The Fibrodysplasia Ossificans Progressiva (FOP) mutation p.R206H in ACVR1 confers an altered ligand response.

Patients with Fibrodysplasia Ossificans Progressiva (FOP) suffer from ectopic bone formation, which progresses during life and results in dramatic movement restrictions. Cause of the disease are point mutations in the Activin A receptor type 1 (ACVR1), with p.R206H being most common. In this study we compared the signalling responses of ACVR1WT and ACVR1R206H to different ligands. ACVR1WT, but not ACVR1R206H inhibited BMP signalling of BMP2 or BMP4 in a ligand binding domain independent manner. Likewise, the basal BMP signalling activity of the receptor BMPR1A or BMPR1B was inhibited by ACVR1WT, but enhanced by ACVR1R206H. In comparison, BMP6 or BMP7 activated ACVR1WT and caused a hyper-activation of ACVR1R206H. These effects were dependent on an intact ligand binding domain. Finally, the neofunction of Activin A in FOP was tested and found to depend on the ligand binding domain for activating ACVR1R206H. We conclude that the FOP mutation ACVR1R206H is more sensitive to a number of natural ligands. The mutant receptor apparently lost some essential inhibitory interactions with its ligands and co-receptors, thereby conferring an enhanced ligand-dependent signalling and stimulating ectopic bone formation as observed in the patients.

ACVR1 p.Q207E causes classic fibrodysplasia ossificans progressiva and is functionally distinct from the engineered constitutively active ACVR1 p.Q207D variant.

Fibrodysplasia ossificans progressiva (FOP) is a disabling genetic disorder of progressive heterotopic ossification (HO). Here, we report a patient with an ultra-rare point mutation [c.619C>G, p.Q207E] located in a codon adjacent to the most common FOP mutation [c.617G>A, p.R206H] of Activin A Receptor, type 1 (ACVR1) and that affects the same intracellular amino acid position in the GS activation domain as the engineered constitutively active (c.a.) variant p.Q207D. It was predicted that both mutations at residue 207 have similar functional effects by introducing a negative charge. Transgenic p.Q207D-c.a. mice have served as a model for FOP HO in several in vivo studies. However, we found that the engineered ACVR1(Q207D-c.a.) is significantly more active than the classic FOP mutation ACVR1(R206H) when overexpressed in chicken limbs and in differentiation assays of chondrogenesis, osteogenesis and myogenesis. Importantly, our studies reveal that the ACVR1(Q207E) resembles the classic FOP receptor in these assays, not the engineered ACVR1(Q207D-c.a.). Notably, reporter gene assays revealed that both naturally occurring FOP receptors (ACVR1(R206H) and ACVR1(Q207E)) were activated by BMP7 and were sensitive to deletion of the ligand binding domain, whereas the engineered ACVR1(Q207D-c.a.) exhibited ligand independent activity. We performed an in silico analysis and propose a structural model for p.Q207D-c.a. that irreversibly relocates the GS domain into an activating position, where it becomes ligand independent. We conclude that the engineered p.Q207D-c.a. mutation has severe limitations as a model for FOP, whereas the naturally occurring mutations p.R206H and p.Q207E facilitate receptor activation, albeit in a reversible manner.

Homozygous missense and nonsense mutations in BMPR1B cause acromesomelic chondrodysplasia-type Grebe.

Acromesomelic chondrodysplasias (ACDs) are characterized by disproportionate shortening of the appendicular skeleton, predominantly affecting the middle (forearms and forelegs) and distal segments (hands and feet). Here, we present two consanguineous families with missense (c.157T>C, p.(C53R)) or nonsense (c.657G>A, p.(W219*)) mutations in BMPR1B. Homozygous affected individuals show clinical and radiographic findings consistent with ACD-type Grebe. Functional analysis of the missense mutation C53R revealed that the mutated receptor was partially located at the cell membrane. In contrast to the wild-type receptor, C53R mutation hindered the activation of the receptor by its ligand GDF5, as shown by reporter gene assay. Further, overexpression of the C53R mutation in an in vitro chondrogenesis assay showed no effect on cell differentiation, indicating a loss of function. The nonsense mutation (c.657G>A, p.(W219*)) introduces a premature stop codon, which is predicted to be subject to nonsense-mediated mRNA decay, causing reduced protein translation of the mutant allele. A loss-of-function effect of both mutations causing recessive ACD-type Grebe is further supported by the mild brachydactyly or even non-penetrance of these mutations observed in the heterozygous parents. In contrast, dominant-negative BMPR1B mutations described previously are associated with autosomal-dominant brachydactyly-type A2.

The ubiquitin-selective chaperone CDC-48/p97, a new player in DNA replication.

Efficient duplication of the genome and its equal distribution into both daughter cells is an essential process for all dividing organisms. To ensure that DNA replication occurs only once during the S phase of the cell cycle, initiation of replication is tightly controlled. Initiation factors are responsible for the recruitment of the replisome, the large molecular machine carrying out DNA synthesis, to origins of replication and license them to start DNA duplication. Remarkably, most of the currently known initiators have been classified as members of the family of AAA(+) ATPases. In our recent study we identified an additional AAA(+) ATPase, CDC-48, to be essential for proper DNA replication in Caenorhabditis elegans. Here, we speculate on the function of CDC-48 (also known as Cdc48p in yeast and p97 in vertebrates) during DNA replication initiation, addressing its ubiquitin-selective chaperone activity.

Cell cycle progression requires the CDC-48UFD-1/NPL-4 complex for efficient DNA replication.

Since cdc48 mutants were isolated by the first genetic screens for cell division cycle (cdc) mutants in yeast, the requirement of the chaperone-like ATPase Cdc48/p97 during cell division has remained unclear. Here, we discover an unanticipated function for Caenorhabditis elegans CDC-48 in DNA replication linked to cell cycle control. Our analysis of the CDC-48(UFD-1/NPL-4) complex identified a general role in S phase progression of mitotic cells essential for embryonic cell division and germline development of adult worms. These developmental defects result from activation of the DNA replication checkpoint caused by replication stress. Similar to loss of replication licensing factors, DNA content is strongly reduced in worms depleted for CDC-48, UFD-1, and NPL-4. In addition, these worms show decreased DNA synthesis and hypersensitivity toward replication blocking agents. Our findings identified a role for CDC-48(UFD-1/NPL-4) in DNA replication, which is important for cell cycle progression and genome stability.