Spondyloepimetaphyseal dysplasia with elevated plasma lysosomal enzymes caused by homozygous variant in MBTPS1

A genetically-encoded fluorescence-based reporter to spatiotemporally investigate mannose-6-phosphate pathway

Maintenance of a pool of active lysosomes with acidic pH and degradative hydrolases is crucial for cell health. Abnormalities in lysosomal function are closely linked to diseases, such as lysosomal storage disorders, neurodegeneration, intracellular infections, and cancer among others. Emerging body of research suggests the malfunction of lysosomal hydrolase trafficking pathway to be a common denominator of several disease pathologies. However, available conventional tools to assess lysosomal hydrolase trafficking are insufficient and fail to provide a comprehensive picture about the trafficking flux and location of lysosomal hydrolases. To address some of the shortcomings, we designed a genetically-encoded fluorescent reporter containing a lysosomal hydrolase tandemly tagged with pH sensitive and insensitive fluorescent proteins, which can spatiotemporally trace the trafficking of lysosomal hydrolases. As a proof of principle, we demonstrate that the reporter can detect perturbations in hydrolase trafficking, that are induced by pharmacological manipulations and pathophysiological conditions like intracellular protein aggregates. This reporter can effectively serve as a probe for mapping the mechanistic intricacies of hydrolase trafficking pathway in health and disease and is a utilitarian tool to identify genetic and pharmacological modulators of this pathway, with potential therapeutic implications.

SPRING licenses S1P-mediated cleavage of SREBP2 by displacing an inhibitory pro-domain

Site-one protease (S1P) conducts the first of two cleavage events in the Golgi to activate Sterol regulatory element binding proteins (SREBPs) and upregulate lipogenic transcription. S1P is also required for a wide array of additional signaling pathways. A zymogen serine protease, S1P matures through autoproteolysis of two pro-domains, with one cleavage event in the endoplasmic reticulum (ER) and the other in the Golgi. We recently identified the SREBP regulating gene, (SPRING), which enhances S1P maturation and is necessary for SREBP signaling. Here, we report the cryo-EM structures of S1P and S1P-SPRING at sub-2.5 Å resolution. SPRING activates S1P by dislodging its inhibitory pro-domain and stabilizing intra-domain contacts. Functionally, SPRING licenses S1P to cleave its cognate substrate, SREBP2. Our findings reveal an activation mechanism for S1P and provide insights into how spatial control of S1P activity underpins cholesterol homeostasis.

A case of MBTPS1-related disorder due to compound heterozygous variants in MBTPS1 gene: Genotype-phenotype expansion and the emergence of a novel syndrome

MBTPS1 (NM_003791.4) encodes Site-1 protease, a serine protease that functions sequentially with Site-2 protease regulating cholesterol homeostasis and endoplasmic reticulum stress response. MBTPS1 pathogenic variants are associated with spondyloepiphyseal dysplasia, Kondo-Fu type (MIM:618392; cataract, alopecia, oral mucosal disorder, and psoriasis-like syndrome, and Silver-Russell-like syndrome). In this report, we describe a 14-year-old female with a complex medical history including white matter volume loss, early-onset cataracts, retrognathia, laryngomalacia, inguinal hernia, joint hypermobility, feeding dysfunction, and speech delay. Additionally, features of ectodermal dysplasia that she has include decreased sweating, heat intolerance, dysplastic nails, chronically dry skin, and abnormal hair growth issues. Exome sequencing analysis identified compound heterozygous variants in the MBTPS1 gene: c.2255G > T p.(Gly752Val) predicted to affect important function of the protein, which was inherited from the mother, and a splice site variant c.2831 + 5G > T, which was inherited from the father. The RNA-seq analysis of the splice variant showed skipping of exon 21, predicted to result in frameshifting p.(Ser901fs28*) leading to non-sense mediated decay. To our knowledge, only eight studies have been published that described the MBPTS1-related disorders. Interestingly, we observed the features of ectodermal dysplasia in our patient that further expands the phenotypic spectrum of MBTPS1 gene-related disorders.

