Penttinen syndrome-associated PDGFRB Val665Ala variant causes aberrant constitutive STAT1 signalling

A germline PDGFRB splice site variant associated with infantile myofibromatosis and resistance to imatinib

PURPOSE

Infantile myofibromatosis is characterized by the development of myofibroblastic tumors in young children. In most cases, the disease is caused by somatic gain-of-function variants in platelet-derived growth factor (PDGF) receptor beta (PDGFRB). Here, we reported a novel germline intronic PDGFRB variant, c.2905-8G>A, in 6 unrelated infants with multifocal myofibromatosis and their relatives.

METHODS

We performed constitutional and tumor DNA and RNA sequencing to identify novel variants, which were subsequently characterized in cellular assays.

RESULTS

All patients had multiple skin nodules, 4 had bone lesions, and 2 had aggressive disease with bowel obstruction. The c.2905-8G>A substitution creates an alternative acceptor splice site in intron 21, inserting 2 codons in the PDGFRB transcript. Functional studies revealed that the splice change induced a partial loss of function, contrasting with previously described variants. In 4 tumor samples, we identified a second somatic hit at position Asp850 in PDGFRB exon 18, triggering constitutive receptor activation and resistance to imatinib. In addition to vinblastine and methotrexate, 2 patients received imatinib without objective response. One of them switched to dasatinib with concomitant improvement.

CONCLUSION

This splice-site PDGFRB variant favors the development of myofibroma, featuring an acquired oncogenic variant in the same gene and resistance to targeted therapy.

Management of Pediatric Patient with Multiple Cranial, Intracranial, and Spinal Manifestations of Penttinen Syndrome: A Case Report

INTRODUCTION

Penttinen premature aging syndrome is caused by mutations in the PDGFRB gene. We describe the case of a 10-year-old girl with a de novo c.1994T>C variant in PDGFRB who developed multiple cranial, intracranial, and spinal manifestations, including macrocephaly, enlarged convexity subarachnoid spaces crossed by numerous vascularized arachnoid trabecule, hydrocephalus, spinal epidural lipomatosis, a low conus medullaris, calvarial thinning with large anterior fontanelle, and a skull fracture with bilateral epidural hematomas. Vascularized arachnoid granulations, spinal epidural lipomatosis, and low conus medullaris have not been previously described in Penttinen syndrome.

CASE PRESENTATION

A female with Penttinen syndrome diagnosed at 9 years of age initially presented as an infant with cutaneous hemangiomas and macrocephaly; imaging showed enlarged convexity subarachnoid spaces. Her convexity subarachnoid spaces continued to expand, leading to subdural shunt placement. At surgery, her enlarged subarachnoid spaces were found to contain numerous abnormally thick, vascularized arachnoid trabecule. Eventually, her subdural shunt failed and her ventricles enlarged, leading to ventricular shunt placement. A large, sunken anterior fontanelle which did not diminish in size led to cranioplasty with a custom implant. She later developed chronic back pain and imaging revealed spinal epidural lipomatosis, a low conus medullaris, and mild scoliosis. At 10 years of age, a fall from a chair resulted in a depressed skull fracture and bilateral parietal epidural hematomas. Emergency left parietal craniotomy was performed for evacuation of the left hematoma, and the patient recovered without complications. Intraoperatively, it was noted that her skull was extremely thin.

CONCLUSION

This case report highlights the clinical presentation and multifaceted neurosurgical management of a patient with Penttinen syndrome. The patient exhibited characteristic features including hypertrophic skin lesions, macrocephaly, and skeletal abnormalities. Our patient's vascularized arachnoid trabecule, spinal epidural lipomatosis, and low conus medullaris have not previously been reported in Penttinen syndrome. Her thin skull potentially contributed to the extent of her depressed skull fracture after her backwards fall and predisposed her toward developing epidural hematomas. Patients with Penttinen syndrome can have multiple cranial, intracranial, and spinal manifestations which may need the attention of a neurosurgeon.

PDGFRA K385 mutants in myxoid glioneuronal tumors promote receptor dimerization and oncogenic signaling

Myxoid glioneuronal tumors (MGNT) are low-grade glioneuronal neoplasms composed of oligodendrocyte-like cells in a mucin-rich stroma. These tumors feature a unique dinucleotide change at codon 385 in the platelet-derived growth factor receptor α (encoded by the PDGFRA gene), resulting in the substitution of lysine 385 into leucine or isoleucine. The functional consequences of these mutations remain largely unexplored. Here, we demonstrated their oncogenic potential in fibroblast and Ba/F3 transformation assays. We showed that the K385I and K385L mutants activate STAT and AKT signaling in the absence of ligand. Co-immunoprecipitations and BRET experiments suggested that the mutations stabilized the active dimeric conformation of the receptor, pointing to a new mechanism of oncogenic PDGF receptor activation. Furthermore, we evaluated the sensitivity of these mutants to three FDA-approved tyrosine kinase inhibitors: imatinib, dasatinib, and avapritinib, which effectively suppressed the constitutive activity of the mutant receptors. Finally, K385 substitution into another hydrophobic amino acid also activated the receptor. Interestingly, K385M was reported in a few cases of brain tumors but not in MGNT. Our results provide valuable insights into the molecular mechanism underlying the activation of PDGFRα by the K385I/L mutations, highlighting their potential as actionable targets in the treatment of myxoid glioneuronal tumors.

Penttinen syndrome-associated PDGFRB Val665Ala variant causes aberrant constitutive STAT1 signalling

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.