The sedlin gene for spondyloepiphyseal dysplasia tarda escapes X-inactivation and contains a non-canonical splice site

Trafficking mechanisms of extracellular matrix macromolecules: insights from vertebrate development and human diseases

Cellular life depends on protein transport and membrane traffic. In multicellular organisms, membrane traffic is required for extracellular matrix deposition, cell adhesion, growth factor release, and receptor signaling, which are collectively required to integrate the development and physiology of tissues and organs. Understanding the regulatory mechanisms that govern cargo and membrane flow presents a prime challenge in cell biology. Extracellular matrix (ECM) secretion remains poorly understood, although given its essential roles in the regulation of cell migration, differentiation, and survival, ECM secretion mechanisms are likely to be tightly controlled. Recent studies in vertebrate model systems, from fishes to mammals and in human patients, have revealed complex and diverse loss-of-function phenotypes associated with mutations in components of the secretory machinery. A broad spectrum of diseases from skeletal and cardiovascular to neurological deficits have been linked to ECM trafficking. These discoveries have directly challenged the prevailing view of secretion as an essential but monolithic process. Here, we will discuss the latest findings on mechanisms of ECM trafficking in vertebrates.

A novel splicing mutation in the SEDL gene causes spondyloepiphyseal dysplasia tarda in a large Chinese pedigree

The X-linked form of spondyloepiphyseal dysplasia tarda (SEDT, OMIM# 313400) is a rare osteochondrodysplasia caused by mutations in the SEDL (TRAPPC2, OMIM# 300202) gene. It is clinically characterized by disproportionate short stature, barrel-shaped chests and early development of degenerative joint disease. We report here a novel mutation in the intron 3 splice-donor site (c. 93+5G>C) segregated in an X-link pattern in a large Chinese family with SEDT. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that the mutation causes an aberrant splicing of exon 3, resulting in the elimination of 31 codons in the exon and a considerable loss function of the SEDL protein. This mutation was not detected in the 100 healthy controls. This novel mutation adds to the spectrum of previously-identified disease-causing mutations. Pre-symptomatic molecular diagnosis and prenatal diagnosis of the pregnant carriers could be helpful to families with SEDT.

A novel COMP mutation in a Chinese patient with pseudoachondroplasia

A 2.75-year-old Chinese boy presented with typical clinical features of pseudoachondroplasia, including disproportionate short-limb short stature, brachydactyly, genu varus and waddling gait. Radiologically, tubular bones were short with widened metaphyses, irregular and small epiphyses; anterior tonguing or beaking of vertebral bodies were characteristic. DNA sequencing analysis of the COMP gene revealed a heterozygous mutation (c.1511G>A, p.Cys504Tyr) in the patient but his parents were unaffected without this genetic change. The missense mutation (c.1511G>A) was not found in 100 healthy controls and has not been reported previously. Our findings expand the spectrum of known mutations in COMP leading to pseudoachondroplasia.

SEDLIN forms homodimers: characterisation of SEDLIN mutations and their interactions with transcription factors MBP1, PITX1 and SF1

BACKGROUND

SEDLIN, a 140 amino acid subunit of the Transport Protein Particle (TRAPP) complex, is ubiquitously expressed and interacts with the transcription factors c-myc promoter-binding protein 1 (MBP1), pituitary homeobox 1 (PITX1) and steroidogenic factor 1 (SF1). SEDLIN mutations cause X-linked spondyloepiphyseal dysplasia tarda (SEDT).

METHODOLOGY/PRINCIPAL FINDINGS

We investigated the effects of 4 missense (Asp47Tyr, Ser73Leu, Phe83Ser and Val130Asp) and the most C-terminal nonsense (Gln131Stop) SEDT-associated mutations on interactions with MBP1, PITX1 and SF1 by expression in COS7 cells. Wild-type SEDLIN was present in the cytoplasm and nucleus and interacted with MBP1, PITX1 and SF1; the SEDLIN mutations did not alter these subcellular localizations or the interactions. However, SEDLIN was found to homodimerize, and the formation of dimers between wild-type and mutant SEDLIN would mask a loss in these interactions. A mammalian SEDLIN null cell-line is not available, and the interactions between SEDLIN and the transcription factors were therefore investigated in yeast, which does not endogenously express SEDLIN. This revealed that all the SEDT mutations, except Asp47Tyr, lead to a loss of interaction with MBP1, PITX1 and SF1. Three-dimensional modelling studies of SEDLIN revealed that Asp47 resides on the surface whereas all the other mutant residues lie within the hydrophobic core of the protein, and hence are likely to affect the correct folding of SEDLIN and thereby disrupt protein-protein interactions.

CONCLUSIONS/SIGNIFICANCE

Our studies demonstrate that SEDLIN is present in the nucleus, forms homodimers and that SEDT-associated mutations cause a loss of interaction with the transcription factors MBP1, PITX1 and SF1.

