Ana Belinda Campos-Xavier

Phenotypic and Molecular Characterization of Bruck Syndrome (Osteogenesis Imperfecta With Contractures of the Large Joints) Caused by a Recessive Mutation in PLOD2

Bruck syndrome (BS) is a recessively‐inherited phenotypic disorder featuring the unusual combination of skeletal changes resembling osteogenesis imperfecta (OI) with congenital contractures of the large joints. Clinical heterogeneity is apparent in cases reported thus far. While the genes coding for collagen 1 chains are unaffected in BS, there is biochemical evidence for a defect in the hydroxylation of lysine residues in collagen 1 telopeptides. One BS locus has been mapped at 17p12, but more recently, two mutations in the lysyl hydroxylase 2 gene (PLOD2, 3q23‐q24) have been identified in BS, showing genetic heterogeneity. The proportion of BS cases linked to 17p22 (BS type 1) or caused by mutations in PLOD2 (BS type 2) is still uncertain, and phenotypic correlations are lacking. We report on a boy who had congenital contractures with pterygia at birth and severe OI‐like osteopenia and multiple fractures. His urine contained high amounts of hydroxyproline but low amounts of collagen crosslinks degradation products; and he was shown to be homozygous for a novel mutation leading to an Arg598His substitution in PLOD2. The mutation is adjacent to the two mutations previously reported (Gly601Val and Thr608Ile), suggesting a functionally important hotspot in PLOD2. The combination of pterygia with bone fragility, as illustrated by this case, is difficult to explain; it suggests that telopeptide lysyl hydroxylation must be involved in prenatal joint formation and morphogenesis. Collagen degradation products in urine and mutation analysis of PLOD2 may be used to diagnose BS and differentiate it from OI.

Mutations in the Heparan-Sulfate Proteoglycan Glypican 6 (GPC6) Impair Endochondral Ossification and Cause Recessive Omodysplasia

Glypicans are a family of glycosylphosphatidylinositol (GPI)-anchored, membrane-bound heparan sulfate (HS) proteoglycans. Their biological roles are only partly understood, although it is assumed that they modulate the activity of HS-binding growth factors. The involvement of glypicans in developmental morphogenesis and growth regulation has been highlighted by Drosophila mutants and by a human overgrowth syndrome with multiple malformations caused by glypican 3 mutations (Simpson-Golabi-Behmel syndrome). We now report that autosomal-recessive omodysplasia, a genetic condition characterized by short-limbed short stature, craniofacial dysmorphism, and variable developmental delay, maps to chromosome 13 (13q31.1-q32.2) and is caused by point mutations or by larger genomic rearrangements in glypican 6 (GPC6). All mutations cause truncation of the GPC6 protein and abolish both the HS-binding site and the GPI-bearing membrane-associated domain, and thus loss of function is predicted. Expression studies in microdissected mouse growth plate revealed expression of Gpc6 in proliferative chondrocytes. Thus, GPC6 seems to have a previously unsuspected role in endochondral ossification and skeletal growth, and its functional abrogation results in a short-limb phenotype.

Chondrodysplasia and Abnormal Joint Development Associated with Mutations in IMPAD1, Encoding the Golgi-Resident Nucleotide Phosphatase, gPAPP

We used whole-exome sequencing to study three individuals with a distinct condition characterized by short stature, chondrodysplasia with brachydactyly, congenital joint dislocations, cleft palate, and facial dysmorphism. Affected individuals carried homozygous missense mutations in IMPAD1, the gene coding for gPAPP, a Golgi-resident nucleotide phosphatase that hydrolyzes phosphoadenosine phosphate (PAP), the byproduct of sulfotransferase reactions, to AMP. The mutations affected residues in or adjacent to the phosphatase active site and are predicted to impair enzyme activity. A fourth unrelated patient was subsequently found to be homozygous for a premature termination codon in IMPAD1. Impad1 inactivation in mice has previously been shown to produce chondrodysplasia with abnormal joint formation and impaired proteoglycan sulfation. The human chondrodysplasia associated with gPAPP deficiency joins a growing number of skeletoarticular conditions associated with defective synthesis of sulfated proteoglycans, highlighting the importance of proteoglycans in the development of skeletal elements and joints.

