Jinoh Kim

A genome-wide association study identifies susceptibility loci for nonsyndromic sagittal craniosynostosis near BMP2 and within BBS9

Sagittal craniosynostosis is the most common form of craniosynostosis, affecting approximately one in 5,000 newborns. We conducted, to our knowledge, the first genome-wide association study for nonsyndromic sagittal craniosynostosis (sNSC) using 130 non-Hispanic case-parent trios of European ancestry (NHW). We found robust associations in a 120-kb region downstream of BMP2 flanked by rs1884302 (P = 1.13 × 10−14, odds ratio (OR) = 4.58) and rs6140226 (P = 3.40 × 10−11, OR = 0.24) and within a 167-kb region of BBS9 between rs10262453 (P = 1.61 × 10−10, OR = 0.19) and rs17724206 (P = 1.50 × 10−8, OR = 0.22). We replicated the associations to both loci (rs1884302, P = 4.39 × 10−31 and rs10262453, P = 3.50 × 10−14) in an independent NHW population of 172 unrelated probands with sNSC and 548 controls. Both BMP2 and BBS9 are genes with roles in skeletal development that warrant functional studies to further understand the etiology of sNSC.

Mutations in SEC24D, Encoding a Component of the COPII Machinery, Cause a Syndromic Form of Osteogenesis Imperfecta

As a result of a whole-exome sequencing study, we report three mutant alleles in SEC24D, a gene encoding a component of the COPII complex involved in protein export from the ER: the truncating mutation c.613C>T (p.Gln205(∗)) and the missense mutations c.3044C>T (p.Ser1015Phe, located in a cargo-binding pocket) and c.2933A>C (p.Gln978Pro, located in the gelsolin-like domain). Three individuals from two families affected by a similar skeletal phenotype were each compound heterozygous for two of these mutant alleles, with c.3044C>T being embedded in a 14 Mb founder haplotype shared by all three. The affected individuals were a 7-year-old boy with a phenotype most closely resembling Cole-Carpenter syndrome and two fetuses initially suspected to have a severe type of osteogenesis imperfecta. All three displayed a severely disturbed ossification of the skull and multiple fractures with prenatal onset. The 7-year-old boy had short stature and craniofacial malformations including macrocephaly, midface hypoplasia, micrognathia, frontal bossing, and down-slanting palpebral fissures. Electron and immunofluorescence microscopy of skin fibroblasts of this individual revealed that ER export of procollagen was inefficient and that ER tubules were dilated, faithfully reproducing the cellular phenotype of individuals with cranio-lentico-sutural dysplasia (CLSD). CLSD is caused by SEC23A mutations and displays a largely overlapping craniofacial phenotype, but it is not characterized by generalized bone fragility and presented with cataracts in the original family described. The cellular and morphological phenotypes we report are in concordance with the phenotypes described for the Sec24d-deficient fish mutants vbi (medaka) and bulldog (zebrafish).

Biogenesis of γ-secretase early in the secretory pathway

γ-Secretase is responsible for proteolytic maturation of signaling and cell surface proteins, including amyloid precursor protein (APP). Abnormal processing of APP by γ-secretase produces a fragment, Aβ42, that may be responsible for Alzheimers disease (AD). The biogenesis and trafficking of this important enzyme in relation to aberrant Aβ processing is not well defined. Using a cell-free reaction to monitor the exit of cargo proteins from the endoplasmic reticulum (ER), we have isolated a transient intermediate of γ-secretase. Here, we provide direct evidence that the γ-secretase complex is formed in an inactive complex at or before the assembly of an ER transport vesicle dependent on the COPII sorting subunit, Sec24A. Maturation of the holoenzyme is achieved in a subsequent compartment. Two familial AD (FAD)–linked PS1 variants are inefficiently packaged into transport vesicles generated from the ER. Our results suggest that aberrant trafficking of PS1 may contribute to disease pathology.