Biochemical consequences of sedlin mutations that cause spondyloepiphyseal dysplasia tarda

SEDT (spondyloepiphyseal dysplasia tarda) is a late-onset X-linked recessive skeletal dysplasia caused by mutations in the gene SEDL coding for sedlin. In the present paper, we investigated four missense mutations observed in SEDT and compare biochemical and cellular characteristics relative to the wild-type protein to address the mechanism of disease and to gain insight into the function of the sedlin protein. In situ hybridization and immunohistochemical experiments in mouse growth plates revealed sedlin to be predominantly expressed in proliferating and hypertrophic chondrocytes. Cell culture studies showed that the wild-type protein localized predominantly in the vicinity of the nucleus and the Golgi, with further localization around the cytoplasm, whereas mutation resulted in mislocalization. The D47Y mutant was expressed similarly to the wild-type, but the S73L, F83S and V130D mutants showed particularly low levels of expression that were rescued in the presence of the proteasome inhibitor MG132 (benzyloxycarbonyl-leucylleucylleucinal). Furthermore, whereas the D47Y mutant folded similarly and had similar stability to the wild-type sedlin as shown by CD and fluorescence, the S73L, F83S and V130D mutants all misfolded during expression. Two independent assays showed that the D47Y mutation resulted in an increased affinity for the transport protein particle component Bet3 compared with the wild-type sedlin. Our results suggest that the sedlin mutations S73L, F83S and V130D cause SEDT by sedlin misfolding, whereas the D47Y mutation may influence normal TRAPP (transport protein particle) dynamics.

TRAPPC2L is a novel, highly conserved TRAPP-interacting protein

Mutations in the trafficking protein particle complex C2 protein (TRAPPC2), a mammalian ortholog of yeast Trs20p and a component of the trafficking protein particle (TRAPP) vesicle tethering complex, have been linked to the skeletal disorder spondyloepiphyseal dysplasia tarda (SEDT). Intriguingly, the X-linked TRAPPC2 is just one of a complement of Trs20-related genes in humans. Here we characterize TRAPPC2L, a novel, highly conserved TRAPP-interacting protein related to TRAPPC2 and the uncharacterized yeast open reading frame YEL048c. TRAPPC2L and TRAPPC2 genes are found in pairs across species and show broad and overlapping expression, suggesting they are functionally distinct, a notion supported by yeast complementation studies and biochemical characterization. RNA interference-mediated knockdown of either TRAPPC2L or TRAPPC2 in HeLa cells leads to fragmentation of the Golgi, implicating both proteins in Golgi dynamics. Gradient fractionation of cellular membranes indicates that TRAPPC2L is found with a portion of cellular TRAPP on very low-density membranes whereas the remainder of TRAPP, but not TRAPPC2L, is found associated with Golgi markers. YEL048c displays genetic interactions with TRAPP II-encoding genes and the gene product co-fractionates with and interacts with yeast TRAPP II. Taken together these results indicate that TRAPPC2L and its yeast ortholog YEL048c are novel TRAPP-interacting proteins that may modulate the function of the TRAPP II complex.

Noncanonical and canonical splice sites: a novel mutation at the rare noncanonical splice-donor cut site (IVS4+1A>G) of SEDL causes variable splicing isoforms in X-linked spondyloepiphyseal dysplasia tarda

X-linked spondyloepiphyseal dysplasia tarda can be caused by mutations in the SEDL gene. This study describes an interesting novel mutation (IVS4+1A>G) located exactly at the rare noncanonical AT-AC consensus splicing donor point of SEDL, which regained the canonical GT-AG consensus splicing junction in addition to several other rarer noncanonical splice patterns. The mutation activated several cryptic splice sites and generated the production of seven erroneous splicing isoforms, which we confirmed by sequencing of RT-PCR products and resequencing of cDNA clones. All the practical splice donors/acceptors were further assessed using FSPLICE 1.0 and SPL(M) Platforms to predict potential splice sites in genomic DNA. Subsequently, the expression levels of SEDL among the affected patients, carriers and controls were estimated using real-time quantitative PCR. Expression analyses showed that the expression levels of SEDL in both patients and carriers were decreased. Taken together, these results illustrated how disruption of the AT donor site in a rare AT-AC intron, leading to a canonical GT donor site, resulted in a multitude of aberrant transcripts, thus impairing exon definition. The unexpected splicing patterns resulting from the special mutation provide additional challenges and opportunities for understanding splicing mechanisms and specificity.

X-linked spondyloepiphyseal dysplasia tarda: molecular cause of a heritable platyspondyly

STUDY DESIGN

Report of a family affected with X-linked spondyloepiphyseal dysplasia tarda with special respect to radiologic alterations of the spine from puberty to the forth decade and to molecular analysis of the underlying genetic defect.

OBJECTIVES

To report the typical radiologic presentation of patients with X-linked spondyloepiphyseal dysplasia tarda and the diagnostic tool of mutation screening for that disease in order to avoid confusion with similar occurrences.

