Fetal akinesia in metatropic dysplasia: The combined phenotype of chondrodysplasia and neuropathy?

Dominant mutations in the receptor calcium channel gene TRPV4 have been associated with a family of skeletal dysplasias (metatropic dysplasia, pseudo-Morquio type 2, spondylometaphyseal dysplasia, Kozlowski type, brachyolmia, and familial digital arthropathy) as well as with dominantly inherited neuropathies (hereditary motor and sensory neuropathy 2C, scapuloperoneal spinal muscular atrophy, and congenital distal spinal muscular atrophy). While there is phenotypic overlap between the various members of each group, the two groups were considered to be totally separate with the former being strictly a structural skeletal condition and the latter group being confined to the peripheral nervous system. We report here on fetal akinesia as the presenting feature of severe metatropic dysplasia, suggesting that certain TRPV4 mutations can cause both a skeletal and a neuropathic phenotype. Three cases were detected on prenatal ultrasound because of absent movements in the second trimester. Case 4 presented with multiple joint contractures and absent limb movements at birth and was diagnosed with "fetal akinesia syndrome". Post-interruption and post-natal X-rays showed typical features of metatropic dysplasia in all four. Sequencing of the TRPV4 gene confirmed the presence of de novo heterozygous mutations predicting G78W (Case 1), T740I (Cases 2 and 3), and K276E (Case 4). Although some degree of restriction of movements is not uncommon in fetuses with skeletal dysplasia, akinesia as leading sign is unusual and suggests that certain TRPV4 mutations produce both chondrodysplasia and a peripheral neuropathy resulting in a severe "overlap" phenotype.     

Don't change that (calcium) channel: mutations in the same calcium channel gene can cause multiple distinct phenotypes

Scapuloperoneal spinal muscular atrophy and CMT2C are allelic disorders caused by alterations in TRPV4 Deng et al. (2010) Nature Genetics 42(2):165–169 Mutations in TRPV4 cause Charcot–Marie–Tooth disease type 2C Landouré et al. (2010) Nature Genetics 42(2):170–174 Alterations in the ankyrin domain of TRPV4 cause congenital distal SMA, scapuloperoneal SMA and HMSN2C Auer‐Grumbach et al. (2010) Nature Genetics 42(2):160–164     

X‐linked spinal muscular atrophy in mice caused by autonomous loss of ATP7A in the motor neuron

ATP7A is a copper‐transporting P‐type ATPase that is essential for cellular copper homeostasis. Loss‐of‐function mutations in the ATP7A gene result in Menkes disease, a fatal neurodegenerative disorder resulting in seizures, hypotonia and failure to thrive, due to systemic copper deficiency. Most recently, rare missense mutations in ATP7A that do not impact systemic copper homeostasis have been shown to cause X‐linked spinal muscular atrophy type 3 (SMAX3), a distal hereditary motor neuropathy. An understanding of the mechanistic and pathophysiological basis of SMAX3 is currently lacking, in part because the disease‐causing mutations have been shown to confer both loss‐ and gain‐of‐function properties to ATP7A, and because there is currently no animal model of the disease. In this study, the Atp7a gene was specifically deleted in the motor neurons of mice, resulting in a degenerative phenotype consistent with the clinical features in affected patients with SMAX3, including the progressive deterioration of gait, age‐dependent muscle atrophy, denervation of neuromuscular junctions and a loss of motor neuron cell bodies. Taken together, these data reveal autonomous requirements for ATP7A that reveal essential roles for copper in the maintenance and function of the motor neuron, and suggest that SMAX3 is caused by a loss of ATP7A function that specifically impacts the spinal motor neuron. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

The phenotypic spectrum of COL2A1 mutations

Heterozygous mutations of COL2A1 create several clinical entities collectively termed type II collagenopathies. These disorders not only impair skeletal growth but also cause ocular and otolaryngological abnormalities. The classical phenotypes include the spondyloepiphyseal dysplasia (SED) spectrum with variable severity, Stickler dysplasia type I (STD-I), and Kniest dysplasia (KND). Most COL2A1 mutations occur in the triple helical region of alpha 1(II) chains: the SED spectrum is mostly attributed to missense mutations that substitute bulky amino acids for glycine residues, STD-I to haploinsufficiency of truncation mutations, and KND to exon skipping due to splice-site mutations. To further elucidate the genotype-phenotype relationship of type II collagenopathies, we examined COL2A1 mutations in 56 families that were suspected of having type II collagenopathies, and found 38 mutations in 41 families. Phenotypes for all 22 missense mutations and one in-frame deletion in the triple helical region fell along the SED spectrum. Glycine to serine substitutions resulted in alternating zones that produce severer and milder skeletal phenotypes. Glycine to nonserine residue substitutions exclusively created more severe phenotypes. The gradient of the SED spectrum did not necessarily correlate with the occurrence of extraskeletal manifestations. All nine truncation or splice-site mutations in the triple helical or N-propeptide region caused STD-I or KND, and extraskeletal changes were inevitable in both phenotypes. All six C-propeptide mutations produced a range of atypical skeletal phenotypes and created ocular, but not otolaryngological, changes.     