Lysosomal enzyme trafficking: from molecular mechanisms to human diseases

Lysosomes degrade and recycle macromolecules that are delivered through the biosynthetic, endocytic, and autophagic routes. Hydrolysis of the different classes of macromolecules is catalyzed by about 70 soluble enzymes that are transported from the Golgi apparatus to lysosomes in a mannose 6-phosphate (M6P)-dependent process. The molecular machinery that generates M6P tags for receptor-mediated targeting of lysosomal enzymes was thought to be understood in detail. However, recent studies on the M6P pathway have identified a previously uncharacterized core component, yielded structural insights in known components, and uncovered functions in various human diseases. Here we review molecular mechanisms of lysosomal enzyme trafficking and discuss its relevance for rare lysosomal disorders, cancer, and viral infection.

Exome Sequencing Reveals Biallelic Mutations in MBTPS1 Gene in a Girl with a Very Rare Skeletal Dysplasia

The Kondo-Fu type of spondyloepiphyseal dysplasia (SEDKF) is a rare skeletal dysplasia caused by homozygous or compound heterozygous mutations in the MBTPS1 gene. The MBTPS1 gene encodes a protein that is involved in the regulation of cholesterol and fatty acid metabolism. Mutations in MBTPS1 can lead to reduced levels of these lipids, which can have a number of effects on development, including skeletal anomalies, growth retardation, and elevated levels of blood lysosomal enzymes. This work reports the case of a 5-year-old girl with SEDKF. The patient had a severely short stature and a number of skeletal anomalies, including kyphosis, pectus carinatum, and reduced bone mineral density. She also had early onset cataracts and inguinal hernias. Genetic testing revealed two novel compound heterozygous variants in the MBTPS1 gene. These variants are predicted to disrupt the function of the MBTPS1 protein, which is consistent with the patient's clinical presentation. This case report adds to the growing body of evidence that mutations in the MBTPS1 gene are causal of SEDKF. We summarized the features of previous reported cases (with age ranges from 4 to 24 years) and identified that 80% had low stature, 70% low weight, 80% had bilateral cataracts and 70% showed Spondyloepiphyseal dysplasia on X-rays. The findings of this study suggest that SEDKF is a clinically heterogeneous disorder that can present with a variety of features. Further studies are needed to better understand the underlying mechanisms of SEDKF and to develop more effective treatments.

A new SPRING in lipid metabolism

PURPOSE OF REVIEW

The SREBP transcription factors are master regulators of lipid homeostasis owing to their role in controlling cholesterol and fatty acid metabolism. The core machinery required to promote their trafficking and proteolytic activation has been established close to 20 years ago. In this review, we summarize the current understanding of a newly identified regulator of SREBP signaling, SPRING (formerly C12ORF49), its proposed mechanism of action, and its role in lipid metabolism.

RECENT FINDINGS

Using whole-genome functional genetic screens we, and others, have recently identified SPRING as a novel regulator of SREBP signaling. SPRING is a Golgi-resident single-pass transmembrane protein that is required for proteolytic activation of SREBPs in this compartment. Mechanistic studies identified regulation of S1P, the protease that cleaves SREBPs, and control of retrograde trafficking of the SREBP chaperone SCAP from the Golgi to the ER as processes requiring SPRING. Emerging studies suggest an important role for SPRING in regulating circulating and hepatic lipid levels in mice and potentially in humans.

SUMMARY

Current studies support the notion that SPRING is a novel component of the core SREBP-activating machinery. Additional studies are warranted to elucidate its role in cellular and systemic lipid metabolism.

