Cellular stress due to impairment of collagen prolyl hydroxylation complex is rescued by the chaperone 4-phenylbutyrate

Osteogenesis imperfecta (OI) types VII, VIII and IX, caused by recessive mutations in cartilage associated protein (CRTAP), prolyl-3-hydroxylase 1 (P3H1), and cyclophilin B (CyPB), respectively, are characterized by the synthesis of overmodified collagen. The genes encode for the components of the endoplasmic reticulum (ER) complex responsible for the 3-hydroxylation of specific proline residues in collagen type I. Our study dissects the effects of mutations in the proteins of the complex on cellular homeostasis, using primary fibroblasts from seven recessive OI patients. In all cell lines the intracellular retention of overmodified type I collagen molecules causes ER enlargement associated to the presence of protein aggregates, activation of the PERK branch of the unfolded protein response and apoptotic death. The administration of 4-phenylbutyrate (4-PBA) alleviates cellular stress by restoring ER cisternae size, normalizing the p-PERK/PERK ratio and the expression of apoptotic marker. The drug has also a stimulatory effect on autophagy. We proved that the rescue of cellular homeostasis following 4-PBA treatment is associated to its chaperone activity, since it increases protein secretion, restoring ER proteostasis and reducing PERK activation and cell survival also in presence of autophagy pharmacological inhibition.Our results provide a novel insight into the mechanism of 4-PBA action and demonstrated that the intracellular stress in recessive OI can be tuned by 4-PBA therapy, similarly to what we recently reported for dominant OI, thus allowing a common target for OI forms characterized by overmodified collagen

Brachydactyly

Brachydactyly ("short digits") is a general term that refers to disproportionately short fingers and toes, and forms part of the group of limb malformations characterized by bone dysostosis. The various types of isolated brachydactyly are rare, except for types A3 and D. Brachydactyly can occur either as an isolated malformation or as a part of a complex malformation syndrome. To date, many different forms of brachydactyly have been identified. Some forms also result in short stature. In isolated brachydactyly, subtle changes elsewhere may be present. Brachydactyly may also be accompanied by other hand malformations, such as syndactyly, polydactyly, reduction defects, or symphalangism

Characterization of greater middle eastern genetic variation for enhanced disease gene discovery

The Greater Middle East (GME) has been a central hub of human migration and population admixture. The tradition of consanguinity, variably practiced in the Persian Gulf region, North Africa, and Central Asia1-3, has resulted in an elevated burden of recessive disease4. Here we generated a whole-exome GME variome from 1,111 unrelated subjects. We detected substantial diversity and admixture in continental and subregional populations, corresponding to several ancient founder populations with little evidence of bottlenecks. Measured consanguinity rates were an order of magnitude above those in other sampled populations, and the GME population exhibited an increased burden of runs of homozygosity (ROHs) but showed no evidence for reduced burden of deleterious variation due to classically theorized ‘genetic purging’. Applying this database to unsolved recessive conditions in the GME population reduced the number of potential disease-causing variants by four- to sevenfold. These results show variegated genetic architecture in GME populations and support future human genetic discoveries in Mendelian and population genetics

A novel homozygous mutation in FGFR3 causes tall stature, severe lateral tibial deviation, scoliosis, hearing impairment, camptodactyly, and arachnodactyly

Most reported mutations in the FGFR3 gene are dominant activating mutations that cause a variety of short-limbed bone dysplasias including achondroplasia and syndromic craniosynostosis. We report the phenotype and underlying molecular abnormality in two brothers, born to first cousin parents. The clinical picture is characterized by tall stature and severe skeletal abnormalities leading to inability to walk, with camptodactyly, arachnodactyly, and scoliosis. Whole exome sequencing revealed a homozygous novel missense mutation in the FGFR3 gene in exon 12 (NM_000142.4:c.1637C>A: p.(Thr546Lys)). The variant is found in the kinase domain of the protein and is predicted to be pathogenic. It is located near a known hotspot for hypochondroplasia. This is the first report of a homozygous loss-of-function mutation in FGFR3 in human that results in a skeletal overgrowth syndrome