Dominant negative mutations in the C-propeptide of COL2A1 cause platyspondylic lethal skeletal dysplasia, torrance type, and define a novel subfamily within the type 2 collagenopathies

Platyspondylic lethal skeletal dysplasia (PLSD) Torrance type (PLSD-T) is a rare skeletal dysplasia characterized by platyspondyly, brachydactyly, and metaphyseal changes. Generally a perinatally lethal disease, a few long-term survivors have been reported. Recently, mutations in the carboxy-propeptide of type II collagen have been identified in two patients with PLSD-T, indicating that PLSD-T is a type 2 collagen-associated disorder. We studied eight additional cases of PLSD-T and found that all had mutations in the C-propeptide domain of COL2A1. The mutational spectrum includes missense, stop codon and frameshift mutations. All non-sense mutations were located in the last exon, where they would escape non-sense-mediated RNA-decay. We conclude that PLSD-T is caused by mutations in the C-propeptide domain of COL2A1, which lead to biosynthesis of an altered collagen chain (as opposed to a null allele). Similar mutations have recently been found to be the cause of spondyloperipheral dysplasia, a non-lethal dominant disorder whose clinical and radiographical features overlap those of the rare long-term survivors with PLSD-T. Thus, spondyloperipheral dysplasia and PLSD-T constitute a novel subfamily within the type II collagenopathies, associated with specific mutations in the C-propeptide domain and characterized by distinctive radiological features including metaphyseal changes and brachydactyly that set them apart from other type 2 collagenopathies associated with mutations in the triple-helical domain of COL2A1. The specific phenotype of C-propeptide mutations could result from a combination of diminished collagen fibril formation, toxic effects through the accumulation of unfolded collagen chains inside the chondrocytes, and alteration of a putative signaling function of the carboxy-propeptide of type 2 collagen.     

Compound heterozygous variants in MAPK8IP3 were detected in severe congenital hypotonia mimicking lethal spinal muscular atrophy

Mitogen-activated protein kinase 8-interacting protein 3 gene (MAPK8IP3) encodes the c-Jun-amino-terminal kinase-interacting protein 3 (JIP3) and is involved in retrograde axonal transport. Heterozygous de novo pathogenic variants in MAPK8IP3 result in a neurodevelopmental disorder with or without brain abnormalities and possible axonal peripheral neuropathy. Whole-exome sequencing was performed on an individual presenting with severe congenital muscle hypotonia of neuronal origin mimicking lethal spinal muscular atrophy. Compound heterozygous rare variants (a splice and a missense) were detected in MAPK8IP3, inherited from the healthy parents. Western blot analysis in a muscle biopsy sample showed a more than 60% decrease in JIP3 expression. Here, we suggest a novel autosomal recessive phenotype of a lower motor neuron disease caused by JIP3 deficiency.     

SMD Kozlowski type caused by p.Arg594His substitution in TRPV4 reveals abnormal ossification and notochordal remnants in discs and vertebrae

Spondylometaphyseal dysplasia Kozlowski type (SMDK) is a monogenic disorder within the TRPV4 dysplasia spectrum and has characteristic spinal and metaphyseal changes. We report skeletal MR imaging in a two-year-old patient who manifested typical clinical and radiographic features of SMDK. The diagnosis was confirmed by molecular analysis which revealed a mutation NM_021625.4:c.1781G > A - p.(Arg594His) in exon 11 of the TRPV4 gene. We have documented abnormalities in endochondral formation of the long and short tubular bones as well as round bones of the wrists and feet. The vertebral bodies had increased thickness of hyaline cartilage which enveloped ossification centers. The vertebrae and discs also had abnormalities in size, shape and structure. These anomalies were most likely the consequence of notochordal remnants presence within the intervertebral discs and in the vertebral bodies. The advantages of MR imaging in bone dysplasias caused by TRPV4 mutations are emphasized in this article.     

