Autosomal dominant brachyolmia in a large Swedish family: phenotypic spectrum and natural course

Comparison of the natural course of clinical and radiologic features in 13 patients with TRPV4-related skeletal dysplasias

BACKGROUND

Heterozygous TRPV4 mutations cause a group of skeletal dysplasias characterized by short stature, short trunk, and skeletal deformities.

OBJECTIVE

The aim of this study is to compare the natural history of clinical and radiologic features of patients with different TRPV4-related skeletal dysplasias.

MATERIALS AND METHODS

Thirteen patients with a mutation in TRPV4 were included in the study, and 11 were followed for a median of 6.5 years. The clinical phenotype of five patients was compatible with spondylometaphyseal dysplasia Kozlowski type, three each with metatropic dysplasia and brachyolmia type 3, and one each with spondyloepiphyseal dysplasia Maroteaux type and congenital distal spinal muscular atrophy.

RESULTS

Short stature and bone pain when running, walking, and climbing stairs occurred in patients with spondylometaphyseal dysplasia Kozlowski type and metatropic dysplasia from the age of 5 years and worsened with increasing age. Kyphosis was more pronounced with increasing age in these two groups of patients, while severe scoliosis occurred in brachyolmia type 3. In the radiographs of patients with spondylometaphyseal dysplasia Kozlowski type and metatropic dysplasia, severe platyspondyly persisted into adulthood or puberty. The patients with spondylometaphyseal dysplasia Kozlowski type exhibited irregular proximal femora leading to destruction of the femoral head towards the end of puberty, whereas metatropic dysplasia showed marked irregularity and widening of the femoral neck. We also observed that metaphyseal dysplasia in long bones other than the proximal femur was so inconspicuous that it could be ignored in patients with spondylometaphyseal dysplasia Kozlowski type.

CONCLUSION

Comparison of radiologic features that change with age in five different TRPV4-related skeletal dysplasias will be of great benefit in the management of this patient group.

Roles for TRPV4 in disease: A discussion of possible mechanisms

The transient receptor potential vanilloid 4 (TRPV4) ion channel is a ubiquitously expressed Ca2+-permeable ion channel that controls intracellular calcium ([Ca2+]i) homeostasis in various types of cells. The physiological roles for TRPV4 are tissue specific and the mechanisms behind this specificity remain mostly unclarified. It is noteworthy that mutations in the TRPV4 channel have been associated to a broad spectrum of congenital diseases, with most of these mutations mainly resulting in gain-of-function. Mutations have been identified in human patients showing a variety of phenotypes and symptoms, mostly related to skeletal and neuromuscular disorders. Since TRPV4 is so widely expressed throughout the body, it comes as no surprise that the literature is growing in evidence linking this protein to malfunction in systems other than the skeletal and neuromuscular. In this review, we summarize the expression patterns of TRPV4 in several tissues and highlight findings of recent studies that address critical structural and functional features of this channel, particularly focusing on its interactions and signaling pathways related to Ca2+ entry. Moreover, we discuss the roles of TRPV4 mutations in some diseases and pinpoint some of the mechanisms underlying pathological states where TRPV4's malfunction is prominent.

The newest TRP channelopathy: Gain of function TRPM3 mutations cause epilepsy and intellectual disability

Transient Receptor Potential Melastatin 3 (TRPM3) is a Ca2+ permeable nonselective cation channel, activated by heat and chemical agonists, such as the endogenous neuro-steroid Pregnenolone Sulfate (PregS) and the chemical compound CIM0216. TRPM3 is expressed in peripheral sensory neurons of the dorsal root ganglia (DRG), and its role in noxious heat sensation in mice is well established. TRPM3 is also expressed in a number of other tissues, including the brain, but its role there has been largely unexplored. Recent reports showed that two mutations in TRPM3 are associated with a developmental and epileptic encephalopathy, pointing to an important role of TRPM3 in the human brain. Subsequent reports found that the two disease-associated mutations increased basal channel activity, and sensitivity of the channel to activation by heat and chemical agonists. This review will discuss these mutations in the context of human diseases caused by mutations in other TRP channels, and in the context of the biophysical properties and physiological functions of TRPM3.

Natural history of TRPV4-Related disorders: From skeletal dysplasia to neuromuscular phenotype

