Bi‐allelic CLPB mutations cause cataract, renal cysts, nephrocalcinosis and 3‐methylglutaconic aciduria, a novel disorder of mitochondrial protein disaggregation

Abstract

Whole exome sequencing was used to investigate the genetic cause of mitochondrial disease in two siblings with a syndrome of congenital lamellar cataracts associated with nephrocalcinosis, medullary cysts and 3-methylglutaconic aciduria. Autosomal recessive inheritance in a gene encoding a mitochondrially targeted protein was assumed; the only variants which satisfied these criteria were c.1882C>T (p.Arg628Cys) and c.1915G>A (p.Glu639Lys) in the CLPB gene, encoding a heat shock protein/chaperonin responsible for disaggregating mitochondrial and cytosolic proteins. Functional studies, including quantitative PCR (qPCR) and Western blot, support pathogenicity of these mutations. Furthermore, molecular modelling suggests that the mutations disrupt interactions between subunits so that the CLPB hexamer cannot form or is unstable, thus impairing its role as a protein disaggregase. We conclude that accumulation of protein aggregates underlies the development of cataracts and nephrocalcinosis in CLPB deficiency, which is a novel genetic cause of 3-methylglutaconic aciduria. A common mitochondrial cause for 3-methylglutaconic aciduria appears to be disruption of the architecture of the mitochondrial membranes, as in Barth syndrome (tafazzin deficiency), Sengers syndrome (acylglycerol kinase deficiency) and MEGDEL syndrome (impaired remodelling of the mitochondrial membrane lipids because of SERAC1 mutations). We now propose that perturbation of the mitochondrial membranes by abnormal protein aggregates leads to 3-methylglutaconic aciduria in CLPB deficiency.

A scoring system predicting the clinical course of CLPB defect based on the foetal and neonatal presentation of 31 patients

“…Both the data of the previously unreported (P16, P17, P24, P25 and P31) as well as of the previously reported individuals (P1, 2, 5, 6, 7, 9, 10, 11, 18-23 are (Wortmann et al 2015) individuals 1-14; P3, 4 are (Kanabus et al 2015) patients 1, 2; P12-15, 26 are (Saunders et al 2015) subjects 1-5; P8 is (Kiykim et al 2016)) were collected via an anonymized online questionnaire completed by the respective physicians of the patients. The data of the patients P 27-30 (individuals II-1 -II-IV, Capo-Chichi et al 2015) were extracted from the literature.…”

“…The alterations in CLPB (RefSeq NM_030813.3) in the previously unreported individuals were identified by exome sequencing followed by Sanger sequencing (P17) or directly by Sanger sequencing (P16, 24, 25, 31) using standard procedures as described previously (Wortmann et al 2015;Saunders et al 2015;Kanabus et al 2015). Alamut® Visual Software and the ExAC Browser (http://exac.broadinstitute.…”

“…Recently autosomal recessive mutations in CLPB have been shown to cause a genetic syndrome with a broad phenotypic spectrum (MIM #616254) (Wortmann et al 2015;Saunders et al 2015;Kanabus et al 2015;Capo-Chichi et al 2015). The neurological presentation ranges from normal development without intellectual deficits to a severe and progressive encephalopathy associated with muscular hypertonia, progressive brain atrophy and movement disorder.…”

Pathogenic variants in HTRA2 cause an early‐onset mitochondrial syndrome associated with 3‐methylglutaconic aciduria

“…Among these are the heat shock protein/chaperonin CLPB (Kanabus et al 2015; Wortmann, Zietkiewicz et al 2015), the mitochondrial matrix peptidase CLPP (Jenkinson et al 2013), subunits of the mitochondrial matrix m-AAA protease AFG3L2 and SPG7 (Cagnoli et al 2010; Pfeffer et al 2014) and the conserved heat shock protein 60 HSPD1 (Hansen et al 2007). Interestingly, patients harbouring variants in CLPB protease are characterised by increased 3-MGA-uria, neutropenia, epilepsy, cataracts and movement disorders, which are some of the prominent features found in the individuals presented here (Kanabus et al 2015; Wortmann et al 2015). It is possible that the protease activity of HTRA2 could contribute to certain post-transcriptional modifications of proteins that are directly associated with inborn errors of metabolism with 3-MGA-uria as discriminative feature.…”

“…Based on the pathological mechanism, 3-MGA-uria syndromes have been classified into two groups, ‘primary 3-MGA-uria’ associated with defects in leucine catabolism and ‘secondary 3-MGA-uria(s)’ that are not related to leucine degradation pathways (Wortmann et al 2013a, b). The secondary 3-MGA-uria syndromes may be distinguished into three different subtypes based on the underlying pathomechanism: (1) defective phospholipid synthesis and remodelling (SERAC1 defect or MEGDEL syndrome, TAZ defect or Barth syndrome and AGK defect or Sengers syndrome) (Barth et al 1983, 2004; Kelley et al 1991; Mayr et al 2012; Wortmann et al 2012); (2) mitochondrial membrane associated diseases (OPA3 defect or Costeff syndrome, DNAJC19 defect or DCMA syndrome and TMEM70 defect) (Anikster et al 2001; Davey et al 2006; Cizkova et al 2008; Magner et al 2015) and (3) other mitochondrial proteins with unknown pathomechanism (CLPB defect) (Kanabus et al 2015; Wortmann et al 2015)…”