Dyggve-Melchior-Clausen syndrome: novel splice mutation with atlanto-axial subluxation

A Novel Homozygous Nonsense Variant in the DYM Underlies Dyggve-Melchior-Clausen Syndrome in Large Consanguineous Family

(1) Background: Dyggve-Melchior-Clausen Syndrome is a skeletal dysplasia caused by a defect in the DYM gene (OMIM number 607461). Pathogenic variants in the gene have been reported to cause Dyggve-Melchior-Clausen (DMC; OMIM 223800) dysplasia and Smith-McCort (SMC; OMIM 607326) dysplasia. (2) Methods: In the present study, large consanguineous families with five affected individuals with osteochondrodysplasia phenotypes were recruited. The family members were analyzed by polymerase chain reaction for homozygosity mapping using highly polymorphic microsatellite markers. Subsequent to linkage analysis, the coding exons and exon intron border of the DYM gene were amplified. The amplified products were then sent for Sanger sequencing. The structural effect of the pathogenic variant was analyzed by different bioinformatics tools. (3) Results: Homozygosity mapping revealed a 9 Mb homozygous region on chromosome 18q21.1 harboring DYM shared by all available affected individuals. Sanger sequencing of the coding exons and exon intron borders of the DYM gene revealed a novel homozygous nonsense variant [DYM (NM_017653.6):c.1205T>A, p.(Leu402Ter)] in affected individuals. All the available unaffected individuals were either heterozygous or wild type for the identified variant. The identified mutation results in loss of protein stability and weekend interactions with other proteins making them pathogenic (4) Conclusions: This is the second nonsense mutation reported in a Pakistani population causing DMC. The study presented would be helpful in prenatal screening, genetic counseling, and carrier testing of other members in the Pakistani community.

Dyggve-Melchior-Clausen Syndrome Caused by a Novel Frameshift Variant in a Japanese Patient

Introduction

Dyggve-Melchior-Clausen syndrome (DMC) is a rare autosomal recessive spondyloepimetaphyseal dysplasia characterized by short stature, microcephaly, intellectual disability, and coarse face. This disorder is caused by pathogenic/likely pathogenic variants of the DYM gene which encodes dymeclin.

Case Presentation

Herein, we report a 60-year-old Japanese man who was born to consanguineous parents. He presented with abdominal distention and rectal prolapse in addition to the common features of DMC. We identified a novel homozygous frameshift variant [c.1670delT, p.(Leu557Argfs*20)] in the DYM gene, which introduces a premature stop codon. Histological analysis revealed disarrangement of actin filaments in cultured fibroblasts.

Discussion

To the best of our knowledge, this is the first Japanese case of DMC with a confirmed variant in the DYM gene. This report provides more information about the geographic distribution and phenotypic spectrum of DMC. Moreover, it presents a novel DYM variant and insights about DMC pathology that may be associated with the disarrangement of actin filaments.

Seven patients with Smith-McCort dysplasia 2: Four novel nonsense variants in RAB33B and follow-up findings

Smith-McCort dysplasia 2 (SMC2) is a rare spondylo-epiphyseal-metaphyseal dysplasia caused by biallelic RAB33B variants. Short trunk dwarfism and radiological findings including the lacy ilia appearance and double bumps of the vertebral bodies are typical features. To date, only eight patients with SMC2 had been reported. The aim of this study is to evaluate the follow-up findings of seven patients from five families with SMC2 and to present four novel variants in RAB33B. The age of diagnosis of the patients was between 4 and 18 years. All patients had variable degrees of short trunk dwarfism with barrel chest, waddling gait, hyperlordosis, genu valgum, elbow and finger joint stiffness, which became more evident with growth. Lacy iliac crest, short ilia with basilar hypoplasia, platyspondyly, dysplastic acetabulum with small and/or laterally displaced femoral heads, and small, irregular carpal bones were detected on skeletal radiographies of all patients. Typical double hump appearance of vertebral bodies was present in patients under 12 years of age, which disappeared after puberty and development of elongated vertebral bodies was also observed. At the time of diagnosis, six patients were able to walk independently; patients who were followed for five to nine years, developed severe hip pain, hip and knee joints stiffness and difficultly of walking after 10 years of age. Only two patients could walk independently during final examination. We detected four novel nonsense variants (p.Gln85Ter, p. Cys48Ter, p. Arg94Ter and p. Gln134Ter) in RAB33B. This study provides important data on long-term skeletal findings of the patients with SMC2.

Additional three patients with Smith-McCort dysplasia due to novel RAB33B mutations

Smith-McCort dysplasia (SMC OMIM 615222) and Dyggve-Melchior-Clausen dysplasia (DMC OMIM 223800) are allelic skeletal dysplasias caused by homozygous or compound heterozygous mutations in DYM (OMIM 607461). Both disorders share the same skeletal phenotypes characterized by spondylo-epi-metaphyseal dysplasia with distinctive lacy ilia. The difference rests on the presence or absence of intellectual disability, that is, intellectual disability in DMC and normal cognition in SMC. However, genetic heterogeneity was suspected in SMC. Recently, RAB33B (OMIM 605950) has been identified as the second gene for SMC. Nevertheless, only two affected families have been reported so far. Here we present three SMC patients with four novel pathogenic variants in RAB33B, including homozygosity for c.211C>T (p.R71*), homozygosity for c.365T>C (p.F122S), and compound heterozygosity for c.48delCGGGGCAG (p.G17Vfs*58) and c.490C>T (p.Q164*). We also summarize the clinical, radiological, and mutation profile of RAB33B after literature mining. This report ascertains the pathogenic relationship between RAB33B and SMC. © 2017 Wiley Periodicals, Inc.

Dymeclin deficiency causes postnatal microcephaly, hypomyelination and reticulum-to-Golgi trafficking defects in mice and humans

Dymeclin is a Golgi-associated protein whose deficiency causes Dyggve-Melchior-Clausen syndrome (DMC, MIM #223800), a rare recessively inherited spondyloepimetaphyseal dysplasia consistently associated with postnatal microcephaly and intellectual disability. While the skeletal phenotype of DMC patients has been extensively described, very little is known about their cerebral anomalies, which result in brain growth defects and cognitive dysfunction. We used Dymeclin-deficient mice to determine the cause of microcephaly and to identify defective mechanisms at the cellular level. Brain weight and volume were reduced in all mutant mice from postnatal day 5 onward. Mutant mice displayed a narrowing of the frontal cortex, although cortical layers were normally organized. Interestingly, the corpus callosum was markedly thinner, a characteristic we also identified in DMC patients. Consistent with this, the myelin sheath was thinner, less compact and not properly rolled, while the number of mature oligodendrocytes and their ability to produce myelin basic protein were significantly decreased. Finally, cortical neurons from mutant mice and primary fibroblasts from DMC patients displayed substantially delayed endoplasmic reticulum to Golgi trafficking, which could be fully rescued upon Dymeclin re-expression. These findings indicate that Dymeclin is crucial for proper myelination and anterograde neuronal trafficking, two processes that are highly active during postnatal brain maturation.