Mutaciones en FAM46A en un paciente con osteogénesis imperfecta

Resumen del póster presentado a la XI Reunión Anual CIBERER, celebrada en Castelldefels, Barcelona del 12 al 14 de marzo de 2018.Osteogénesis imperfecta (OI) es una enfermedad asociada a fragilidad ósea y osteoporosis, producida principalmente por mutaciones heterocigotas en los genes codificantes de las cadenas polipeptídicas de colágeno tipo I, COL1A1 y COL1A2. Asimismo, también existen otras formas recesivas y dominantes de esta enfermedad, las cuales se caracterizan por una alta variabilidad genética. Hasta la fecha se han descrito 19 loci de OI, cuya función está relacionada con el metabolismo del colágeno I; la mineralización de la matriz ósea; o la diferenciación de los osteoblastos. Aquí presentamos un paciente de origen egipcio con una mutación en homocigosis en FAM46A, un nuevo gen recientemente identificado como causante de esta enfermedad, y en cuya publicación ha colaborado nuestro laboratorio. FAM46A codifica una proteína perteneciente a la superfamilia de las nucleotidil-transferasas, pero su función molecular a día de hoy permanece desconocida. No obstante, existen evidencias, incluido un modelo animal generado por mutagénesis ENU publicado con anterioridad, que indican que FAM46A desempeña un papel esencial en el desarrollo óseo.Peer reviewe

OSX/SP7 mutations and osteogenesis imperfecta

Osteogenesis imperfecta or >brittle bone disease> is a collagen type I-related condition associated with osteoporosis and increased risk of bone fractures. Using a combination of homozygosity mapping and candidate gene approach we identified a homozygous single basepair deletion (c.1052delA) in SP7/Osterix (OSX) in a consanguineous patient with recessive osteogenesis imperfecta. The clinical findings of this patient include recurrent fractures, mild bone deformities, delayed tooth eruption, normal hearing, and white sclera. OSX encodes a transcription factor containing three Cys2-His2 zinc-finger DNA-binding domains at its C-terminus, which in mice has been shown to be essential for bone formation. The frameshift caused by the c.1052delA deletion removes the last 81 amino acids of the protein including the third zinc-finger motif. This finding adds a new locus to the spectrum of genes associated with osteogenesis imperfecta and reveals that SP7/OSX also plays a key role in human bone development.Peer Reviewe

BMP1 mutations in autosomal recessive osteogenesis imperfecta

Mutations in BMP1 cause osteogenesis imperfect (OI) type XIII (MIM 614856), a probably not so uncommon form of autosomal recessive OI. (Editor's Note: See Chapters 1 and 2 for discussion of OI types related to specific mutations.) We have studied an Egyptian consanguineous family with two children initially diagnosed as OI type III. They had blue sclera, severe kyphoscoliosis and large umbilical hernia. Genetic analysis of this family performed by homozygosity mapping showed lack of linkage to any of the previously known AR-OI loci but identified a homozygous 10.27. Mb DNA segment on chromosome 8p in both affected children comprising the type I collagen C-propeptide protease gene BMP1. Direct sequencing of BMP1 in the proband revealed a Phe249Leu homozygous missense change within the BMP1/mTLD astacin protease domain. Phe249 is a residue which is invariable in all invertebrate and vertebrate members of the astacin family of metalloproteases. A few months later, using a combination of whole-exome sequencing and filtering for homozygous stretches of identified variants, another mutation was reported by other authors in two affected sibs of a consanguineous family from Turkey. They showed increased bone mineral density and multiple recurrent fractures. The mutation in BMP1, a 34. G>C transversion in exon 1 resulting in a Gly12Arg (G12R) substitution within the signal peptide, impaired protein's localization to the endoplasmic reticulum, the correct post-translational glycosylation and its secretion. BMP1 expands the number of growing genes involved in AR-OI and demonstrates the high genetic complexity of the collagen I pathway disorders.Peer Reviewe