Kosaki overgrowth syndrome (KOGS) is a rare genetic disorder linked to germline variants in the platelet-derived growth factor receptor beta gene ( PDGFRB ) that is characterized by postnatal overgrowth, hyperelastic skin, lipodystrophy, and craniofacial anomalies. This study aimed to summarize clinical and genetic data from reported KOGS cases and investigate the molecular consequences of two recurrent KOGS-associated PDGFRB variants. Clinical and genetic information from 17 previously published KOGS cases was reviewed. Molecular studies were performed using patient-derived fibroblasts and genetically modified cells transduced with the recurrent PDGFRB variants Trp566Arg and Pro584Arg. Downstream signaling and phosphorylation of PDGFRβ tyrosine residues were assessed by immunoblotting and immunocytochemistry. Both variants induced phosphorylation of specific PDGFRβ tyrosine residues and activated downstream signaling pathways in the absence of ligand stimulation, which could contribute to the phenotypic overgrowth observed in KOGS. Immunocytochemistry revealed vesicle-like structures of phosphorylated PDGFRβ (pY740), resembling wild-type cells stimulated with growth factor, thereby supporting the constitutive activation of PDGFRβ in patient fibroblasts. Both variants showed sensitivity to the tyrosine kinase inhibitor imatinib. Recurrent PDGFRB variants in KOGS cause ligand-independent activation of PDGFRβ, contributing to the overgrowth phenotype. Imatinib may represent a potential targeted therapeutic option.

Platelet-derived growth factor receptor-beta (PDGFRβ) is a receptor tyrosine kinase that plays significant roles in cell growth, proliferation, and differentiation. Germline variants of PDGFRB can lead to several different diseases, e.g. infantile myofibromatosis, Kosaki overgrowth syndrome, Penttinen premature aging syndrome, ocular pterygium - digital keloid dysplasia, primary familial brain calcification, and others. Some variants cause the kinase to be constitutively active, even in the absence of ligand, while others lead to inactivation of signaling transduction mechanisms. Constitutive activation of PDGFRβ leads to increased cell growth, proliferation, and differentiation, which can lead to the development of tumors or other abnormal growths. The development of new therapies that target PDGFRβ is an active area of research, primarily in cancer treatment. However, these therapies have the potential to also provide effective treatment options for patients with germline variants of PDGFRB. Here, we provide a summary of recurrent activating germline variants reported in PDGFRB and examine their sensitivity to different tyrosine kinase inhibitors. We show that the respective amino acid substitutions respond differently to treatment with tyrosine kinase inhibitors that correlate with previous in vivo data. Our data may assist healthcare providers when deciding personalized treatment of patients with disorders associated with activating variants in PDGFRB.

Syndromic forms of craniosynostosis occur as a result of dysregulation of various molecular signaling cascades. In humans, a specific gain-of-function mutation (W566R) in PDGFRB causes a distinctive overgrowth syndrome (OMIM # 616592). Affected individuals exhibit distinctive facial features and craniosynostosis. Using CRISPR/Cas9 gene editing, we generated a mouse model carrying the same pathogenic variant of PDGFRB. The Pdgfrb+/W565R mice exhibited craniosynostosis with skull-base malformation: thus, we successfully recapitulated the human disease phenotype. In humans, haploinsufficiency of RUNX2, a critical transcription factor in osteogenesis, results in defects of the skull and clavicles due to insufficient membranous ossification. Such phenotypes have been well reproduced in Runx2+/- mice. To delineate the molecular mechanisms underlying the development of Pdgfrb-related craniosynostosis, we crossed the Pdgfrb+/W565R mice with Runx2+/- mice. It is noteworthy that the double- mutant mice, i.e. Pdgfrb+/W565R  Runx2+/- mice, exhibited near complete restoration of the cranial sutures and skull base. The present observation provides in vivo evidence that overactivation of Pdgfrb signaling leads to craniosynostosis through the effect of Runx2. The phenotypic reversal of the cranial structures suggests that modification of the Pdgfrb-Runx2 signaling cascade might offer a novel therapeutic opportunity for craniosynostosis.

Penttinen syndrome is a rare progeroid disorder caused by mutations in platelet-derived growth factor (PDGF) receptor beta (encoded by the PDGFRB proto-oncogene) and characterized by a prematurely aged appearance with lipoatrophy, skin lesions, thin hair and acro-osteolysis. Activating mutations in PDGFRB have been associated with other human diseases, including Kosaki overgrowth syndrome, infantile myofibromatosis, fusiform aneurysms, acute lymphoblastic leukaemia and myeloproliferative neoplasms associated with eosinophilia. The goal of the present study was to characterize the PDGFRB p.Val665Ala variant associated with Penttinen syndrome at the molecular level. This substitution is located in a conserved loop of the receptor tyrosine kinase domain. We observed that the mutant receptor was expressed at a lower level but showed constitutive activity. In the absence of ligand, the mutant activated STAT1 and elicited an interferon-like transcriptional response. Phosphorylation of STAT3, STAT5, AKT and phospholipase Cγ was weak or undetectable. It was devoid of oncogenic activity in two cell proliferation assays, contrasting with classical PDGF receptor oncogenic mutants. STAT1 activation was not sensitive to ruxolitinib and did not rely on interferon-JAK2 signalling. Another tyrosine kinase inhibitor, imatinib, blocked signalling by the p.Val665Ala variant at a higher concentration compared with the wild-type receptor. Importantly, this concentration remained in the therapeutic range. Dasatinib, nilotinib and ponatinib also inhibited the mutant receptor. In conclusion, the p.Val665Ala variant confers unique features to PDGF receptor β compared with other characterized gain-of-function mutants, which may in part explain the particular set of symptoms associated with Penttinen syndrome.