The most 5' truncating homozygous mutation of WNT1 in siblings with osteogenesis imperfecta with a variable degree of brain anomalies: a case report

Osteogenesis imperfecta: shifting paradigms in pathophysiology and care in children

The formation of functional bone requires a delicate interplay between osteogenesis and osteolysis. Disturbances in this subtle balance result in an increased risk for fractures. Besides its mechanical function, bone tissue represents a key player in the regulation of calcium homeostasis. Impaired bone formation results in bone fragility, which is especially pronounced in osteogenesis imperfecta (OI). This rare genetic disorder is characterized by frequent fractures as well as extraskeletal manifestations. The current classification of OI includes 23 distinct types. In recent years, several new mutations in different genes have been identified, although the exact pathomechanisms leading to the clinical presentation of OI often remain unclear. While bisphosphonates are still the standard of care, novel therapeutic approaches are emerging. Especially, targeted antibody therapies, originally developed for osteoporosis, are increasingly being investigated in children with OI and represent a promising approach to alleviate the consequences of impaired osteogenesis and improve quality of life in OI patients. This review aims to provide insight into the pathophysiology of OI and the consequences of distinct disease-causing mutations affecting the regulation of bone homeostasis. In this context, we describe the four most recently identified OI-causing genes and provide an update on current approaches for diagnosis and treatment.

Genotypic and Phenotypic Spectrum and Pathogenesis of WNT1 Variants in a Large Cohort of Patients With OI/Osteoporosis

CONTEXT

Mutations in WNT1 can cause rare inherited disorders such as osteogenesis imperfecta (OI) and early-onset osteoporosis (EOOP). Owing to its rarity, the clinical characteristics and pathogenic mechanism of WNT1 mutations remain unclear.

OBJECTIVE

We aimed to explore the phenotypic and genotypic spectrum and treatment responses of a large cohort of patients with WNT1-related OI/OP and the molecular mechanisms of WNT1 variants.

METHODS

The phenotypes and genotypes of patients and their responses to bisphosphonates or denosumab were evaluated. Western blot analysis, quantitative polymerase chain reaction, and immunofluorescence staining were used to evaluate the expression levels of WNT1, total β-catenin, and type I collagen in the tibial bone or skin from one patient.

RESULTS

We included 16 patients with 16 mutations identified in WNT1, including a novel mutation. The types of WNT1 mutations were related to skeletal phenotypes, and biallelic nonsense mutations or frameshift mutations could lead to an earlier occurrence of fragility fractures and more severe skeletal phenotypes. Some rare comorbidities were identified in this cohort, including cerebral abnormalities, hematologic diseases, and pituitary adenoma. Bisphosphonates and denosumab significantly increased the spine and proximal hip BMD of patients with WNT1 mutations and reshaped the compressed vertebrae. We report for the first time a decreased β-catenin level in the bone of patient 10 with c.677C > T and c.502G > A compared to the healthy control, which revealed the potential mechanisms of WNT1-induced skeletal phenotypes.

CONCLUSION

Biallelic nonsense mutations or frameshift mutations of WNT1 could lead to an earlier occurrence of fragility fractures and a more severe skeletal phenotype in OI and EOOP induced by WNT1 mutations. The reduced osteogenic activity caused by WNT pathway downregulation could be a potential pathogenic mechanism of WNT1-related OI and EOOP.

Hypothesis: By-products of vascular disruption carried in the CSF affect prenatal brain development

Prenatal CNS disruptions can be associated with physically separate findings. Examples include cognitive issues in septo-optic dysplasia and sporadic and WNT1-related unilateral cerebellar hypoplasia, and physical findings such as thinning of the corpus callosum, ventriculomegaly, hippocampal abnormalities, olfactory tract and bulb hypoplasia, and distant cortical dysplasias with schizencephaly. Similar effects to toxicities with intraventricular hemorrhage in prematurity could occur earlier in development. CSF transportation of disruption by-products would provide access to vulnerable areas through inflammatory effects on blood-brain barrier permeability. Outcomes are influenced by location and volume of byproducts in the CSF, timing, transport, and inflammatory responses. A particular association of vermis disruption with cognitive issues may be related to CSF flow distortions that avoid toxin dilutions in the third ventricle. Symmetrical contralateral cortical dysplasia with schizencephaly may reflect immunovascular field-related vulnerabilities seen in situations such as vitiligo.

Osteogenesis Imperfecta: Mechanisms and Signaling Pathways Connecting Classical and Rare OI Types

Osteogenesis imperfecta (OI) is a phenotypically and genetically heterogeneous skeletal dysplasia characterized by bone fragility, growth deficiency, and skeletal deformity. Previously known to be caused by defects in type I collagen, the major protein of extracellular matrix, it is now also understood to be a collagen-related disorder caused by defects in collagen folding, posttranslational modification and processing, bone mineralization, and osteoblast differentiation, with inheritance of OI types spanning autosomal dominant and recessive as well as X-linked recessive. This review provides the latest updates on OI, encompassing both classical OI and rare forms, their mechanism, and the signaling pathways involved in their pathophysiology. There is a special emphasis on mutations in type I procollagen C-propeptide structure and processing, the later causing OI with strikingly high bone mass. Types V and VI OI, while notably different, are shown to be interrelated by the interferon-induced transmembrane protein 5 p.S40L mutation that reveals the connection between the bone-restricted interferon-induced transmembrane protein-like protein and pigment epithelium-derived factor pathways. The function of regulated intramembrane proteolysis has been extended beyond cholesterol metabolism to bone formation by defects in regulated membrane proteolysis components site-2 protease and old astrocyte specifically induced-substance. Several recently proposed candidate genes for new types of OI are also presented. Discoveries of new OI genes add complexity to already-challenging OI management; current and potential approaches are summarized.

