Rac1 is required for Prkar1a-mediated Nf2 suppression in Schwann cell tumors

Abstract

Schwannomas are peripheral nerve sheath tumors that often occur in the setting of an inherited tumor predisposition syndrome, including Neurofibromatosis Types 1 (NF1) and 2 (NF2), Familial Schwannomatosis (FS) and Carney Complex (CNC). Loss of the NF2 tumor suppressor (encoding NF2, or Merlin) is associated with upregulation of the Rac1 small GTPase, which is thought to play a key role in mediating tumor formation. In prior studies, we generated a mouse model of schwannomas by performing tissue-specific knockout of the CNC gene Prkar1a, which encodes the type 1A regulatory subunit of Protein Kinase A. These tumors exhibited down-regulation of Nf2 protein and an increase in activated Rac1. To assess the requirement for Rac1 in schwannoma formation, we generated a double knockout of Prkar1a and Rac1 in Schwann cells and monitored tumor formation. Loss of Rac1 reduced tumor formation by reducing proliferation and enhancing apoptosis. Surprisingly, the reduction of tumor formation was accompanied by re-expression of the Nf2 protein. Furthermore, activated Rac1 was able to downregulate Nf2 in vitro in a Pak-dependent manner. These in vivo data indicate that activation of Rac1 is responsible for suppression of Nf2 protein production; deficiency of Nf2 in Schwann cells leads to loss of cellular growth control and tumor formation.. Further, PKA activation through mutation in Prkar1a is sufficient to initiate Rac1 signaling, with subsequent reduction of Nf2 and schwannomagenesis. Although in vitro evidence has shown that loss of Nf2 activates Rac1, our data indicates that signaling between Nf2 and Rac1 occurs in a bidirectional fashion, and these interactions are modulated by PKA.

NF2 Loss Promotes Oncogenic RAS-Induced Thyroid Cancers via YAP-Dependent Transactivation of RAS Proteins and Sensitizes Them to MEK Inhibition

“…Although germline or somatic NF2 mutations are primarily implicated in the pathogenesis of nervous system tumors, mesotheliomas, melanomas and renal cell carcinomas, mice with heterozygous Nf2 mutation develop a broader range of tumors, which lose the wild-type allele (52). Loss-of function of merlin also occurs in the absence of homozygous deletion or mutation, through aberrant splicing, mRNA loss, calpain-mediated proteolysis, proteasomal degradation, or phosphorylation (27–30, 53). Consistent with this, several thyroid cancer cell lines that were merlin-null or low did not have homozygous genomic NF2 mutations.…”

Schwannomas and Their Pathogenesis

“…PRKAR1a appears to act as a tumor suppressor gene, and inactivation results in increased cyclic adenosine monophosphate (cAMP) activity and Rac 1 activation , leading to suppression of merlin activity (see later) and the development of Schwann cell tumors.…”

Tumors of the neural crest: Common themes in development and cancer

“…Total kinase activity is increased in these patients. Importantly, this includes the inappropriate phosphorylation and inactivation of Merlin (Manchanda et al, ). Repression of merlin signaling links the pathogenesis of schwann cell tumors across multiple tumor predisposition syndromes.…”