Hepatic SREBP signaling requires SPRING to govern systemic lipid metabolism in mice and humans

The sterol regulatory element binding proteins (SREBPs) are transcription factors that govern cholesterol and fatty acid metabolism. We recently identified SPRING as a post-transcriptional regulator of SREBP activation. Constitutive or inducible global ablation of Spring in mice is not tolerated, and we therefore develop liver-specific Spring knockout mice (LKO). Transcriptomics and proteomics analysis reveal attenuated SREBP signaling in livers and hepatocytes of LKO mice. Total plasma cholesterol is reduced in male and female LKO mice in both the low-density lipoprotein and high-density lipoprotein fractions, while triglycerides are unaffected. Loss of Spring decreases hepatic cholesterol and triglyceride content due to diminished biosynthesis, which coincides with reduced very-low-density lipoprotein secretion. Accordingly, LKO mice are protected from fructose diet-induced hepatosteatosis. In humans, we find common genetic SPRING variants that associate with circulating high-density lipoprotein cholesterol and ApoA1 levels. This study positions SPRING as a core component of hepatic SREBP signaling and systemic lipid metabolism in mice and humans.

Case Report: Recombinant human growth hormone therapy in a patient with spondyloepiphyseal dysplasia, Kondo-Fu type

BACKGROUND

Variants in membrane-bound transcription factor peptidase, site 1 (MBTPS1) gene, can result in clinically rare spondyloepiphyseal dysplasia of Kondo-fu type (OMIM #618392, SEDKF), Silver-Russell syndrome, and CAOP (cataract, alopecia, oral mucosal disorder, and psoriasis-like) syndrome.

CASE PRESENTATION

A 6-year-old Chinese male child diagnosed with SEDKF underwent 3 years of growth hormone therapy. A genetic examination revealed two new nonsense variants in the MBTPS1 gene on chromosome 16q23-q24 with compound heterozygotes c.1589(exon12)A > G and c.163(exon2)G > A.

CONCLUSION

The MBTPS1 gene c.1589(exon12)A > G and c.163(exon2)G > A on chromosome 16q23-q24 is associated with SEDKF. Growth hormone therapy can repair growth retardation in patients with spondyloepiphyseal dysplasia, Kondo-Fu type; however, more evidence of such patient cases is required to support this hypothesis.

Identification of a New Variant of the MBTPS1 Gene of the Kondo-Fu Type of Spondyloepiphyseal Dysplasia (SEDKF) in a Saudi Patient

Spondyloepiphyseal dysplasia (SEDKF) is a rare skeletal dysplasia associated with kyphosis and low bone mineral density, significantly delayed growth, and skeletal deformities. Blood lysosomal enzyme levels have also been shown to be elevated with a delay in development. The first variant described was compound heterozygosity for mutations in the MBTPS1 gene: a 1-bp duplication and a missense mutation. In the current study, we examined a Saudi consanguineous family. Clinical features like spondyloepiphyseal dysplasia, indicative of characteristic skeletal abnormalities, and impaired cognitive abilities were observed. Our patient has dysmorphic facial features, short stature, and significant skeletal deformities. A homozygous missense MBTPS1 (c.2634C > A p. (Ser878Arg)) with unknown significance was discovered in the whole exome; pathogenic MBTPS1 variants cause the autosomal recessive Kondo-Fu type of spondyloepiphyseal dysplasia (SEDKF, OMIM®: 618392). The whole exome sequence, which described a homozygous missense variant of unknown clinical significance (VUS, class 3 variant) in the MBTPS1 gene, was heterozygous in both asymptomatic parents. We are mindful that changing the classification of a variant of unknown significance is challenging. Considering clinical phenotypes and radiological findings produced by the pathogenic mutation in the MBTPS1 gene, the identified c.2634C > A variant is supported and may be categorized as likely pathogenic based on clinical symptoms.

S1P defects cause a new entity of cataract, alopecia, oral mucosal disorder, and psoriasis-like syndrome

In this report, we discovered a new entity named cataract, alopecia, oral mucosal disorder, and psoriasis‐like (CAOP) syndrome in two unrelated and ethnically diverse patients. Furthermore, patient 1 failed to respond to regular treatment. We found that CAOP syndrome was caused by an autosomal recessive defect in the mitochondrial membrane‐bound transcription factor peptidase/site‐1 protease (MBTPS1, S1P). Mitochondrial abnormalities were observed in patient 1 with CAOP syndrome. Furthermore, we found that S1P is a novel mitochondrial protein that forms a trimeric complex with ETFA/ETFB. S1P enhances ETFA/ETFB flavination and maintains its stability. Patient S1P variants destabilize ETFA/ETFB, impair mitochondrial respiration, decrease fatty acid β‐oxidation activity, and shift mitochondrial oxidative phosphorylation (OXPHOS) to glycolysis. Mitochondrial dysfunction and inflammatory lesions in patient 1 were significantly ameliorated by riboflavin supplementation, which restored the stability of ETFA/ETFB. Our study discovered that mutations in MBTPS1 resulted in a new entity of CAOP syndrome and elucidated the mechanism of the mutations in the new disease.

