Patrick Frosk

Limb-Girdle Muscular Dystrophy Type 2H Associated with Mutation in TRIM32, a Putative E3-Ubiquitin–Ligase Gene

Limb-girdle muscular dystrophy type 2H (LGMD2H) is a mild autosomal recessive myopathy that was first described in the Manitoba Hutterite population. Previous studies in our laboratory mapped the causative gene for this disease to a 6.5-Mb region in chromosomal region 9q31-33, flanked by D9S302 and D9S1850. We have now used additional families and a panel of 26 microsatellite markers to construct haplotypes. Twelve recombination events that reduced the size of the candidate region to 560 kb were identified or inferred. This region is flanked by D9S1126 and D9S737 and contains at least four genes. Exons of these genes were sequenced in one affected individual, and four sequence variations were identified. The families included in our study and 100 control individuals were tested for these variations. On the basis of our results, the mutation in the tripartite-motif-containing gene (TRIM32) that replaces aspartate with asparagine at position 487 appears to be the causative mutation of LGMD2H. All affected individuals were found to be homozygous for D487N, and this mutation was not found in any of the controls. This mutation occurs in an NHL (named after the proteins NCL1, HT2A, and LIN-41) domain at a position that is highly conserved. NHL domains are known to be involved in protein-protein interactions. Although the function of TRIM32 is unknown, current knowledge of the domain structure of this protein suggests that it may be an E3-ubiquitin ligase. If proven, this represents a new pathogenic mechanism leading to muscular dystrophy.

Cardiac and respiratory failure in limb-girdle muscular dystrophy 2I.

Mutations in the gene encoding fukutin‐related protein cause limb‐girdle muscular dystrophy 2I. In this multicenter retrospective analysis of 38 patients, 55.3% had cardiac abnormalities, of which 24% had developed cardiac failure. Heterozygotes for the common C826A mutation developed cardiac involvement earlier than homozygotes. All patients initially improved while receiving standard therapy. Independent of cardiac status, forced vital capacity was below 75% in 44.4% of the patients. There was no absolute correlation between skeletal muscle weakness and cardiomyopathy or respiratory insufficiency. These complications are a primary part of this specific type of limb‐girdle muscular dystrophy, with important implications for management. Ann Neurol 2004;56:738–741

An siRNA-based functional genomics screen for the identification of regulators of ciliogenesis and ciliopathy genes

Defects in primary cilium biogenesis underlie the ciliopathies, a growing group of genetic disorders. We describe a whole-genome siRNA-based reverse genetics screen for defects in biogenesis and/or maintenance of the primary cilium, obtaining a global resource. We identify 112 candidate ciliogenesis and ciliopathy genes, including 44 components of the ubiquitin–proteasome system, 12 G-protein-coupled receptors, and 3 pre-mRNA processing factors (PRPF6, PRPF8 and PRPF31) mutated in autosomal dominant retinitis pigmentosa. The PRPFs localize to the connecting cilium, and PRPF8- and PRPF31-mutated cells have ciliary defects. Combining the screen with exome sequencing data identified recessive mutations in PIBF1, also known as CEP90, and C21orf2, also known as LRRC76, as causes of the ciliopathies Joubert and Jeune syndromes. Biochemical approaches place C21orf2 within key ciliopathy-associated protein modules, offering an explanation for the skeletal and retinal involvement observed in individuals with C21orf2 variants. Our global, unbiased approaches provide insights into ciliogenesis complexity and identify roles for unanticipated pathways in human genetic disease.

Commonality of TRIM32 mutation in causing sarcotubular myopathy and LGMD2H

Sarcotubular myopathy (OMIM 268950) is a rare autosomal recessive myopathy first described in two Hutterite brothers from South Dakota and in two non‐Hutterite brothers from Germany. We report that sarcotubular myopathy (STM) is caused by mutation in TRIM32, the gene encoding the tripartite motif‐containing protein 32. TRIM32 was found to be the gene mutated in limb girdle muscular dystrophy type 2H (LGMD2H [OMIM 254110]), a disorder that has been confined to the Hutterite population. The TRIM32 mutation found in the STM patients is identical to the causative mutation for LGMD2H (D487N), Haplotype analysis shows that the disease chromosomes share common ancestry. Ann Neurol 2005;57:591–595

The Interaction of Piasy with Trim32, an E3-Ubiquitin Ligase Mutated in Limb-girdle Muscular Dystrophy Type 2H, Promotes Piasy Degradation and Regulates UVB-induced Keratinocyte Apoptosis through NFκB

