Severe dystonia, cerebellar atrophy, and cardiomyopathy likely caused by a missense mutation in TOR1AIP1

Abstract

Background: Dystonia, cerebellar atrophy, and cardiomyopathy constitute a rare association.

Methods: We used homozygosity mapping and whole exome sequencing to determine the mutation, western blot and immunolabelling on cultured fibroblasts to demonstrate the lower expression and the mislocalization of the protein.

Results: We report on a boy born from consanguineous healthy parents, who presented at three years of age with rapidly progressing dystonia, progressive cerebellar atrophy, and dilated cardiomyopathy. We identified regions of homozygosity and performed whole exome sequencing that revealed a homozygous missense mutation in TOR1AIP1. The mutation, absent in controls, results in a change of a highly conserved glutamic acid to alanine. TOR1AIP1 encodes lamina-associated polypeptide 1 (LAP1), a transmembrane protein ubiquitously expressed in the inner nuclear membrane. LAP1 interacts with torsinA, the protein mutated in DYT1-dystonia. In vitro studies in fibroblasts of the patient revealed reduced expression of LAP1 and its mislocalization and aggregation in the endoplasmic reticulum as underlying pathogenic mechanisms.

Conclusions and relevance: The pathogenic role of TOR1AIP1 mutation is supported by a) the involvement of a highly conserved amino acid, b) the absence of the mutation in controls, c) the functional interaction of LAP1 with torsinA, and d) mislocalization of LAP1 in patient cells. Of note, cardiomyopathy has been reported in LAP1-null mice and in patients with the TOR1AIP1 nonsense mutation. Other cases will help delineate the clinical spectrum of LAP1-related mutations.

Figure 1

Familial pedigree and brain MRI of the patient. A. Familial pedigree. B. Sagittal 3DT1 gradient-echo image shows enlargement of the vermian fissures, which demonstrated cerebellar atrophy (mainly anterior). Reformatted axial 3DT1 gradient-echo image shows small lentiform nuclei compared to caudate nuclei volume. Slight enlargement of the lateral ventricles was also present. C. Sanger sequencing of TOR1AIP1 shows a homozygous A to C variant at position 179,887,067 on chromosome 1 in the patient (V1). Both parents are heterozygous carriers (IV 1 and IV2). D. The mutated glutamic acid (surrounded by blue lines) is conserved across a broad range of species.

Figure 2

Sub-expression and mislocalization of LAP1. A. Three bands were observed using anti-LAP1 antibody in control fibroblasts. Fibroblasts from the patient had significantly less of the larger isoforms, whereas the expression of the shorter isoform was less affected (LAP1 antibodies: courtesy of Dr. W.T. Dauer, anti-GAPDH: Santa Cruz Biotechnology; Olympus FV-1000 confocal microscope). B. Fibroblasts from patient show a strong reduction or total absence of LAP1 at the nuclear envelope. A faint staining for LAP1 was observed at the ER (stained with calnexin antibodies, Santa Cruz Biotechnology; secondary Alexa-conjugates antibodies, Life Technologies; HRP-conjugated antibodies: Jackson ImmunoResearch), and strong staining was observed in some regions (see magnification in the inset). Scale bar: 10 μm. C. Electron micrograph of patient fibroblast did not reveal alterations at the nuclear envelope. Scale bar: 5 μm; inset: 500 nm. Imaging as described before [3].