The genetic basis of the osteochondrodysplasias

Should We Stop Calling Thanatophoric Dysplasia a Lethal Condition? A Case Report of a Long-Term Survivor

Thanatophoric dysplasia (TD) is a rare skeletal dysplasia commonly thought to be lethal. In this case report, we discuss a nine-year-old male with TD and review his parents' decision making shortly after their son was born, the technology needed to sustain him, and his parents' perception of his quality of life. We also summarize the clinical course of published long-term survivors with TD. Pediatric Palliative Care teams, especially those conducting perinatal palliative care consultations, are often asked to support families in the face of prognostic uncertainty. Our case report and review of the literature adds to the uncertainty of prognosis in TD and suggests that pediatric palliative care providers should be wary of the label “lethal.”

The impairment of MAGMAS function in human is responsible for a severe skeletal dysplasia

Impairment of the tightly regulated ossification process leads to a wide range of skeletal dysplasias and deciphering their molecular bases has contributed to the understanding of this complex process. Here, we report a homozygous mutation in the mitochondria-associated granulocyte macrophage colony stimulating factor-signaling gene (MAGMAS) in a novel and severe spondylodysplastic dysplasia. MAGMAS, also referred to as PAM16 (presequence translocase-associated motor 16), is a mitochondria-associated protein involved in preprotein translocation into the matrix. We show that MAGMAS is specifically expressed in trabecular bone and cartilage at early developmental stages and that the mutation leads to an instability of the protein. We further demonstrate that the mutation described here confers to yeast strains a temperature-sensitive phenotype, impairs the import of mitochondrial matrix pre-proteins and induces cell death. The finding of deleterious MAGMAS mutations in an early lethal skeletal dysplasia supports a key role for this mitochondrial protein in the ossification process.

Skeletal dysplasias (SD) refer to a complex group of rare genetic disorders affecting the growth and development of the skeleton. The identification of the molecular basis of a considerable number of SD has greatly expanded our knowledge of the ossification process. Among SD, spondylodysplastic dysplasia is a generic term describing different conditions characterized by severe vertebral abnormalities and distinct by additional specific features. Several entities within this group are well defined. However, a few cases remain unclassified, of which a novel autosomal recessive spondylometaphyseal dysplasia recently reported by Mégarbané et al. in two Lebanese families. Here, we identified MAGMAS as a candidate gene responsible for this severe SD. MAGMAS, also referred to as PAM16, is a mitochondria-associated protein, involved in pre-proteins import into mitochondria and essential for cell growth and development. We demonstrated that MAGMAS is expressed in bone and cartilage in early developmental stages underlining its specific role in skeletogenesis. We also give strong evidence of the deleterious effect of the identified mutation on the in-vivo activity of Magmas and the viability of yeast strains. Reporting deleterious MAGMAS mutation in a SD supports a key and specific role for this mitochondrial protein in ossification.

Connexin 40, a target of transcription factor Tbx5, patterns wrist, digits, and sternum

Haploinsufficiency of T-box transcription factor 5 (TBX5) causes human Holt-Oram syndrome (HOS), a developmental disorder characterized by skeletal and heart malformations. Mice carrying a Tbx5 null allele (Tbx5(+/Delta)) have malformations in digits, wrists, and sternum joints, regions where Tbx5 is expressed. We demonstrate that mice deficient in connexin 40 (Cx40), a Tbx5-regulated gap junction component, shared axial and appendicular skeletal malformations with Tbx5(+/Delta) mice. Although no role in skeleton patterning has been described for gap junctions, we demonstrate here that Cx40 is involved in formation of specific joints, as well as bone shape. Even a 50% reduction in either Tbx5 or Cx40 produces bone abnormalities, demonstrating their crucial control over skeletal development. Further, we demonstrate that Tbx5 exerts in part its key regulatory role in bone growth and maturation by controlling via Cx40 the expression of Sox9 (a transcription factor essential for chondrogenesis and skeleton growth). Our study strongly suggests that Cx40 deficiency accounts for many skeletal malformations in HOS and that Tbx5 regulation of Cx40 plays a critical role in the exquisite developmental patterning of the forelimbs and sternum.