Dual diagnosis of UQCRFS1-related mitochondrial complex III deficiency and recessive GJA8-related cataracts

Abstract

Biallelic pathogenic variants in UQCRFS1 underlie a rare form of isolated mitochondrial complex III deficiency associated with lactic acidosis and a distinctive scalp alopecia previously described in two unrelated probands. Here, we describe a participant in the Undiagnosed Diseases Network (UDN) with a dual diagnosis of two autosomal recessive disorders revealed by genome sequencing: UQCRFS1-related mitochondrial complex III deficiency and GJA8-related cataracts. Both pathogenic variants have been reported before: UQCRFS1 (NM_006003.3:c.215-1 G>C, p.Val72_Thr81del10) in a case with mitochondrial complex III deficiency and GJA8 (NM 005267.5:c.736 G>T, p.Glu246*) as a somatic change in aged cornea leading to decreased junctional coupling. A multi-modal approach combining enzyme assays and cellular proteomics analysis provided clear evidence of complex III respiratory chain dysfunction and low abundance of the Rieske iron-sulfur protein, validating the pathogenic effect of the UQCRFS1 variant. This report extends the genotypic and phenotypic spectrum for these two rare disorders and highlights the utility of deep phenotyping and genomics data to achieve diagnosis and insights into rare disease.

Keywords: GJA8; UQCRFS1; alopecia; cataracts; mitochondrial complex III; phenotypic spectrum; rare disease.

Fig. 1.. Proband pedigree and clinical images.

Panel A: Pedigree depicting family history and UQCRFS1 and GJA8 genotypes. The proband’s parents are first cousins through their maternal lines. Panel B: Photo of the proband at nearly 2 years of age illustrating non-dysmorphic craniofacial features, normal proportionate stature, and scalp alopecia.

Fig. 2.. Diagram of the proband’s GJA8 variant in context of known pathogenic variants.

The proband’s Q246Ter variant is shown in relation to variants underlying either recessive (top) or dominant (bottom) cataracts. Transmembrane domains (TM) are shown in brown.

Fig. 3. :. Analysis of mitochondrial function in fibroblasts.

Panel A: Mitochondrial complex I (CI) assembly is followed on non-denaturing gel after western blotting and identification with an antibody against NDUFS2. Normal fibroblasts show a large amount of fully assembled complex I at 1000 kDa with a small band at 230 kDa. The proband’s cells show the normal amount of the holocomplex and the 230 kDa band. There is a faint band at 950 kDa (arrow) which represents an intermediate without the incorporation of the N-module. Huh-7cells treated with chloramphenicol are shown as a positive control. Panel B: The activity and assembly of the respiratory chain enzyme complexes were analyzed by BN-PAGE followed by in-gel activity staining. The activities of all enzyme complexes were normal, including assembly of complex V. Abbreviations: CI = complex I, CII = complex II, CIII = complex III, CIV = complex IV, CV = complex V.

Fig. 4. :. Proteomics analysis of the fibroblasts of the patient with UQCRS1 variants compared to controls.

Panel A: Volcano plot of differential mitochondrial protein abundance results after correction of mitochondrial abundance. The x-axis represents the difference between the patient and control mean values. This shows a decrease in the abundance of the subunits of complex III (red), complex I (blue), and the SQOR protein (black). Proteins encoding subunits of complex I or III are shifted to the left, indicating reduced abundance in the proband. The horizontal line shows a cutoff p-value <0.05. Panel B: Quantification of mitochondrial complex abundance shows a decrease in complex III, and a smaller decrease in complex I, and an increase in the mitoribosomal subunits. These data represent an aggregation of the data presented in Fig. 4A. The p-value and the relative % of the complex in the proband compared to the controls are provided in sequential rows above the figure. Abbreviations: * p<0.05, ** p<0.01, *** p<0.001, **** p<0.0001, ns = non-significant. CI = complex I, CII = complex II, CIII = complex III, CIV = complex IV, CV = complex V, mtLSU = large subunit of the mitoribosome, mtSSU = small subunit of the mitoribosome, PDH = pyruvate dehydrogenase complex.