Protein inhibitors of activated STATs (PIAS) family members are ubiquitin-protein isopeptide ligase-small ubiquitin-like modifier ligases for diverse transcription factors. However, the regulation of PIAS protein activity in cells is poorly understood. Previously, we reported that expression of Trim32, a RING domain ubiquitin-protein isopeptide ligase-ubiquitin ligase mutated in human limb-girdle muscular dystrophy type 2H (LGMD2H) and Bardet-Biedl syndrome, is elevated during mouse skin carcinogenesis, protecting keratinocytes from apoptosis induced by UVB and tumor necrosis factor-α (TNFα). Here we report that Trim32 interacts with Piasy and promotes Piasy ubiquitination and degradation. Ubiquitination of Piasy by Trim32 could be reproduced in vitro using purified components. Their interaction was induced by treatment with UVB/TNFα and involved redistribution of Piasy from the nucleus to the cytoplasm, where it accumulated in cytoplasmic granules that colocalized with Trim32. Piasy destabilization and ubiquitination required an intact RING domain in Trim32. The LGMD2H-associated missense point mutation prevented Trim32 binding to Piasy, and human Piasy failed to colocalize with human Trim32 in fibroblasts isolated from an LGMD2H patient. Trim32 expression increased the transcriptional activity of NFκB in epidermal keratinocytes, both under basal treatment and after UVB/TNFα treatment. Conversely, Piasy inhibited NFκB activity under the same conditions and sensitized keratinocytes to apoptosis induced by TNFα and UVB. Our results indicate that, by controlling Piasy stability, Trim32 regulates UVB-induced keratinocyte apoptosis through induction of NFκB and suggests loss of function of Trim32 in LGMD2H.

Hutterite brothers both affected with two forms of limb girdle muscular dystrophy: LGMD2H and LGMD2I

Limb girdle muscular dystrophy (LGMD) is very common in the Hutterite population of the North American Prairies. We have recently reported the homozygous c.1459G>A mutation in TRIM32 associated with LGMD2H. We have also identified Hutterite patients with LGMD2I, homozygous for the common c.826C>A mutation in FKRP. To date, all Hutterites with LGMD have been shown to be homozygous for either the TRIM32 or FKRP mutation. We now report a Hutterite family in which both parents and five sons were all found to be homozygous for the TRIM32 mutation. The father had slowly progressive proximal muscle weakness, whereas three sons and their mother, all currently asymptomatic, had normal physical examinations. The remaining two sons (7 and 10 years old), presented with mild decrease in stamina, had normal neuromuscular examinations and were found to be homozygous for the FKRP mutation in addition to the TRIM32 mutation. These two boys do not differ in age at or mode of presentation, physical findings, or serum CK levels compared to age-matched individuals affected with LGMD2I alone. This suggests that the effects of these two mutations are not acting synergistically at this time. It remains to be seen whether there will be signs of interaction between these two mutations as the patients get older.

SCN1A Mutation Associated With Intractable Myoclonic Epilepsy and Migraine Headache

Mutations in the SCN1A gene are associated with a variety of epilepsy syndromes and more recently with familial hemiplegic migraine. The spectrum of phenotypes can be quite broad even within the same family and with the same mutation. Here we describe a child with intractable myoclonic epilepsy and autism spectrum disorder who carries an inherited mutation in SCN1A (c.3521C>G, p.T1174S). Previous reports suggest this mutation causes familial hemiplegic migraine and interestingly both the patient’s mother, who also carries the mutation, and the patient’s maternal grandmother, have frequent migraines with aura.

A truncating mutation in CEP55 is the likely cause of MARCH, a novel syndrome affecting neuronal mitosis

Background Hydranencephaly is a congenital anomaly leading to replacement of the cerebral hemispheres with a fluid-filled cyst. The goals of this work are to describe a novel autosomal-recessive syndrome that includes hydranencephaly (multinucleated neurons, anhydramnios, renal dysplasia, cerebellar hypoplasia and hydranencephaly (MARCH)); to identify its genetic cause(s) and to provide functional insight into pathomechanism. Methods We used homozygosity mapping and exome sequencing to identify recessive mutations in a single family with three affected fetuses. Immunohistochemistry, RT-PCR and imaging in cell lines, and zebrafish models, were used to explore the function of the gene and the effect of the mutation. Results We identified a homozygous nonsense mutation in CEP55 segregating with MARCH. Testing the effect of this allele on patient-derived cells indicated both a reduction of the overall CEP55 message and the production of a message that likely gives rise to a truncated protein. Suppression or ablation of cep55l in zebrafish embryos recapitulated key features of MARCH, most notably renal dysplasia, cerebellar hypoplasia and craniofacial abnormalities. These phenotypes could be rescued by full-length but not truncated human CEP55 message. Finally, we expressed the truncated form of CEP55 in human cells, where we observed a failure of truncated protein to localise to the midbody, leading to abscission failure and multinucleated daughter cells. Conclusions CEP55 loss of function mutations likely underlie MARCH, a novel multiple congenital anomaly syndrome. This association expands the involvement of centrosomal proteins in human genetic disorders by highlighting a role in midbody function.

The importance of functional validation after next-generation sequencing: evaluation of a novel CARD11 variant

The advent of next-generation sequencing (NGS) techniques such as whole exome sequencing (WES) has greatly accelerated the identification and treatment of rare Mendelian diseases. In particular, these approaches have demonstrated immense utility for identifying inborn errors of immunity (primary immunodeficiencies [PIDs]). This impact is emphasized by the fact that of the estimated 350 gene defects linked to inborn errors of immunity, ~100 have been described since the implementation of NGS techniques in 2010 (1). This article is protected by copyright. All rights reserved.