The diagnostic challenge of progressive pseudorheumatoid dysplasia (PPRD): A review of clinical features, radiographic features, and WISP3 mutations in 63 affected individuals

Progressive pseudorheumatoid dysplasia (PPRD) is a genetic, non‐inflammatory arthropathy caused by recessive loss of function mutations in WISP3 (Wnt1‐inducible signaling pathway protein 3; MIM 603400), encoding for a signaling protein. The disease is clinically silent at birth and in infancy. It manifests between the age of 3 and 6 years with joint pain and progressive joint stiffness. Affected children are referred to pediatric rheumatologists and orthopedic surgeons; however, signs of inflammation are absent and anti‐inflammatory treatment is of little help. Bony enlargement at the interphalangeal joints progresses leading to camptodactyly. Spine involvement develops in late childhood and adolescence leading to short trunk with thoracolumbar kyphosis. Adult height is usually below the 3rd percentile. Radiographic signs are relatively mild. Platyspondyly develops in late childhood and can be the first clue to the diagnosis. Enlargement of the phalangeal metaphyses develops subtly and is usually recognizable by 10 years. The femoral heads are large and the acetabulum forms a distinct “lip” overriding the femoral head. There is a progressive narrowing of all articular spaces as articular cartilage is lost. Medical management of PPRD remains symptomatic and relies on pain medication. Hip joint replacement surgery in early adulthood is effective in reducing pain and maintaining mobility and can be recommended. Subsequent knee joint replacement is a further option. Mutation analysis of WISP3 allowed the confirmation of the diagnosis in 63 out of 64 typical cases in our series. Intronic mutations in WISP3 leading to splicing aberrations can be detected only in cDNA from fibroblasts and therefore a skin biopsy is indicated when genomic analysis fails to reveal mutations in individuals with otherwise typical signs and symptoms. In spite of the first symptoms appearing in early childhood, the diagnosis of PPRD is most often made only in the second decade and affected children often receive unnecessary anti‐inflammatory and immunosuppressive treatments. Increasing awareness of PPRD appears to be essential to allow for a timely diagnosis.

A novel mutation in the sulfate transporter gene SLC26A2 (DTDST) specific to the Finnish population causes de la Chapelle dysplasia

Background: Mutations in the sulfate transporter gene SLC26A2 (DTDST) cause a continuum of skeletal dysplasia phenotypes that includes achondrogenesis type 1B (ACG1B), atelosteogenesis type 2 (AO2), diastrophic dysplasia (DTD), and recessive multiple epiphyseal dysplasia (rMED). In 1972, de la Chapelle et al reported two siblings with a lethal skeletal dysplasia, which was denoted “neonatal osseous dysplasia” and “de la Chapelle dysplasia” (DLCD). It was suggested that DLCD might be part of the SLC26A2 spectrum of phenotypes, both because of the Finnish origin of the original family and of radiographic similarities to ACG1B and AO2. Objective: To test the hypothesis whether SLC26A2 mutations are responsible for DLCD. Methods: We studied the DNA from the original DLCD family and from seven Finnish DTD patients in whom we had identified only one copy of IVS1+2T>C, the common Finnish mutation. A novel SLC26A2 mutation was found in all subjects, inserted by site-directed mutagenesis in a vector harbouring the SLC26A2 cDNA, and expressed in sulfate transport deficient Chinese hamster ovary (CHO) cells to measure sulfate uptake activity. Results: We identified a hitherto undescribed SLC26A2 mutation, T512K, homozygous in the affected subjects and heterozygous in both parents and in the unaffected sister. T512K was then identified as second pathogenic allele in the seven Finnish DTD subjects. Expression studies confirmed pathogenicity. Conclusions: DLCD is indeed allelic to the other SLC26A2 disorders. T512K is a second rare “Finnish” mutation that results in DLCD at homozygosity and in DTD when compounded with the milder, common Finnish mutation.

Post-axial polydactyly type A2, overgrowth and autistic traits associated with a chromosome 13q31.3 microduplication encompassing miR-17-92 and GPC5.

Genomic rearrangements at chromosome 13q31.3q32.1 have been associated with digital anomalies, dysmorphic features, and variable degree of mental disability. Microdeletions leading to haploinsufficiency of miR17∼92, a cluster of micro RNA genes closely linked to GPC5 in both mouse and human genomes, has recently been associated with digital anomalies in the Feingold like syndrome. Here, we report on a boy with familial dominant post-axial polydactyly (PAP) type A, overgrowth, significant facial dysmorphisms and autistic traits who carries the smallest germline microduplication known so far in that region. The microduplication encompasses the whole miR17∼92 cluster and the first 5 exons of GPC5. This report supports the newly recognized role of miR17∼92 gene dosage in digital developmental anomalies, and suggests a possible role of GPC5 in growth regulation and in cognitive development.

Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia†

Spondylo‐megaepiphyseal‐metaphyseal dysplasia (SMMD; OMIM 613330) is a dysostosis/dysplasia caused by recessive mutations in the homeobox‐containing gene, NKX3‐2 (formerly known as BAPX1). Because of the rarity of the condition, its diagnostic features and natural course are not well known. We describe clinical and radiographic findings in six patients (five of which with homozygous mutations in the NKX3‐2 gene) and highlight the unusual and severe changes in the cervical spine and the neurologic complications. In individuals with SMMD, the trunk and the neck are short, while the limbs, fingers and toes are disproportionately long. Radiographs show a severe ossification delay of the vertebral bodies with sagittal and coronal clefts, missing ossification of the pubic bones, large round “balloon‐like” epiphyses of the long bones, and presence of multiple pseudoepiphyses at all metacarpals and phalanges. Reduced or absent ossification of the cervical vertebrae leads to cervical instability with anterior or posterior kinking of the cervical spine (swan neck‐like deformity, kyknodysostosis). As a result of the cervical spine instability or deformation, five of six patients in our series suffered cervical cord injury that manifested clinically as limb spasticity. Although the number of individuals observed is small, the high incidence of cervical spine deformation in SMMD is unique among skeletal dysplasias. Early diagnosis of SMMD by recognition of the radiographic pattern might prevent of the neurologic complications via prophylactic cervical spine stabilization.

MMP13 mutations are the cause of recessive metaphyseal dysplasia, Spahr type

Metaphyseal dysplasia, Spahr type (MDST; OMIM 250400) was described in 1961 based on the observation of four children in one family who had rickets‐like metaphyseal changes but normal blood chemistry and moderate short stature. Its molecular basis and nosologic status remained unknown. We followed up on those individuals and diagnosed the disorder in an additional member of the family. We used exome sequencing to ascertain the underlying mutation and explored its consequences on three‐dimensional models of the affected protein. The MDST phenotype is associated with moderate short stature and knee pain in adults, while extra‐skeletal complications are not observed. The sequencing showed that MDST segregated with a c.619T>G single nucleotide transversion in MMP13. The predicted non‐conservative amino acid substitution, p.Trp207Gly, disrupts a crucial hydrogen bond in the calcium‐binding region of the catalytic domain of the matrix metalloproteinase, MMP13. The MDST phenotype is associated with recessive MMP13 mutations, confirming the importance of this metalloproteinase in the metaphyseal growth plate. Dominant MMP13 mutations have been associated with metaphyseal anadysplasia (OMIM 602111), while a single child homozygous for a MMP13 mutation had been previously diagnosed as “recessive metaphyseal anadysplasia,” that we conclude is the same nosologic entity as MDST. Molecular confirmation of MDST allows distinction of it from dominant conditions (e.g., metaphyseal dysplasia, Schmid type; OMIM # 156500) and from more severe multi‐system conditions (such as cartilage‐hair hypoplasia; OMIM # 250250) and to give precise recurrence risks and prognosis.

When should clinicians search for GLUT1 deficiency syndrome in childhood generalized epilepsies

UNLABELLED GLUT1 deficiency (GLUT1D) has recently been identified as an important cause of generalized epilepsies in childhood. As it is a treatable condition, it is crucial to determine which patients should be investigated. METHODS We analyzed SLC2A1 for mutations in a group of 93 unrelated children with generalized epilepsies. Fasting lumbar puncture was performed following the identification of a mutation. We compared our results with a systematic review of 7 publications of series of patients with generalized epilepsies screened for SLC2A1 mutations. RESULTS We found 2/93 (2.1%) patients with a SLC2A1 mutation. One, carrying a novel de novo deletion had epilepsy with myoclonic-atonic seizures (MAE), mild slowing of head growth, choreiform movements and developmental delay. The other, with a paternally inherited missense mutation, had childhood absence epilepsy with atypical EEG features and paroxysmal exercise-induced dyskinesia (PED) initially misdiagnosed as myoclonic seizures. Out of a total of 1110 screened patients with generalized epilepsies from 7 studies, 2.4% (29/1110) had GLUT1D. This rate was higher (5.6%) among 303 patients with early onset absence epilepsy (EOAE) from 4 studies. About 50% of GLUT1D patients had abnormal movements and 41% a family history of seizures, abnormal movements or both. CONCLUSION GLUT1D is most likely to be found in MAE and in EOAE. The probability of finding GLUT1D in the classical idiopathic generalized epilepsies is very low. Pointers to GLUT1D include an increase in seizures before meals, cognitive impairment, or PED which can easily be overlooked.

Corner fracture type spondylometaphyseal dysplasia: Overlap with type II collagenopathies

Spondylometaphyseal dysplasia (SMD) corner fracture type (also known as SMD “Sutcliffe” type, MIM 184255) is a rare skeletal dysplasia that presents with mild to moderate short stature, developmental coxa vara, mild platyspondyly, corner fracture‐like lesions, and metaphyseal abnormalities with sparing of the epiphyses. The molecular basis for this disorder has yet to be clarified. We describe two patients with SMD corner fracture type and heterozygous pathogenic variants in COL2A1. These two cases together with a third case of SMD corner fracture type with a heterozygous COL2A1 pathogenic variant previously described suggest that this disorder overlaps with type II collagenopathies. The finding of one of the pathogenic variants in a previously reported case of spondyloepimetaphyseal dysplasia (SEMD) Strudwick type and the significant clinical similarity suggest an overlap between SMD corner fracture and SEMD Strudwick types.