Case report: Early-onset osteoporosis in a patient carrying a novel heterozygous variant of the WNT1 gene

The WNT1 gene is crucial for bone development and homeostasis. Homozygous mutations in WNT1 cause severe bone fragility known as osteogenesis imperfecta type XV. Moreover, heterozygous WNT1 mutations have been found in adults with early-onset osteoporosis. We identified a 35 year-old Caucasian woman who experienced multiple vertebral fractures two months after her second pregnancy. There was no history of risk factors for secondary osteoporosis or family history of osteoporosis. Dual-energy X-ray absorptiometry confirmed a marked reduction of bone mineral density (BMD) at the lumbar spine (0.734 g/cm², Z-score -2.8), femoral neck (0.48 g/cm², Z-score -3.5), and total hip (0.589 g/cm², Z-score -3.0). Blood tests excluded secondary causes of bone fragility. Genetic analysis revealed a heterozygous missense mutation (p.Leu370Val) in the WNT1 gene. Varsome classified it as a variant of uncertain significance. However, the fact that the Leucine residue at position 370 is highly conserved among vertebrate species and the variant has a very low allelic frequency in the general population would exclude the possibility of a polymorphism. The patient was treated for two years with teriparatide therapy associated with calcium and vitamin D supplements. During the follow-up period she did not report further clinical fractures. After 24 months of teriparatide, BMD increased at lumbar spine (+14.6%), femoral neck (+8.3%) and total hip (+4.9%) compared to baseline. We confirm that the heterozygous WNT1 mutation could cause a variable bone fragility and low turnover osteoporosis. We suggest that teriparatide is one of the most appropriate available therapies for this case.

MBTPS2, a membrane bound protease, underlying several distinct skin and bone disorders

The MBTPS2 gene on the X-chromosome encodes the membrane-bound transcription factor protease, site-2 (MBTPS2) or site-2 protease (S2P) which cleaves and activates several signaling and regulatory proteins from the membrane. The MBTPS2 is critical for a myriad of cellular processes, ranging from the regulation of cholesterol homeostasis to unfolded protein responses. While its functional role has become much clearer in the recent years, how mutations in the MBTPS2 gene lead to several human disorders with different phenotypes including Ichthyosis Follicularis, Atrichia and Photophobia syndrome (IFAP) with or without BRESHECK syndrome, Keratosis Follicularis Spinulosa Decalvans (KFSD), Olmsted syndrome, and Osteogenesis Imperfecta type XIX remains obscure. This review presents the biological role of MBTPS2 in development, summarizes its mutations and implicated disorders, and discusses outstanding unanswered questions.

A patient with combined pituitary hormone deficiency and osteogenesis imperfecta associated with mutations in LHX4 and COL1A2

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The mutations in two different genes should be sought in the patients with complex phenotypes.

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The c.1531G>T in COL1A2 leading to OI and c.364C>T (p.R122W) in LHX4 to CPHD were found in a Thai boy.

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The incomplete penetrance and loss-of-function are the features of p.R122W mutation in LHX4.

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The mutation spectra of COL1A2 and LHX4 and pathomechanism of LHX4 are expanded.

Genetic disorders have been shown to co-occur in individual patient. A Thai boy with features of osteogenesis imperfecta (OI) and combined pituitary hormone deficiency (CPHD) was identified. The causative mutations were investigated by whole exome and Sanger sequencing. Pathogenicity and pathomechanism of the variants were studied by luciferase assay. The proband was found to harbor a novel de novo heterozygous missense mutation, c.1531G > T (p.G511C), in COL1A2 leading to OI and a heterozygous missense variant, c.364C > T (p.R122W), in LHX4. The LHX4 p.R122W has never been reported to cause CPHD. The variant was predicted to be deleterious and found in the highly conserved LIM2 domain of LHX4. The luciferase assays revealed that the p.R122W was unable to activate POU1F1, GH1, and TSHB promoters, validating its pathogenic effect in CPHD. Moreover, the variant did not alter the function of wild-type LHX4, indicating its hypomorphic pathomechanism. In conclusion, the novel de novo heterozygous p.G511C mutation in COL1A2 and the heterozygous pathogenic p.R122W mutation in LHX4 were demonstrated in a patient with OI and CPHD. This study proposes that the mutations in two different genes should be sought in the patients with clinical features unable to be explained by a mutation in one gene.

Ptosis as a unique hallmark for autosomal recessive WNT1-associated osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder, mainly characterized by bone fragility and low bone mass. Defects in the type I procollagen-encoding genes account for the majority of OI, but increasingly more rare autosomal recessive (AR) forms are being identified, which are caused by defects in genes involved in collagen metabolism, bone mineralization, or osteoblast differentiation. Bi-allelic mutations in WNT1 have been associated with a rare form of AR OI, characterized by severe osteoporosis, vertebral compression, scoliosis, fractures, short stature, and variable neurological problems. Heterozygous WNT1 mutations have been linked to autosomal dominant early-onset osteoporosis. In this study, we describe the clinical and molecular findings in 10 new patients with AR WNT1-related OI. Thorough revision of the clinical symptoms of these 10 novel patients and previously published AR WNT1 OI cases highlight ptosis as a unique hallmark in the diagnosis of this OI subtype.