Clinical and molecular characterization of a patient with MBTPS1 related spondyloepiphyseal dysplasia: Evidence of pathogenicity for a synonymous variant

BACKGROUND

A novel autosomal recessive skeletal dysplasia resulting from pathogenic variants in membrane-bound transcription factor peptidase, site 1 (MBTPS1) has been recently delineated. To date, only three patients have been reported.

METHODS

In this study, we reported the clinical and molecular features of a Chinese boy who was diagnosed with spondyloepiphyseal dysplasia. The effects of variants on mRNA splicing were analyzed through transcript analysis in vivo and minigene splice assay in vitro.

RESULTS

The proband mainly showed short stature, special facial features, cataract, hernias, and serious sleep apnea syndrome. Growth hormone stimulation tests suggested the boy had growth hormone deficiency. Imaging examinations suggested abnormal thoracolumbar vertebrae and severely decreased bone mineral density. Genetic analysis of MBTPS1 gene revealed two novel heterozygous variants, a nonsense mutation c.2656C > T (p.Q886*, 167) in exon 20 and a synonymous variant c.774C > T (p.A258=) in exon 6. The transcript analysis in vivo exhibited that the synonymous variant c.774C > T caused exon 6 skipping. The minigene splice assay in vitro confirmed the alteration of MBTPS1 mRNA splicing and the exon skipping was partially restored by an antisense oligonucleotide (ASO) treatment.

CONCLUSION

Notably, we report a Chinese rare case of spondyloepiphyseal dysplasia and validate its pathogenic synonymous variant in the MBTPS1 gene.

Site-1 and site-2 proteases: A team of two in regulated proteolysis

The site-1 and site-2 proteases (S1P and S2P) were identified over 20 years ago, and the functions of both have been addressed in numerous studies ever since. Whereas S1P processes a set of substrates independently of S2P, the latter acts in concert with S1P in a mechanism, called regulated intramembrane proteolysis, that controls lipid metabolism and response to unfolded proteins. This review summarizes the molecular roles that S1P and S2P jointly play in these processes. As S1P and S2P deficiencies mainly affect connective tissues, yet with varying phenotypes, we discuss the segregated functions of S1P and S2P in terms of cell homeostasis and maintenance of the connective tissues. In addition, we provide experimental data that point at S2P, but not S1P, as a critical regulator of cell adaptation to proteotoxicity or lipid imbalance. Therefore, we hypothesize that S2P can also function independently of S1P activity.

The role of the Golgi apparatus in disease (Review)

The Golgi apparatus is known to underpin many important cellular homeostatic functions, including trafficking, sorting and modifications of proteins or lipids. These functions are dysregulated in neurodegenerative diseases, cancer, infectious diseases and cardiovascular diseases, and the number of disease-related genes associated with Golgi apparatus is on the increase. Recently, many studies have suggested that the mutations in the genes encoding Golgi resident proteins can trigger the occurrence of diseases. By summarizing the pathogenesis of these genetic diseases, it was found that most of these diseases have defects in membrane trafficking. Such defects typically result in mislocalization of proteins, impaired glycosylation of proteins, and the accumulation of undegraded proteins. In the present review, we aim to understand the patterns of mutations in the genes encoding Golgi resident proteins and decipher the interplay between Golgi resident proteins and membrane trafficking pathway in cells. Furthermore, the detection of Golgi resident protein in human serum samples has the potential to be used as a diagnostic tool for diseases, and its central role in membrane trafficking pathways provides possible targets for disease therapy. Thus, we also introduced the clinical value of Golgi apparatus in the present review.