POPDC3 Gene Variants Associate with a New Form of Limb Girdle Muscular Dystrophy

Abstract

Objective The Popeye domain containing 3 (POPDC3) gene encodes a membrane protein involved in cyclic adenosine monophosphate (cAMP) signaling. Besides gastric cancer, no disease association has been described. We describe a new muscular dystrophy associated with this gene. Methods We screened 1,500 patients with unclassified limb girdle weakness or hyperCKemia for pathogenic POPDC3 variants. Five patients carrying POPDC3 variants were examined by muscle magnetic resonance imaging (MRI), muscle biopsy, and cardiac examination. We performed functional analyses in a zebrafish popdc3 knockdown model and heterologous expression of the mutant proteins in Xenopus laevis oocytes to measure TREK‐1 current. Results We identified homozygous POPDC3 missense variants (p.Leu155His, p.Leu217Phe, and p.Arg261Gln) in 5 patients from 3 ethnically distinct families. Variants affected highly conserved residues in the Popeye (p.Leu155 and p.Leu217) and carboxy‐terminal (p.Arg261) domains. The variants were almost absent from control populations. Probands’ muscle biopsies were dystrophic, and serum creatine kinase levels were 1,050 to 9,200U/l. Muscle weakness was proximal with adulthood onset in most patients and affected lower earlier than upper limbs. Muscle MRI revealed fat replacement of paraspinal and proximal leg muscles; cardiac investigations were unremarkable. Knockdown of popdc3 in zebrafish, using 2 different splice‐site blocking morpholinos, resulted in larvae with tail curling and dystrophic muscle features. All 3 mutants cloned in Xenopus oocytes caused an aberrant modulation of the mechano‐gated potassium channel, TREK‐1. Interpretation Our findings point to an important role of POPDC3 for skeletal muscle function and suggest that pathogenic variants in POPDC3 are responsible for a novel type of autosomal recessive limb girdle muscular dystrophy. ANN NEUROL 2019;86:832–843

The Role of the Popeye Domain Containing Gene Family in Organ Homeostasis

“…A strongly reduced cytosolic localization of POPDC2 was observed in all patients carrying missense or nonsense mutations of POPDC1 [14,23]. Interestingly, while POPDC3 mutations have also been linked to LGMD, the biopsies of patients carrying any one of the three identified POPDC3 mutations do not display aberrant membrane localization of the mutant protein, nor were there any alterations observed in POPDC1 or POPDC2 [17] suggesting that differences exist between the pathogenic processes that are trigged by different POPDC isoforms.…”

“…In contrast, patients carrying a POPDC2 mutation develop a severe third-degree AV-block, but muscle appears to be normal [31]. The reverse is true for POPDC3, as in this case only skeletal muscle is affected and patients develop a severe LGMD, but the heart is normal [17]. The differential effect of POPDC mutations on heart and skeletal muscle maybe related to the expression level of each isoform, or alternatively that the different isoforms have unique functions, specific to cardiac or skeletal muscle, respectively.…”

“…Electron microscopy of the popdc1 mutants revealed a lack of the electron dense matrix proteins (mainly a network of collagen type, I, III and IV), which accumulates in the MTJ [14]. As a putative consequence of the impaired MTJ development, myofiber detachment was observed in popdc1, popdc2 and, although rare, also in popdc3 morphants [14,17,30]. The common phenotype seen in case of popdc1-3 morphants suggests an important role of POPDC proteins in MTJ formation.…”

“…The DSPE and FQVT motifs of the PBC are shown in pink. The positions of the three pathological mutations in POPDC3 reported by Vissing et al [17] are shown as red spheres. The model was produced using the Phyre2 algorithm and the cAMP binding site was predicted using the 3DLigandSite predictor [16,18].…”

“…However, insensitivity to cAMP binding seems to be unable to explain the effect of all known POPDC family mutations. The three recently reported pathological mutations in POPDC3 (L155H, L217F and R261Q), which were discovered in patients suffering from limb-girdle muscular dystrophy show variability in their effect on cAMP binding [17] (Figure 1). A repeat of the co-expression experiment with TREK1 in Xenopus oocytes was run.…”