To identify the gene underlying Fanconi anemia (FA) complementation group I we studied informative FA-I families by a genome-wide linkage analysis, which resulted in 4 candidate regions together encompassing 351 genes. Candidates were selected via bioinformatics and data mining on the basis of their resemblance to other FA genes/proteins acting in the FA pathway, such as: degree of evolutionary conservation, presence of nuclear localization signals and pattern of tissue-dependent expression. We found a candidate, KIAA1794 on chromosome 15q25-26, to be mutated in 8 affected individuals previously assigned to complementation group I. Western blots of endogenous FANCI indicated that functionally active KIAA1794 protein is lacking in FA-I individuals. Knock-down of KIAA1794 expression by siRNA in HeLa cells caused excessive chromosomal breakage induced by mitomycin C, a hallmark of FA cells. Furthermore, phenotypic reversion of a patient-derived cell line was associated with a secondary genetic alteration at the KIAA1794 locus. These data add up to two conclusions. First, KIAA1794 is a FA gene. Second, this gene is identical to FANCI, since the patient cell lines found mutated in this study included the reference cell line for group I, EUFA592.

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Osteogenesis imperfecta (OI) is characterized primarily by susceptibility to fractures with or without bone deformation. OI is genetically heterogeneous: over 20 genetic causes are recognized. We identified bi-allelic pathogenic KDELR2 variants as a cause of OI in four families. KDELR2 encodes KDEL endoplasmic reticulum protein retention receptor 2, which recycles ER-resident proteins with a KDEL-like peptide from the cis-Golgi to the ER through COPI retrograde transport. Analysis of patient primary fibroblasts showed intracellular decrease of HSP47 and FKBP65 along with reduced procollagen type I in culture media. Electron microscopy identified an abnormal quality of secreted collagen fibrils with increased amount of HSP47 bound to monomeric and multimeric collagen molecules. Mapping the identified KDELR2 variants onto the crystal structure of G. gallus KDELR2 indicated that these lead to an inactive receptor resulting in impaired KDELR2-mediated Golgi-ER transport. Therefore, in KDELR2-deficient individuals, OI most likely occurs because of the inability of HSP47 to bind KDELR2 and dissociate from collagen type I. Instead, HSP47 remains bound to collagen molecules extracellularly, disrupting fiber formation. This highlights the importance of intracellular recycling of ER-resident molecular chaperones for collagen type I and bone metabolism and a crucial role of HSP47 in the KDELR2-associated pathogenic mechanism leading to OI.Osteogenesis imperfecta (OI) (MIM: PS166200) is a clinically and genetically heterogeneous connective tissue disorder characterized by liability to fractures with or without bone deformation. Secondary features include blue sclerae, dentinogenesis imperfecta (DI), progressive hearing loss, and joint hypermobility. OI is divided into five clinical types, and it has long been estimated that 90% of individuals with OI have dominant pathogenic variants in COL1A1 (MIM: 120150) or COL1A2 (MIM: 120160) encoding the a1 and a2 chains of collagen type I. However, there is growing evidence that autosomal-recessive forms of OI can be more common in consanguineous populations. 1,2 OI type 1 (MIM: 166200) is usually caused by pathogenic variants resulting in haploinsufficiency of COL1A1, and OI type 5 (MIM: 610967) is always caused by the dominant c.À14C>T variant in the 5 0 UTR of IFITM5. 1 To date,

