Attenuated clinical and osteoclastic phenotypes of Paget’s disease of bone linked to the p.Pro392Leu/SQSTM1 mutation by a rare variant in the DOCK6 gene

Abstract

Background We identified two families with Paget's disease of bone (PDB) linked to the p.Pro392Leu mutation within the SQSTM1 gene displaying a possible digenism. This study aimed at identifying this second genetic variant cosegregating with the p.Pro392Leu mutation and at characterizing its impact on the clinical and cellular phenotypes of PDB. Methods Whole exome sequencing was performed in one patient per family and two healthy controls. We compared clinical characteristics of PDB in 14 relatives from the two families. The osteoclastic phenotype was compared in in vitro differentiated osteoclasts from 31 participants carrying the DOCK6 and/or SQSTM1 variants. Tridimensional models of SQSTM1 and DOCK6 proteins were generated to evaluate the impact of these variants on their stability and flexibility. Statistical analyses were performed with Graphpad prism. Results Whole-exome sequencing allowed us to identify the p.Val45Ile missense variant in the DOCK6 gene in patients. In both families, the mean age at PDB diagnosis was delayed in pagetic patients carrier of the p.Val45Ile variant alone compared to those carrying the p.Pro392Leu mutation alone (67 vs. 44 years, P = 0.03). Although both p.Val45Ile and p.Pro392Leu variants gave rise to a pagetic phenotype of osteoclast versus healthy controls, the p.Val45Ile variant was found to attenuate the severity of the osteoclastic phenotype of PDB caused by the p.Pro392Leu mutation when both variants were present. The DOCK6 mRNA expression was higher in carriers of the p.Val45Ile variant than in pagetic patients without any mutations and healthy controls. Structural bioinformatics analyses suggested that the p.Pro392Leu mutation might rigidify the UBA domain and thus decrease its possible intramolecular interaction with a novel domain, the serum response factor–transcription factor (SRF-TF)-like domain, whereas the p.Val45Ile variant may decrease SRF-TF-like activity. Conclusion The p.Val45Ile variant may attenuate the severity of the clinical phenotype of PDB in patient carriers of both variants. In vitro, the rare variant of the DOCK6 may have a modifier effect on the p.Pro392Leu mutation, possibly via its effect on the SRF-TF-like.

Update on the pathogenesis and genetics of Paget’s disease of bone

“…Always remaining in the context of modifying genes, very recently Dessay et al (2022) elegantly identified, by whole exome sequencing in two large unrelated French Canadian PDB families with germline P392L SQSTM1 mutation, a clinically attenuating effect on PDB severity exerted by a V45I rare variant in the DOCK6 gene. Remarkably, albeit both variants were separately found in PDB patients and gave rise to a pagetic phenotype of osteoclasts in vitro versus healthy controls, the DOCK6 variant was found to delay the onset of PDB and attenuate the severity of osteoclast phenotype of PDB caused by the P392L mutation of SQSTM1, when both variants were present.…”

“…DOCK6 protein belongs to the dedicator of cytokinesis (DOCK) family of atypical guanine nucleotide exchange factors, components of intracellular signaling networks and by interacting with small GTPases they probably play a role in actin cytoskeletal reorganization by activating the Rho GTPases Cdc42 and Rac1 (Miyamoto et al, 2007). The same Authors performed a structural bioinformatics analyses showing that the SQSTM1 P392L mutation may decrease its possible intramolecular interaction with the serum response factor-transcription factor (SRF-TF)-like domain, while at the same time the V45I variant of DOCK6 gene may decrease SRF-TF-like activity (Dessay et al, 2022). Indeed, homozygous or dominant negative heterozygous mutations of DOCK6 gene account for the Adams-Oliver syndrome-2, an autosomal recessive multiple congenital anomaly syndrome featured by aplasia cutis congenita and terminal transverse limb defects, in association with variable involvement of the brain, eyes, and cardiovascular system (Shaheen et al, 2011).…”