SUMMARY OF BACKGROUND DATA

Spondyloepiphyseal dysplasia tarda is a genetically heterogeneous disorder that frequently manifests itself with back pain starting around puberty. The X-linked recessive form (X-linked spondyloepiphyseal dysplasia tarda) affects males and is clinically characterized by an arm span markedly exceeding total height, a barrel chest deformity, and early development of degenerative joint disease. The disorder is caused by mutations in the SEDL gene located on Xp22.12-p22.31.

METHODS

Radiologic alterations of the cervical, thoracal, and lumbar spine were assessed in the affected family members and one suspected female carrier in correlation to age. All 6 exon codings for the SEDL gene were analyzed by primer cycle sequencing.

RESULTS

In 3 male patients from a French family, we identified a 5 base pair deletion in SEDL, exon 5 at position 267-271 (delAAGAC). Carrier status for the mutation could be confirmed in one female member of the family, which is inconspicuous in terms of spine and joint diseases. Radiologic abnormalities of the patients comprised generalized platyspondyly, a hump-shaped deformity of cervical, thoracal, and lumbar vertebral bodies as well as signs of retrospondylophytes, osteochondrosis, and spondylarthrosis.

CONCLUSIONS

X-linked spondyloepiphyseal dysplasia tarda should be kept in mind as a differential diagnosis in men with early onset of back pain and radiologic abnormalities of the vertebral bodies comprising platyspondyly and a central hump.

Membrane traffic fuses with cartilage development

The ability of cells to synthesize and secrete proteins is essential for numerous cellular functions. Therefore, when mutations in one component of the secretory pathway result in a tissue-specific defect, a unique opportunity arises to examine the molecular mechanisms at play. The recent finding that a defect in the protein sedlin, whose yeast counterpart is involved in the first step of the secretory pathway, leads to a cartilage-specific disorder in humans raises numerous questions and interesting possibilities for understanding both the pathobiology involved and the role of membrane traffic in normal cartilage development.

Modulation of human luteinizing hormone beta gene transcription by MIP-2A

MIP-2A was recently identified as a MBP-1 interacting cellular protein. We have shown previously that MBP-1 acts as a transcriptional repressor. Functional association between MIP-2A and MBP-1 suggests that MIP-2A can act as a cofactor and relieves MBP-1-mediated transcriptional repression. In this study, we report the tissue-specific expression of MIP-2A and its role in the regulation of gene transcription. RNA dot blot analysis of human multiple tissue expression array suggested that MIP-2A is highly abundant in right cerebellum, pituitary, adrenal, and testis but barely detectable in skeletal muscle. Predominant expression of MIP-2A in pituitary tissue led us to investigate whether MIP-2A can transcriptionally regulate luteinizing hormone beta (LHbeta), a pituitary-specific hormone synthesized and secreted from gonadotropic cells. The LHbeta promoter is regulated by the orphan nuclear receptor SF-1 and homeodomain protein Ptx1. Although each factor enhances the LHbeta promoter, coexpression of both results in a strong synergistic activation. Therefore, we examined whether MIP-2A can modulate SF-1- and Ptx1-mediated transcriptional activation. Our results suggested that MIP-2A expression inhibits SF-1- and Ptx1-mediated transactivation of LHbeta promoter. Subsequent analysis demonstrated that MIP-2A physically interacts with both SF-1 and Ptx1, thereby inhibiting transactivation of the LHbeta promoter. Taken together, our results indicate that MIP-2A preferentially expresses in certain tissues, including the pituitary gland, and negatively regulates the LHbeta gene transcription.

Preonset studies of spondyloepiphyseal dysplasia tarda caused by a novel 2-base pair deletion in SEDL encoding sedlin

Spondyloepiphyseal dysplasia tarda (SEDT), an X-linked recessive skeletal disorder, presents with disproportionate short stature and "barrel-chest" deformity in affected (hemizygous) adolescent boys. In four reported families to date, mutations in a gene designated SEDL (spondyloepiphyseal dysplasia late) cosegregate with SEDT. We diagnosed SEDT in a short-stature, kyphotic 15-year-old boy because of his characteristic vertebral malformations. Clinical manifestations of SEDT were evident in at least four previous generations. A novel 2-base pair (bp) deletion in exon 5 of SEDL was found in the propositus by polymerase chain reaction (PCR) amplification and sequencing of all four coding exons. The mutation ATdel241-242 cosegregated with the kindred's skeletal disease. The deletion is adjacent to a noncanonical splice site for exon 5 but does not alter splicing. Instead, it deletes 2 bp from the coding sequence, causing a frameshift. A maternal aunt and her three young sons were investigated subsequently. Radiographs showed subtle shaping abnormalities of her pelvis and knees, suggesting heterozygosity. X-rays of the spine and pelvis of her 8-year-old son revealed characteristic changes of SEDT, but her younger sons (aged 6 years and 3 years) showed no abnormalities. SEDL analysis confirmed that she and only her eldest boy had the 2-bp deletion. Molecular testing of SEDL enables carrier detection and definitive diagnosis before clinical or radiographic expression of SEDT. Although there is no specific treatment for SEDT, preexpression molecular testing of SEDL could be helpful if avoiding physical activities potentially injurious to the spine and the joints proves beneficial.