Progress on the study of Popeye domain-containing 3 (POPDC3) in malignancies and striated muscle function and homeostasis.

The Popeye domain-containing protein 3 (POPDC3), a transmembrane protein with a unique cyclic adenosine monophosphate (cAMP) binding site, is widely expressed in mammalian tissues, with the highest levels of expression in skeletal muscle. POPDC3 plays a key role in many physiological and pathological processes and is considered a candidate biomarker and potential therapeutic target of cancer. In addition, POPDC3 gene variants have been associated with limb-girdle muscular dystrophy (LGMD) type 26. However, there are only a few studies on the biological role of POPDC3, interacting proteins, potential downstream targets, and regulated signaling pathways. Therefore, this review focuses on the structure of POPDC3 protein, interacting molecules, and the role and mechanism in cancer, and in cardiac and skeletal muscle, and to review the current research progress of POPDC3 and propose possible future study directions.

Homozygous missense variant in POPDC3 causes recessive limb-girdle muscular dystrophy type 26.

BACKGROUND: Limb-girdle muscular dystrophy (LGMD) comprises a heterogeneous group of diseases, affecting different muscles, predominantly skeletal muscles and cardiac muscles of the body. LGMD is classified into two main subtypes A and B, which are further subclassified into eight dominant and thirty recessive subtypes. Three genes, namely POPDC1, POPDC2 and POPDC3, encode popeye domain-containing protein (POPDC), and the variants of POPDC1 and POPDC3 genes have been associated with LGMD. METHODS: In the present study, we performed whole-exome sequencing (WES) analysis on a single-family to investigate the hallmark features of LGMD. The results of WES were further confirmed by Sanger sequencing and 3D protein modeling was also conducted. RESULTS: WES data analysis and Sanger sequencing revealed a homozygous missense variant (c.460A>G; p.Lys154Glu) at a highly conserved amino acid position in the POPDC3. Mutations in the POPDC3 gene have been previously associated with recessive limb-girdle muscular dystrophy type 26. 3D protein modeling further suggested that the identified variant might affect the POPDC3 structure and proper function. CONCLUSIONS: The present study confirms the role of POPDC3 in LGMD, and will facilitate genetic counseling of the family to mitigate the risks of the carrier or affects on future pregnancies.

A novel splice site variant in the POPDC3 causes autosomal recessive limb-girdle muscular dystrophy type 26.

Limb-Girdle muscular dystrophy (LGMD) is a group of muscle disorders with highly heterogeneous genetic patterns and clinical phenotypes, and this group includes multiple subtypes. Different LGMD subtypes have similar phenotypes and clinical overlaps, these subtypes are difficult to distinguish by clinical symptoms alone and can only be accurately diagnosed by analysis in combination with definitive genetic test results. Here, we report a female presenting features of LGMD. After analysis of whole-exome sequencing data, a novel homozygous POPDC3 variant c.486-1G>A (rs113419658) located in the acceptor splice site of intron 2 was identified in the proband. The variant effect on splicing were analyzed by genetic analysis based on cDNA synthesized by the patient's RNA. cDNA analysis indicated that the novel homozygous POPDC3 splice variant disrupted original acceptor splice site, which can cause a frameshift in the mRNA of the POPDC3 gene, thereby producing a truncated POPDC3 protein and ultimately affecting its normal function. POPDC3 variant was recently associated with recessive limb-girdle muscular dystrophy type 26 (LGMDR26). Based on the above results, we hypothesize that this variant is probably a pathogenic variant, and expand the gene variant spectrum of POPDC3.

The Transition from Gastric Intestinal Metaplasia to Gastric Cancer Involves POPDC1 and POPDC3 Downregulation.