De Novo Mutations in the Motor Domain of KIF1A Cause Cognitive Impairment,Spastic Paraparesis,Axonal Neuropathy,and Cerebellar Atrophy

KIF1A is a neuron‐specific motor protein that plays important roles in cargo transport along neurites. Recessive mutations in KIF1A were previously described in families with spastic paraparesis or sensory and autonomic neuropathy type‐2. Here, we report 11 heterozygous de novo missense mutations (p.S58L, p.T99M, p.G102D, p.V144F, p.R167C, p.A202P, p.S215R, p.R216P, p.L249Q, p.E253K, and p.R316W) in KIF1A in 14 individuals, including two monozygotic twins. Two mutations (p.T99M and p.E253K) were recurrent, each being found in unrelated cases. All these de novo mutations are located in the motor domain (MD) of KIF1A. Structural modeling revealed that they alter conserved residues that are critical for the structure and function of the MD. Transfection studies suggested that at least five of these mutations affect the transport of the MD along axons. Individuals with de novo mutations in KIF1A display a phenotype characterized by cognitive impairment and variable presence of cerebellar atrophy, spastic paraparesis, optic nerve atrophy, peripheral neuropathy, and epilepsy. Our findings thus indicate that de novo missense mutations in the MD of KIF1A cause a phenotype that overlaps with, while being more severe, than that associated with recessive mutations in the same gene.     

Mutation in The Nuclear‐Encoded Mitochondrial Isoleucyl–tRNA Synthetase IARS2 in Patients with Cataracts,Growth Hormone Deficiency with Short Stature,Partial Sensorineural Deafness,and Peripheral Neuropathy or with Leigh Syndrome

Mutations in the nuclear‐encoded mitochondrial aminoacyl–tRNA synthetases are associated with a range of clinical phenotypes. Here, we report a novel disorder in three adult patients with a phenotype including cataracts, short‐stature secondary to growth hormone deficiency, sensorineural hearing deficit, peripheral sensory neuropathy, and skeletal dysplasia. Using SNP genotyping and whole‐exome sequencing, we identified a single likely causal variant, a missense mutation in a conserved residue of the nuclear gene IARS2, encoding mitochondrial isoleucyl–tRNA synthetase. The mutation is homozygous in the affected patients, heterozygous in carriers, and absent in control chromosomes. IARS2 protein level was reduced in skin cells cultured from one of the patients, consistent with a pathogenic effect of the mutation. Compound heterozygous mutations in IARS2 were independently identified in a previously unreported patient with a more severe mitochondrial phenotype diagnosed as Leigh syndrome. This is the first report of clinical findings associated with IARS2 mutations.     

Expansion of the clinical spectrum of frontometaphyseal dysplasia 2 caused by the recurrent mutation p.Pro485Leu in MAP3K7

Frontometaphyseal dysplasia 2 (FMD2) is a skeletal dysplasia with supraorbital hyperostosis combined with undermodeling of the bones, joint contractures and some extraskeletal features. It is caused by heterozygous mutations in MAP3K7, encoding the Mitogen-Activated Protein 3-Kinase 7. MAP3K7 is activated by TGF-β and plays an important role in osteogenesis. Less than 20 patients with FMD2 and MAP3K7 mutations have been described thus far. The majority of the patients harbor a recurrent missense mutation, NM_003188.3: c.1454C?>?T [NP_003179.1: p.(Pro485Leu)], which leads to a more severe phenotype than mutations in other domains. Here we describe an additional patient with FMD2 caused by the recurrent c.1454C?>?T MAP3K7 mutation, identified as a de novo variant by whole-genome sequencing. The 17-year-old boy has the characteristic skeletal and facial features of FMD2. However, some novel features were also observed, including growth retardation and spina bifida occulta. In line with other patients harboring the same mutation he also showed keloid scars and had no intellectual disability. This report expands the clinical spectrum of FMD2 caused by the recurrent c.1454C?>?T [p.(Pro485Leu)] mutation in MAP3K7.     

GRS defective axonal distribution as a potential contributor to distal spinal muscular atrophy type V pathogenesis in a new model of GRS-associated neuropathy

Distal spinal muscular atrophy type V (dSMA-V), a hereditary axonal neuropathy, is a glycyl-tRNA synthetase (GRS)-associated neuropathy caused by a mutation in GRS. In this study, using an adenovirus vector system equipped with a neuron-specific promoter, we constructed a new GRS-associated neuropathy mouse model. We found that wild-type GRS (WT) is distributed in peripheral axons, dorsal root ganglion (DRG) cell bodies, central axon terminals and motor neuron cell bodies in the mouse model. In contrast, the L129P mutant GRS was localized in DRG and motor neuron cell bodies. Thus, we propose that the disease-causing L129P mutant is linked to a distribution defect in peripheral nerves in vivo.     

