PSMC1 variant causes a novel neurological syndrome

Abstract

Proteasome 26S, the eukaryotic proteasome, serves as the machinery for cellular protein degradation. It is composed of the 20S core particle and one or two 19S regulatory particles, composed of a base and a lid. To date, several human diseases have been associated with mutations within the 26S proteasome subunits; only one of them affects a base subunit. We now delineate an autosomal recessive syndrome of failure to thrive, severe developmental delay and intellectual disability, spastic tetraplegia with central hypotonia, chorea, hearing loss, micropenis and undescended testes, as well as mild elevation of liver enzymes. None of the affected individuals achieved verbal communication or ambulation. Ventriculomegaly was evident on MRI. Homozygosity mapping combined with exome sequencing revealed a disease-associated p.I328T PSMC1 variant. Protein modeling demonstrated that the PSMC1 variant is located at the highly conserved putative ATP binding and hydrolysis domain, and is suggested to interrupt a hydrophobic core within the protein. Fruit flies in which we silenced the Drosophila ortholog Rpt2 specifically in the eye exhibited an apparent phenotype that was highly rescued by the human wild-type PSMC1, yet only partly by the mutant PSMC1, proving the functional effect of the p.I328T disease-causing variant.

Keywords: PSMC1; Rpt2; monogenic disease; neurological syndrome; proteasome 26S; protein homeostasis.

FIGURE 1

Pedigree, linkage analysis, and PSMC1 variant. (A) Pedigree of the Bedouin studied kindred. Affected individuals are marked in black. (B) Sanger sequencing demonstrating the c.983T>C, p.I328T PSMC1 missense variant: unaffected individual (V:1), obligatory carrier (IV:1), and affected individual (V:2). (C) Homozygosity Mapper plot showing single homozygosity locus shared by affected individuals, marked in red. (D) LOD score analysis plot of chromosome 14 [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 2

PSMC1 conservation and structure. (A) Protein multiple sequence alignment (MSA) for PSMC1. Black rectangle marks the p.I328 isoleucine residue. (B) Schematic model of PSMC1. In orange, an oligonucleotide binding (OB) domain, in green AAA+ motor domain (AAA). Red flag marks the location of the amino acid substitution. (C) Structure of the PSMC ring of the regulatory particle (19S). PSMC1 is marked in green. Gray arrow indicates on the hydrophobic core, where the substitution is located. (D) Zoom‐in on the hydrophobic core. The p.I328T substitution of isoleucine to threonine is labeled in blue; oxygen is marked in red [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 3

In vivo Drosophila assay. (A–E) Representative bright‐field microscope images of fly eyes displaying eye‐specific expression of GMR‐Gal4 in the presence of pUASt β‐tubulin‐60D (A, F, F′), RNAi‐Rpt2 (B, G, G'), pUASt β‐tubulin‐60D and RNAi‐Rpt2 (C, H, H′), RNAi‐Rpt2 and PSMC1‐WT (D, I, I′), and RNAi‐Rpt2 and PSMC1‐I255T (E, J, J') from flies collected on zero day (same day of hatching). Black spots indicate necrotic areas and light areas indicate depigmentation. Bars of the whole fly eye represents 100 μm. Zoom‐in bars represent 10 μm. [Colour figure can be viewed at wileyonlinelibrary.com]