Intestinal metaplasia (IM) is an intermediate step in the progression from premalignant to malignant stages of gastric cancer (GC). The Popeye domain containing (POPDC) gene family encodes three transmembrane proteins, POPDC1, POPDC2, and POPDC3, initially described in muscles and later in epithelial and other cells, where they function in cell-cell interaction, and cell migration. POPDC1 and POPDC3 downregulation was described in several tumors, including colon and gastric cancers. We questioned whether IM-to-GC transition involves POPDC gene dysregulation. Gastric endoscopic biopsies of normal, IM, and GC patients were examined for expression levels of POPDC1-3 and several suggested IM biomarkers, using immunohistochemistry and qPCR. Immunostaining indicated lower POPDC1 and POPDC3 labeling in IM compared with normal tissues. Significantly lower POPDC1 and POPDC3 mRNA levels were measured in IM and GC biopsies and in GC-derived cell lines. The reduction in focal IM was smaller than in extensive IM that resembled GC tissues. POPDC1 and POPDC3 transcript levels were highly correlated with each other and inversely correlated with LGR5, OLFM4, CDX2, and several mucin transcripts. The association of POPDC1 and POPDC3 downregulation with IM-to-GC transition implicates a role in tumor suppression and highlights them as potential biomarkers for GC progression and prospective treatment targets.

A homozygous loss of function variant in POPDC3: From invalidating exercise intolerance to a limb-girdle muscular dystrophy phenotype.

Recessive pathogenic variants in POPDC3 have recently been associated with the rare limb-girdle muscular dystrophy (LGMD) subtype LGMDR26. We studied three siblings and a distantly related individual with a skeletal muscle disorder, harboring the c.486-6T>A splice site variant in POPDC3 in homozygosity. Immunohistochemistry, western blot, and mRNA experiments on patients' skeletal muscle tissue as well as on patients' myoblasts were performed to study the pathogenicity of the predicted loss of function mechanism of the variant. Patients mainly presented with invalidating myalgia and exercise intolerance and limited to no segmentary muscle weakness. CK levels were markedly elevated in all patients. A loss of function mechanism at the RNA level was shown (r.485_486insauag, p.Ile163*). Muscle biopsies performed in three out of four patients showed non-specific myopathic features with a marked type 2 fiber predominance and the presence of a large number of severely atrophic fibers with pyknotic nuclear clumps. We show that skeletal muscle symptoms in LGMDR26 may range from an overt late juvenile to young adult-onset limb-girdle muscular dystrophy phenotype to severe exercise intolerance and myalgia, with consistently highly elevated CK levels. We further prove a clear LOF mechanism of POPDC3 in this rare disorder.

POPDC1 is suppressed in human breast cancer tissues and is negatively regulated by EGFR in breast cancer cell lines.

Breast cancer molecular heterogeneity has resulted in disparities in therapeutic response and targeting of molecular subtypes of breast cancer. This necessitates identification and validation of novel therapeutic targets for breast cancer treatment. Suppression of Popeye domain-containing (POPDC) proteins is hypothesized to promote malignant cell behaviour and poor clinical outcomes in various cancers. We aimed to determine whether POPDC proteins are suppressed in human ductal carcinoma tissues and if this correlates to clinical progression and Her2 status. We further assessed if the EGFR regulated POPDC1 in breast cancer. Here we show significant suppression of POPDC1 in malignant breast cancer tissues without correlation to clinical progression. Interestingly, POPDC2 and POPDC3 were highly expressed in malignant breast tissues. Furthermore, HER2+ status significantly correlated with high POPDC2 and POPDC3, but not POPDC1 expression. We further show for the first time that low POPDC1 correlates to high EGFR expression in breast cancer tissues and that EGFR negatively regulates POPDC1 expression in MCF7, MDA231 and SKBR3 breast cancer cells. Furthermore, overexpression of POPDC1 in MCF7, MDA231 and SKBR3 cells attenuated EGF-mediated cell migration and proliferation. These findings show that POPDC1 is suppressed in breast cancer and can potentially be targeted to inhibit EGFR-mediated cell migration and proliferation.