Osteogenesis Imperfecta (OI) comprises a heterogeneous group of patients who share bone fragility and deformities as the main characteristics, albeit with different degrees of severity. Phenotypic variation also exists in other connective tissue aspects of the disease, complicating disease classification and disease course prediction. Although collagen type I defects are long established as the primary cause of the bone pathology, we are still far from comprehending the complete mechanism. In the last years, the advent of next generation sequencing has triggered the discovery of many new genetic causes for OI, helping to draw its molecular landscape. It has become clear that, in addition to collagen type I genes, OI can be caused by multiple proteins connected to different parts of collagen biosynthesis. The production of collagen entails a complex process, starting from the production of the collagen Iα1 and collagen Iα2 chains in the endoplasmic reticulum, during and after which procollagen is subjected to a plethora of posttranslational modifications by chaperones. After reaching the Golgi organelle, procollagen is destined to the extracellular matrix where it forms collagen fibrils. Recently discovered mutations in components of the retrograde transport of chaperones highlight its emerging role as critical contributor of OI development. This review offers an overview of collagen regulation in the context of recent gene discoveries, emphasizing the significance of transport disruptions in the OI mechanism. We aim to motivate exploration of skeletal fragility in OI from the perspective of these pathways to identify regulatory points which can hint to therapeutic targets.

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder leading to progressive heterotopic ossifications (HO) of muscles, tendons, and ligaments, which can be induced by trauma or by surgery. Despite strong medical advice to the contrary, an FOP patient insisted on surgery to alleviate her complete trismus, which caused an unbearable impact on her quality of life (QOL). The entire trismus history of this FOP patient is presented. [18F]‐NaF position emission tomography/computed tomography (PET/CT) scans were introduced as an imaging method for heterotopic bone formation activity. To place our findings into context, a systematic review on jaw surgery in FOP was performed. After falling down the stairs, a 9‐year‐old patient developed mobility impairment of her left‐sided jaw. During the following 13 years bone scintigraphy showed persistent activity of the disease leading to progressive left‐sided zygomatico‐mandibular fusion by HO, resulting in complete trismus. Within 1 month after HO removal on the left side and a matching right coronoidectomy, [18F]‐NaF PET/CT demonstrated a substantial flare‐up activity followed by new HO in both masseter and temporalis muscles. Despite recurrent HO and trismus her QOL increased due to a stable increased interincisal opening of 5.5 mm. Although systematic review reveals a 100% risk of HO recurrence after jaw surgery, information on improved QOL is scarce. In conclusion, surgery in FOP may be beneficial for QOL despite new HO formation. Assessment of disease activity using [18F]‐NaF PET/CT is possible before HO is evident on CT and may serve as a new and quantitative marker of the disease. © 2017 The Authors. JBMR Plus Published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Mutations in genes encoding proteins of the smooth muscle cell (SMC) contractile apparatus contribute to familial aortic aneurysms. To investigate the pathogenicity of these mutations, SMC are required. We demonstrate a novel method to generate SMC-like cells from human dermal fibroblasts by transdifferentiation to study the effect of variants in genes encoding proteins of the SMC contractile apparatus (ACTA2 and MYH11) in patients with aortic aneurysms. Dermal fibroblasts from seven healthy donors and cells from seven patients with MYH11 or ACTA2 variants were transdifferentiated into SMC-like cells within a 2-week duration using 5 ng/ml TGFβ1 on a scaffold containing collagen and elastin. The induced SMC were comparable to primary human aortic SMC in mRNA expression of SMC markers which was confirmed on the protein level by immunofluorescence quantification analysis and Western blotting. In patients with MYH11 or ACTA2 variants, the effect of intronic variants on splicing was demonstrated on the mRNA level in the induced SMC, allowing classification into pathogenic or nonpathogenic variants. In conclusion, direct conversion of human dermal fibroblasts into SMC-like cells is a highly efficient method to investigate the pathogenicity of variants in proteins of the SMC contractile apparatus.

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Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare progressive genetic disease effecting one in a million individuals. During their life, patients with FOP progressively develop bone in the soft tissues resulting in increasing immobility and early death. A mutation in the ACVR1 gene was identified as the causative mutation of FOP in 2006. After this, the pathophysiology of FOP has been further elucidated through the efforts of research groups worldwide. In 2015, a workshop was held to gather these groups and discuss the new challenges in FOP research. Here we present an overview and update on these topics.