Zinc metalloproteinase,ZMPSTE24, is mutated in mandibuloacral dysplasia

Mandibuloacral dysplasia (MAD; OMIM 248370) is a rare, genetically and phenotypically heterogeneous, autosomal recessive disorder characterized by skeletal abnormalities including hypoplasia of the mandible and clavicles, acro-osteolysis, cutaneous atrophy and lipodystrophy. A homozygous missense mutation, Arg527His, in the LMNA gene which encodes nuclear lamina proteins lamins A and C has been reported in patients with MAD and partial lipodystrophy. We studied four patients with MAD who had no mutations in the LMNA gene. We now show compound heterozygous mutations, Phe361fsX379 and Trp340Arg, in the zinc metalloproteinase (ZMPSTE24) gene in one of the four patients who had severe MAD associated with progeroid appearance and generalized lipodystrophy. ZMPSTE24 is involved in post-translational proteolytic cleavage of carboxy terminal residues of farnesylated prelamin A in two steps to form mature lamin A. Deficiency of Zmpste24 in mice causes accumulation of prelamin A and phenotypic features similar to MAD. The yeast homolog, Ste24, has a parallel role in processing of prenylated mating pheromone a-factor. Since human ZMPSTE24 can also process a-factor when expressed in yeast, we assessed the functional significance of the two ZMPSTE24 mutations in the yeast to complement the mating defect of the haploid MATa yeast lacking STE24 and Ras-converting enzyme 1 (RCE1; another prenylprotein-specific endoprotease) genes. The ZMPSTE24 mutant construct, Phe361fsX379, was inactive in complementing the yeast a-factor but the mutant, Trp340Arg, was partially active compared to the wild type ZMPSTE24 construct. We conclude that mutations in ZMPSTE24 may cause MAD by affecting prelamin A processing.     

Frontometaphyseal dysplasia: mutations in FLNA and phenotypic diversity

Frontometaphyseal dysplasia is an X-linked trait primarily characterized by a skeletal dysplasia comprising hyperostosis of the skull and modeling anomalies of the tubular bones. Extraskeletal features include tracheobronchial, cardiac, and urological malformations. A proportion of individuals have missense mutations or small deletions in the X-linked gene, FLNA. We report here our experience with comprehensive screening of the FLNA gene in a group of 23 unrelated probands (11 familial instances, 12 simplex cases; total affected individuals 32) with FMD. We found missense mutations leading to substitutions in the actin-binding domain and within filamin repeats 9, 10, 14, 16, 22, and 23 of filamin A in 13/23 (57%) of individuals in this cohort. Some mutations present with a male phenotype that is characterized by a severe skeletal dysplasia, cardiac, and genitourinary malformations that leads to perinatal death. Although no phenotypic feature consistently discriminates between females with FMD who are heterozygous for FLNA mutations and those in whom no FLNA mutation can be identified, there is a difference in the degree of skewing of X-inactivation between these two groups. This observation suggests that locus heterogeneity may exist for this disorder.     

Autosomal Recessive Multiple Epiphyseal Dysplasia in a Korean Girl Caused by Novel Compound Heterozygous Mutations in the DTDST (SLC26A2) Gene

Multiple epiphyseal dysplasia is caused by heterogenous genotypes involving more than six genes. Recessive mutations in the DTDST gene cause a phenotype of recessive multiple epiphyseal dysplasia (rMED). The authors report a 9-yr old Korean girl with the rMED phenotype having novel compound heterozygous mutations in the DTDST gene, which were inherited from both parents. This is the first Korean rMED case attributed to DTDST mutations, and expands the spectrum of diseases caused by DTDST mutations.     

SLC26A2 disease spectrum in Sweden – high frequency of recessive multiple epiphyseal dysplasia (rMED)

Diastrophic dysplasia (DTD) is an autosomal recessive skeletal dysplasia caused by SLC26A2 mutations. Clinical features include short stature, joint contractures, spinal deformities, and cleft palate. SLC26A2 mutations also result in other skeletal dysplasias, including the milder recessive multiple epiphyseal dysplasia (rMED). DTD is overrepresented in Finland and we speculated that this may have influenced the prevalence and spectrum of SLC26A2‐related skeletal conditions also in Sweden. We reviewed the patient registry at Department of Clinical Genetics, Karolinska University Hospital, Stockholm to identify subjects with SLC26A2 mutations. Seven patients from six families were identified; clinical data were available for six patients. All but one patient had one or two copies of the Finnish SLC26A2 founder mutation IVS1+2T>C. Arg279Trp mutation was present in compound heterozygous form in five patients with phenotypes consistent with rMED. Their heights ranged from ?2.6 to ?1.4 standard deviation units below normal mean and radiographic features included generalised epiphyseal dysplasia and double‐layered patellae. Two rMED patients had hypoplastic C2 and cervical kyphosis, a severe manifestation previously described only in DTD. Our study confirms a high prevalence of rMED in Sweden and expands the phenotypic manifestations of rMED.     