Cranio-lenticulo-sutural dysplasia associated with defects in collagen secretion

Boyadjiev SA, Kim S‐D, Hata A, Haldeman‐Englert C, Zackai EH, Naydenov C, Hamamoto S, Schekman RW, Kim J. Cranio‐lenticulo‐sutural dysplasia associated with defects in collagen secretion.

SEC23-SEC31 the Interface Plays Critical Role for Export of Procollagen from the Endoplasmic Reticulum

Background: SEC23 plays a critical role in generating transport vesicles from the endoplasmic reticulum (ER). Results: SEC23A M702V mutation at the SEC23-SEC31 interface causes a selective retention of procollagen in the ER. We correlate this defect with an enhanced SAR1 GTPase activity by M702V SEC23A through SEC13-SEC31. Conclusion: The SEC23-SEC31 interface plays a critical role in capturing various cargo molecules. Significance: SAR1 GTP hydrolysis is critical for cargo selection. COPII proteins are essential for exporting most cargo molecules from the endoplasmic reticulum. The membrane-facing surface of the COPII proteins (especially SEC23-SEC24) interacts directly or indirectly with the cargo molecules destined for exit. As we characterized the SEC23A mutations at the SEC31 binding site identified from patients with cranio-lenticulo-sutural dysplasia, we discovered that the SEC23-SEC31 interface can also influence cargo selection. Remarkably, M702V SEC23A does not compromise COPII assembly, vesicle size, and packaging of cargo molecules into COPII vesicles that we have tested but induces accumulation of procollagen in the endoplasmic reticulum when expressed in normal fibroblasts. We observed that M702V SEC23A activates SAR1B GTPase more than wild-type SEC23A when SEC13-SEC31 is present, indicating that M702V SEC23A causes premature dissociation of COPII from the membrane. Our results indicate that a longer stay of COPII proteins on the membrane is required to cargo procollagen than other molecules and suggest that the SEC23-SEC31 interface plays a critical role in capturing various cargo molecules.

Mice deficient in LMAN1 exhibit FV and FVIII deficiencies and liver accumulation of α1-antitrypsin

The type 1-transmembrane protein LMAN1 (ERGIC-53) forms a complex with the soluble protein MCFD2 and cycles between the endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment (ERGIC). Mutations in either LMAN1 or MCFD2 cause the combined deficiency of factor V (FV) and factor VIII (FVIII; F5F8D), suggesting an ER-to-Golgi cargo receptor function for the LMAN1-MCFD2 complex. Here we report the analysis of LMAN1-deficient mice. Levels of plasma FV and FVIII, and platelet FV, are all reduced to ∼ 50% of wild-type in Lman1(-/-) mice, compared with the 5%-30% levels typically observed in human F5F8D patients. Despite previous reports identifying cathepsin C, cathepsin Z, and α1-antitrypsin as additional potential cargoes for LMAN1, no differences were observed between wild-type and Lman1(-/-) mice in the levels of cathepsin C and cathepsin Z in liver lysates or α1-antitrypsin levels in plasma. LMAN1 deficiency had no apparent effect on COPII-coated vesicle formation in an in vitro assay. However, the ER in Lman1(-/-) hepatocytes is slightly distended, with significant accumulation of α1-antitrypsin and GRP78. An unexpected, partially penetrant, perinatal lethality was observed for Lman1(-/-) mice, dependent on the specific inbred strain genetic background, suggesting a potential role for other, as yet unidentified LMAN1-dependent cargo proteins.

ALX4 gain-of-function mutations in nonsyndromic craniosynostosis

Craniosynostosis is the early fusion of one or more sutures of the infant skull and is a common defect occurring in approximately 1 of every 2,500 live births. Nonsyndromic craniosynostosis (NSC) accounts for approximately 80% of all cases and is thought to have strong genetic determinants that are yet to be identified. ALX4 is a homeodomain transcription factor with known involvement in osteoblast regulation. By direct sequencing of the ALX4 coding region in sagittal or sagittal‐suture‐involved nonsyndromic craniosynostosis probands, we identified novel, nonsynonymous, familial variants in three of 203 individuals with NSC. Using dual‐luciferase assay we show that two of these variants (V7F and K211E) confer a significant gain‐of‐function effect on ALX4. Our results suggest that ALX4 variants may have an impact on the genetic etiology of NSC. Hum Mutat 33:1626–1629, 2012.

