Clinical and genomic elucidation of skeletal dysplasias

Skeletal dysplasias are clinically and genetically diverse group of disorders of bone and cartilage development. 461 different conditions caused by variants in 437 genes have been enumerated in the recent nosology of the genetic disorders of skeleton. Clinical and radiographic descriptions have aided the phenotyping of known skeletal dysplasias with a genetic diagnosis and facilitated gene discovery by next generation sequencing, mostly by whole exome sequencing. We aimed to systematically phenotype patients with skeletal dysplasias by clinical and radiological evaluation and perform suitable genetic testing (targeted mutation analysis and exome sequencing) for establishing a molecular diagnosis. The first manuscript describes delineation of a novel skeletal dysplasia caused by variants in PISD in two children with a spondyloepimetaphyseal dysplasia. Both of them showed similar platyspondyly, large epiphyses and metaphyseal changes and child-parents trio whole exome sequencing returned non-diagnostic results. Both of them shared the homozygous missense variant c.797G>A/p.(Cys266Tyr) in PISD. The also shared the same region of homozygosity on chromosome 22 encompassing this gene suggesting a remote inbreeding. PISD codes for phosphatidylserine (PS) decarboxylase that is localized in the inner mitochondrial membrane. It catalyzes the decarboxylation of PS to phosphatidylethanolamine (PE) in mammalian cells. PE occurs at high abundance in mitochondrial membranes. Fibroblasts from one of the patients showed fragmented mitochondria. Patient cells showed decreased cell viability when treated with MG-132 or staurosporine that induce activation of the intrinsic apoptosis pathway. There was an increased caspase-3 and caspase-7 activation too. Ethanolamine supplementation mostly restored cell viability and enhanced apoptosis in MG-132-stressed patient cells. Our findings demonstrate biallelic hypomorphic PISD variant p.(Cys266Tyr) is associated with a novel spondyloepimetaphyseal dysplasia. Our second manuscript characterizes recessive type 2 collagenopathies in two families with skeletal dysplasias. The phenotypic spectrum of Type 2 collagenopathies ranges from the lethal achondrogenesis type 2 to milder osteoarthritis with mild chondrodysplasia. All of them are monoallelic except for the two recent reports on biallelic variants in COL2A1 in two children with spondyloepiphyseal dysplasia congenita. In this work, we describe two additional families with homozygous variants, c.4135C>T (p.Arg1379Cys) and c.3190C>T (p.Arg1133Cys) in COL2A1 resulting in two distinct skeletal dysplasia phenotypes of intermediate severity. Patients with biallelic variants in type 2 collagen gene exhibit a spondyloepimetaphyseal dysplasia and display a wide variation in the severity of short stature and skeletal affliction. We hypothesize that these variants are likely to be hypomorphic and provide further evidence to the existence of autosomal recessive type 2 collagenopathies. In the third manuscript, we expand the allelic and phenotypic spectrum of Steel syndrome, a rare recessive skeletal dysplasia, characterized by dislocations of the hips and radial heads, carpal coalition, short stature, facial dysmorphism, and scoliosis. Until date 47 patients have been reported. However, disease causing variants have been identified only in twenty Puerto Rican and nine non-Puerto Rican families. In our report, we describe two monozygotic twins and a boy from two families with novel missense variants, c.295G >A p.(Ala99 Thr), c.3056C >A p.(Pro1019His) and c.2521G >A p.(Gly841Arg) in COL27A1. This is the first documentation of cleft palate and delayed carpal bone ossification as features of Steel syndrome. We also performed a review of clinical features in all mutation-proven Steel syndrome patients. Short stature and dislocation/subluxation of hip joint are consistently observed in Steel syndrome. Other features include dislocated radial heads, scoliosis, lordosis, carpal coalition, facial dysmorphism, hearing loss, bilateral fifth finger clinodactyly, knee deformities and developmental delay. Seven missense variants and eight null variants are reported in COL27A1 until date. We also looked into the genotype-phenotype correlation in Puerto Rican and non-Puerto Rican patients in this study. These three papers add a new skeletal dysplasia (PISD-related), describe recessive collagen type 2 disease, and further characterize Steel syndrome and thus enhance our current understanding of skeletal dysplasias.

Publication year:2022

Keywords:Doctoral thesis

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