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.

Citation
Alsaif, H. S., Al-Owain, M., Barrios-Llerena, M. E., Gosadi, G., binamer, Y., Devadason, D., … Alkuraya, F. S. (2019). Homozygous Loss-of-Function Mutations in AP1B1, Encoding Beta-1 Subunit of Adaptor-Related Protein Complex 1, Cause MEDNIK-like Syndrome. The American Journal of Human Genetics. doi:10.1016/j.ajhg.2019.09.020

Acknowledgements
We thank the study families for their enthusiastic participation. We thank Mais Hashem, Firdous Abdulwahab, and Niema Ibrahim for their help as clinical coordinators, Eman Al-Obeid, Tarfa Alshidi, and Mona Alanazi for their help with the tissue culture, and the Genotyping and Sequencing Core Facilities at King Faisal Specialist Hospital and Research Center for their technical help. This work was supported in part by King Salman Center for Disability Research.

Publisher
Elsevier BV

Journal
The American Journal of Human Genetics

DOI
10.1016/j.ajhg.2019.09.020

Additional Links
https://linkinghub.elsevier.com/retrieve/pii/S000292971930360X

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