Marinesco-Sjögren Syndrome in an Emirati Child with a Novel Mutation in SIL1 Affecting the 5' Untranslated Region

Nucleotide exchange factor SIL1 promotes the progress of breast cancer cells via regulating the cell cycle and apoptosis

Breast cancer, as one of the most malignant tumors, poses a serious threat to the lives of females. Nucleotide exchange factor SIL1 is an important regulator of endoplasmic reticulum function that might have a specific role in tumor progression. In this study, we aimed to investigate the effect of SIL1 on the proliferation, apoptosis, and metastasis of human breast cancer. SIL1-specific small interfering RNA was transfected into two breast cancer cell lines, MCF7 and MDA-MB-231, to generate SIL1 knockdown cells. Clone formation and Cell Counting Kit-8 assays were performed to determine cell proliferation. Wound healing and transwell assays were used to detect the cell migration and invasion, respectively. Cell cycle and apoptosis were determined by flow cytometry. The messenger RNA and protein levels of target genes were analyzed using quantitative real-time PCR and western blot. According to the results of TCGA and GTEx database analysis, we determined that SIL1 was overexpressed in 1085 breast cancer samples compared with 291 normal samples. Knockdown of SIL1 inhibited the proliferation, migration, and invasion of MCF7 and MDA-MB-231 cells, accordingly. The cell cycle was blocked at the G1 phase following transfection of SIL1-specific small interfering RNA through the inhibition of Cyclin D1, CDK4, and CDK6. SIL1 knockdown induced apoptosis and also promoted the activity of Caspase9 and Bax. Furthermore, SIL1 was able to promote phosphorylation of ERK1/2. Based on these results, SIL1 might act as an oncogene and accelerate the progression of human breast cancer.

Identification of Cellular Pathogenicity Markers for SIL1 Mutations Linked to Marinesco-Sjögren Syndrome

Background and objective: Recessive mutations in the SIL1 gene cause Marinesco-Sjögren syndrome (MSS), a rare neuropediatric disorder. MSS-patients typically present with congenital cataracts, intellectual disability, cerebellar ataxia and progressive vacuolar myopathy. However, atypical clinical presentations associated with SIL1 mutations have been described over the last years; compound heterozygosity of SIL1 missense mutations even resulted in a phenotype not fulfilling the clinical diagnostic criteria of MSS. Thus, a read-out system to evaluate reliably the pathogenicity of amino acid changes in SIL1 is needed. Here, we aim to provide suitable cellular biomarkers enabling the robust evaluation of pathogenicity of SIL1 mutations. Methods: Five SIL1 variants including one polymorphism (p.K132Q), three known pathogenic mutations (p.V231_I232del, p.G312R, and p.L457P) and one ambiguous missense variant (p.R92W) were studied along with the wild-type proteins in Hek293 in vitro models by cell biological assays, immunoprecipitation, immunoblotting, and immunofluorescence as well as electron microscopy. Moreover, the SIL1-interactomes were interrogated by tandem-affinity-purification and subsequent mass spectrometry. Results: Our combined studies confirmed the pathogenicity of p.V231_I232del, p.G312R, and p.L457P by showing instability of the proteins as well as tendency to form aggregates. This observation is in line with altered structure of the ER-Golgi system and vacuole formation upon expression of these pathogenic SIL1-mutants as well as the presence of oxidative or ER-stress. Reduced cellular fitness along with abnormal mitochondrial architecture could also be observed. Notably, both the polymorphic p.K132Q and the ambiguous p.R92W variants did not elicit such alterations. Study of the SIL1-interactome identified POC1A as a novel binding partner of wild-type SIL1; the interaction is disrupted upon the presence of pathogenic mutants but not influenced by the presence of benign variants. Disrupted SIL1-POC1A interaction is associated with centrosome disintegration. Conclusions: We developed a combination of cellular outcome measures to evaluate the pathogenicity of SIL1 variants in suitable in vitro models and demonstrated that the p. R92W missense variant is a polymorphism rather than a pathogenic mutation leading to MSS.