Mutational analysis of the ACVR1 gene in Italian patients affected with fibrodysplasia ossificans progressiva: confirmations and advancements

Abstract

Fibrodysplasia ossificans progressiva (FOP, MIM 135100) is a rare genetic disorder characterized by congenital great toe malformations and progressive heterotopic ossification transforming skeletal muscles and connective tissues to bone following a well-defined anatomic pattern of progression. Recently, FOP has been associated with a specific mutation of ACVR1, the gene coding for a bone morphogenetic protein type I receptor. The identification of ACVR1 as the causative gene for FOP now allows the genetic screening of FOP patients to identify the frequency of the identified recurrent ACVR1 mutation and to investigate genetic variability that may be associated with this severely debilitating disease. We report the screening for mutations in the ACVR1 gene carried out in a cohort of 17 Italian patients. Fifteen of these displayed the previously described c.617G>A mutation, leading to the R206H substitution in the GS domain of the ACVR1 receptor. In two patients, we found a novel mutation c.774G>C, leading to the R258S substitution in the kinase domain of the ACVR1 receptor. In the three-dimensional model of protein structure, R258 maps in close proximity to the GS domain, a key regulator of ACVR1 activity, where R206 is located. The GS domain is known to bind the regulatory protein FKBP12 and to undergo multiple phosphorylation events that trigger a signaling cascade inside the cell. The novel amino-acid substitution is predicted to influence either the conformation/stability of the GS region or the binding affinity with FKBP12, resulting in a less stringent inhibitory control on the ACVR1 kinase activity.

Figure 1

Electropherograms of the two point mutations found in our case series: the c.617G>A substitution, recurrent in FOP (a), and the new c.774G>C transition (b) causing the R206H and R258S residue change, respectively.

Figure 2

The R258 residue, boxed in the figure, is highly conserved among species (a) and among the members of the ALK protein family (b). According to the ClustalX software (see Materials and methods), the asterisks underneath sequences in panel b indicate positions with a single, fully conserved residue.

Figure 3

Representation of the molecular surface of the ACVR1: FKPB12 complex in the proximity of the mutated site. (a) Wild-type ACVR1. On account of the strong similarity between the catalytic domain of TGFBR1 and ACVR1, the quaternary arrangement of the ACVR1: FKPB12 complex is predicted to be the same as that observed in the crystal structure of the TGFBR1: FKPB12 complex. The surfaces of the two proteins were calculated independently with the MSMS program suite^, with a conventional probe radius of 1.4 Å. The residue, R258, is represented in ball-and-stick, visible under the semitransparent surface, together with the main chain in its close proximity. The main chain of the αGS1 helix and the GS loop are schematically represented and indicated, the circle encloses the guanidium group of the R258 residue. (b) R258S mutation. The shorter serine side chain is no longer in interaction with the GS region.