Sequence analyses of presenilin mutations linked to familial Alzheimer’s disease

Familial Alzheimer’s disease (FAD)-linked presenilin (PS) mutations show gain-of-toxic-function characteristics. These FAD PS mutations are scattered throughout the PS molecule, reminiscent of the distribution of cystic fibrosis transmembrane conductance regulator and p53 mutations. Because of the scattered distribution of PS mutations, it is difficult to infer mechanistic insights about how these mutations cause the disease similarly. Recent careful reexamination of γ-secretase activity indicates that some PS mutations decrease the proteolytic activity of γ-secretase, suggesting a loss-of-function nature of PS mutations. To extend this observation to all known PS mutations, a large number of PS mutations were evaluated using bioinformatic tools. The analyses reveal that as many as one third of PS1 residues are highly conserved, that about 75% of FAD mutations are located to the highly conserved residues, and that most PS mutations likely damage the activity of PS. These results are consistent with the idea that the majority of PS mutations lower the activity of PS/γ-secretase.

Keutel syndrome: Report of two novel MGP mutations and discussion of clinical overlap with arylsulfatase E deficiency and relapsing polychondritis

Keutel syndrome is a rare, autosomal recessive disorder characterized by diffuse cartilage calcification, peripheral pulmonary artery stenosis, midface retrusion, and short distal phalanges. To date, 28 patients from 18 families have been reported, and five mutations in the matrix Gla protein gene (MGP) have been identified. The matrix Gla protein (MGP) is a vitamin K‐dependent extracellular protein that functions as a calcification inhibitor through incompletely understood mechanisms. We present the clinical manifestations of three affected siblings from a consanguineous Turkish family, in whom we detected the sixth MGP mutation (c.79G>T, which predicts p.E27X) and a fourth unrelated patient in whom we detected the seventh MGP mutation, a partial deletion of exon 4. Both mutations predict complete loss of MGP function. One of the patients presented initially with a working diagnosis of relapsing polychondritis. Clinical features suggestive of Keutel syndrome were also observed in one additional unrelated patient who was later found to have a deletion of arylsulfatase E, consistent with a diagnosis of X‐linked recessive chondrodysplasia punctata. Through a discussion of these cases, we highlight the clinical overlap of Keutel syndrome, X‐linked chondrodysplasia punctata, and the inflammatory disease relapsing polychondritis.

Sec24 interaction is essential for localization and virulence-associated function of the bacterial effector protein NleA.

Enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC) are food‐borne pathogens that cause severe diarrhoeal disease in humans. Citrobacter rodentium is a related mouse pathogen that serves as a small animal model for EPEC and EHEC infections. EPEC, EHEC and C. rodentium translocate bacterial virulence proteins directly into host cells via a type III secretion system (T3SS). Non‐LEE‐encoded effector A (NleA) is a T3SS effector that is common to EPEC, EHEC and C. rodentium and is required for bacterial virulence. NleA localizes to the host cell secretory pathway and inhibits vesicle trafficking by interacting with the Sec24 subunit of mammalian coatamer protein II complex (COPII). Mammalian cells express four paralogues of Sec24 (Sec24A–D), which mediate selection of cargo proteins for transport and possess distinct, but overlapping cargo specificities. Here, we show that NleA binds Sec24A–D with two distinct mechanisms. An NleA protein variant with greatly diminished interaction with all Sec24 paralogues does not properly localize, does not inhibit COPII‐mediated vesicle budding, and does not confer virulence in the mouse infection model. Together, this work provides strong evidence that the interaction and inhibition of COPII by NleA is an important aspect of EPEC‐ and EHEC‐mediated disease.