TRPV4-related disorders constitute a broad spectrum of clinical phenotypes including several genetic skeletal and neuromuscular disorders, in which clinical variability and somewhat overlapping features are present. These disorders have previously been considered to be clinically distinct phenotypes before their molecular basis was discovered. However, with the identification of TRPV4 variants in the etiology, they are referred as TRPV4-related disorders (TRPV4-pathies), and are now mainly grouped into skeletal dysplasias and neuromuscular disorders. The skeletal dysplasia group includes metatropic dysplasia, parastremmatic dysplasia, spondyloepiphyseal dysplasia Maroteaux type, spondylometaphyseal dysplasia Kozlowski type, autosomal dominant brachyolmia, and familial digital arthropathy-brachydactyly, whereas the neuromuscular group includes congenital distal spinal muscular atrophy (SMA), scapuloperoneal SMA and Charcot-Marie-Tooth neuropathy type 2C with common manifestations of peripheral neuropathy, joint contractures, and respiratory system involvement. Apart from familial digital arthropathy-brachydactyly, skeletal dysplasia associated with TRPV4 pathogenic variants share some clinical features such as short stature with short trunk, spinal and pelvic changes with varying degrees of long bone involvement. Of note, there is considerable phenotypic overlap within and between both groups. Herein, we report on the clinical and molecular spectrum of 11 patients from six different families diagnosed with TRPV4-related disorders. This study yet represents the largest cohort of patients with TRPV4 variants from a single center in Turkey.

Transient Receptor Potential Vanilloid in the Brain Gliovascular Unit: Prospective Targets in Therapy

The gliovascular unit (GVU) is composed of the brain microvascular endothelial cells forming blood–brain barrier and the neighboring surrounding “mural” cells (e.g., pericytes) and astrocytes. Modulation of the GVU/BBB features could be observed in a variety of vascular, immunologic, neuro-psychiatric diseases, and cancers, which can disrupt the brain homeostasis. Ca²⁺ dynamics have been regarded as a major factor in determining BBB/GVU properties, and previous studies have demonstrated the role of transient receptor potential vanilloid (TRPV) channels in modulating Ca²⁺ and BBB/GVU properties. The physiological role of thermosensitive TRPV channels in the BBB/GVU, as well as their possible therapeutic potential as targets in treating brain diseases via preserving the BBB are reviewed. TRPV2 and TRPV4 are the most abundant isoforms in the human BBB, and TRPV2 was evidenced to play a main role in regulating human BBB integrity. Interspecies differences in TRPV2 and TRPV4 BBB expression complicate further preclinical validation. More studies are still needed to better establish the physiopathological TRPV roles such as in astrocytes, vascular smooth muscle cells, and pericytes. The effect of the chronic TRPV modulation should also deserve further studies to evaluate their benefit and innocuity in vivo.

Autosomal dominant brachyolmia: transient metaphyseal striations

We report transient proximal and distal femoral metaphyseal striations that have not previously been described in autosomal dominant brachyolmia. The pelvis/hip radiograph of a 13-year-old boy demonstrated bilaterally symmetrical proximal femoral metaphyseal vertical striations. Additional vertical striations were also observed at the distal femur and proximal tibia metaphysis. Radiography of the thoracolumbar spine demonstrated platyspondyly with irregular endplates and overfaced pedicles. TRPV4 mutations were confirmed in this patient. Similar proximal femoral metaphyseal vertical striations were noted in the patient's sibling. Those streaks disappeared on the follow-up radiographs, and we considered it a unique radiologic finding transiently observed in autosomal dominant brachyolmia.

Autosomal recessive brachyolmia: early radiological findings

Brachyolmia (BO) is a heterogeneous group of skeletal dysplasias with skeletal changes limited to the spine or with minimal extraspinal features. BO is currently classified into types 1, 2, 3, and 4. BO types 1 and 4 are autosomal recessive conditions caused by PAPSS2 mutations, which may be merged together as an autosomal recessive BO (AR-BO). The clinical and radiological signs of AR-BO in late childhood have already been reported; however, the early manifestations and their age-dependent evolution have not been well documented. We report an affected boy with AR-BO, whose skeletal abnormalities were detected in utero and who was followed until 10 years of age. Prenatal ultrasound showed bowing of the legs. In infancy, radiographs showed moderate platyspondyly and dumbbell deformity of the tubular bones. Gradually, the platyspondyly became more pronounced, while the bowing of the legs and dumbbell deformities of the tubular bones diminished with age. In late childhood, the overall findings were consistent with known features of AR-BO. Genetic testing confirmed the diagnosis. Being aware of the initial skeletal changes may facilitate early diagnosis of PAPSS2-related skeletal dysplasias.

Mutations in the latent TGF-beta binding protein 3 (LTBP3) gene cause brachyolmia with amelogenesis imperfecta

Inherited dental malformations constitute a clinically and genetically heterogeneous group of disorders. Here, we report on four families, three of them consanguineous, with an identical phenotype, characterized by significant short stature with brachyolmia and hypoplastic amelogenesis imperfecta (AI) with almost absent enamel. This phenotype was first described in 1996 by Verloes et al. as an autosomal recessive form of brachyolmia associated with AI. Whole-exome sequencing resulted in the identification of recessive hypomorphic mutations including deletion, nonsense and splice mutations, in the LTBP3 gene, which is involved in the TGF-beta signaling pathway. We further investigated gene expression during mouse development and tooth formation. Differentiated ameloblasts synthesizing enamel matrix proteins and odontoblasts expressed the gene. Study of an available knockout mouse model showed that the mutant mice displayed very thin to absent enamel in both incisors and molars, hereby recapitulating the AI phenotype in the human disorder.