of AP1B1 in family 1. (B) RT-PCR of family 1 showing complete absence of AP1B1 transcript as a result of the deletion. (C) Sequence chromatogram of the RT-PCR product showing the resulting 1 bp deletion at the cDNA level in family 2.

Figure 3

MEDNIK-like Mutation in AP1B1 Results in Abnormal ATP7A Trafficking Dermal fibroblasts were co-stained with antibodies against ATP7A (red) and giantin as a TGN marker (green), as well as DAPI nuclear counterstain (blue). (A) Under the regular culture conditions, the near complete colocalization of ATP7A and giantin in controls is virtually completely abrogated in cells from individuals 2 and 3. (B) The same experiment is repeated with added copper (200 μM), which revealed redistribution of ATP7A away from TGN in controls but no change from baseline in cells from individuals 2 and 3.