Perspectives and diagnostic considerations in spinal muscular atrophy

Spinal muscular atrophy is an autosomal recessive neurodegenerative disease and the most common genetic cause of infant mortality. The disease results in motor neuron loss and skeletal muscle atrophy. Spinal muscular atrophy is caused by mutations in the telomeric copy of the survival motor neuron 1 (SMN1) gene, but all patients retain a centromeric copy of the gene, SMN2. In the majority of cases, the disease severity correlates inversely with an increased SMN2 gene copy number. Because spinal muscular atrophy is both a severe and common disorder, a direct carrier testing has been beneficial to many families. The survival motor neuron protein is ubiquitously expressed and performs a role in the assembly of the spliceosome. It is still not understood why mutations in the SMN1 gene only seem to affect motor neurons. Progress has been made by developing therapeutic strategies based on understanding the pathogenesis of the disease. This review attempts to highlight some of the recent advances in the understanding of the disease with a focus on molecular diagnostics.     

TRPV6 compound heterozygous variants result in impaired placental calcium transport and severe undermineralization and dysplasia of the fetal skeleton

Transient receptor potential vanilloid 6 (TRPV6) functions in tetramer form for calcium transport. Until now, TRPV6 has not been linked with skeletal development disorders. An infant with antenatal onset thoracic insufficiency required significant ventilatory support. Skeletal survey showed generalized marked undermineralization, hypoplastic fractured ribs, metaphyseal fractures, and extensive periosteal reaction along femoral, tibial, and humeral diaphyses. Parathyroid hormone (PTH) elevation (53.4–101 pmol/L) initially suggested PTH signaling disorders. Progressively, biochemical normalization with radiological mineralization suggested recovery from in utero pathophysiology. Genomic testing was undertaken and in silico protein modeling of variants. No abnormalities in antenatal CGH array or UPD14 testing. Postnatal molecular genetic analysis found no causative variants in CASR, GNA11, APS21, or a 336 gene skeletal dysplasia panel investigated by whole exome sequencing. Trio exome analysis identified compound heterozygous TRPV6 likely pathogenic variants: novel maternally inherited missense variant, c.1978G > C p.(Gly660Arg), and paternally inherited nonsense variant, c.1528C > T p.(Arg510Ter), confirming recessive inheritance. p.(Gly660Arg) generates a large side chain protruding from the C‐terminal hook into the interface with the adjacent TRPV6 subunit. In silico protein modeling suggests steric clashes between interface residues, decreased C‐terminal hook, and TRPV6 tetramer stability. The p.(Gly660Arg) variant is predicted to result in profound loss of TRPV6 activity. This first case of a novel dysplasia features severe but improving perinatal abnormalities. The TRPV6 compound heterozygous variants appear likely to interfere with fetoplacental calcium transfer crucial for in utero skeletal development. Astute clinical interpretation of evolving perinatal abnormalities remains valuable in complex calcium and bone pathophysiology and informs exome sequencing interpretation.     

Gene therapy for peripheral nervous system diseases

Peripheral nerve diseases, also known as peripheral neuropathies, affect 15-20 million of Americans and diabetic neuropathy is the most common condition. Currently, the treatment of peripheral neuropathies is more focused on managing pain rather than providing permissive conditions for regeneration. Despite advances in microsurgical techniques, including nerve grafting and reanastomosis, axonal regeneration after peripheral nerve injury remains suboptimal. Also, no satisfactory treatments are available at this time for peripheral neurodegeneration occurring in motor neuron diseases (MND), including amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). Peripheral nerves have the inherent capacity of regeneration. Gene therapy strategies focused on neuroprotection may help optimizing axonal regrowth. A better understanding of the cellular and molecular events involved in axonal degeneration and regeneration have helped researchers to identify targets for intervention. This review summarizes the current state on the clinical experience as well as gene therapy strategies for peripheral neuropathies, including MND, peripheral nerve injury, neuropathic pain, and diabetic neuropathy.