Palmitoylation of the small GTPase Cdc42 by DHHC5 modulates spine formation and gene transcription.

The small GTPase Cdc42 exists in the form of two alternatively spliced variants that are modified by hydrophobic chains: the ubiquitously expressed Cdc42-prenyl and a brain-specific isoform that can be palmitoylated, Cdc42-palm. Our previous work demonstrated that Cdc42-palm can be palmitoylated at two cysteine residues, Cys188 and Cys189, while Cys188 can also be prenylated. We showed that palmitoylation of Cys188 is essential for the plasma membrane localization of Cdc42-palm and is critically involved in Cdc42-mediated regulation of gene transcription and neuronal morphology. However, the abundance and regulation of this modification was not investigated. In the present study, we found that only a minor fraction of Cdc42 undergoes monopalmitoylation in neuroblastoma cells and in hippocampal neurons. In addition, we identified DHHC5 as one of the major palmitoyl acyltransferases that could physically interact with Cdc42-palm. We demonstrate that overexpression of dominant negative DHHC5 mutant decreased palmitoylation and plasma membrane localization of Cdc42-palm. In addition, knockdown of DHHC5 significantly reduced Cdc42-palm palmitoylation, leading to a decrease of Cdc42-mediated gene transcription and spine formation in hippocampal neurons. We also found that the expression of DHHC5 in the brain is developmentally regulated. Taken together, these findings suggest that DHHC5-mediated palmitoylation of Cdc42 represents an important mechanism for the regulation of Cdc42 functions in hippocampus.

Lack of noncanonical RAS mutations in cytogenetically normal acute myeloid leukemia.

Transforming mutations in RAS genes are commonly found in human malignancies, including myeloid leukemias. To investigate the incidence, spectrum, and distribution of activating K- and N-RAS mutations in cytogenetically normal acute myeloid leukemia (CN-AML) patients, 204 CN-AML patients were screened. Activating K- and N-RAS mutations were detected in 3 of 204 (1.5 %) and 22 of 204 (10.8 %) CN-AML samples, respectively. RAS mutated patients presented with a lower percentage of bone marrow blasts (65 vs 80 %, P = 0.022). RAS mutations tended to occur with nucleophosmin-1 (NPM1) mutations (P = 0.079), and all three samples containing K-RAS mutations had concomitant NPM1 mutations. There was no significant overlap between K-RAS mutations and N-RAS, FLT3, CEBPA, IDH1/2, WT1 or MLL mutations. RAS mutation status did not impact relapse-free or overall survival of CN-AML patients. In contrast to reports of noncanonical RAS mutations in other cancers, including some leukemia subtypes, we only observed K- and N-RAS mutations in codons 12, 13, or 61 in CN-AML samples. Our findings suggest that while K-RAS mutations are infrequent in CN-AML, activating K-RAS mutations may cooperate with mutated NPM1 to induce leukemia.