Somatic mutations in a subset of growth hormone (GH)-secreting pituitary tumors convert the gene for the alpha polypeptide chain (alpha s) of Gs into a putative oncogene, termed gsp. These mutations, which activate alpha s by inhibiting its guanosine triphosphatase (GTPase) activity, are found in codons for either of two amino acids, each of which is completely conserved in all known G protein alpha chains. The likelihood that similar mutations would activate other G proteins prompted a survey of human tumors for mutations that replace either of these two amino acids in other G protein alpha chain genes. The first gene so far tested, which encodes the alpha chain of Gi2, showed mutations that replaced arginine-179 with either cysteine or histidine in 3 of 11 tumors of the adrenal cortex and 3 of 10 endocrine tumors of the ovary. The mutant alpha i2 gene is a putative oncogene, referred to as gip2. In addition, gsp mutations were found in 18 of 42 GH-secreting pituitary tumors and in an autonomously functioning thyroid adenoma. These findings suggest that human tumors may harbor oncogenic mutations in various G protein alpha chain genes.

Decitabine (5-aza-2-deoxycytidine) inhibits DNA methylation and has dual effects on neoplastic cells, including the reactivation of silenced genes and differentiation at low doses and cytotoxicity at high doses. We evaluated, in a phase 1 study, low-dose prolonged exposure schedules of decitabine in relapsed/refractory leuke- 19 or 11%). There was no correlation between P15 methylation at baseline or after therapy and response to decitabine. We conclude that decitabine is effective in myeloid malignancies, and low doses are as or more effective than higher doses.

We discuss the biology of Ras signal transduction and the epidemiology of ras mutations in association with disease as a background for the development of a Raf kinase inhibitor, BAY 43-9006. Knowledge of Ras effector pathways has permitted genetic validation of numerous targets involved in the Ras signaling cascade. A key Ras effector pathway involves the kinase cascade RAF/MEK/ ERK (MEK: MAP/ERK kinase; ERK: extracellular signal related kinase). Indeed, we present studies of cell lines stably expressing mutant MEK constructs, which point to Raf kinase as a target for therapeutics with selective anti-tumor activity. Finally, a small molecule drug discovery program based on inhibition of Raf kinase activity is outlined and the initial pre-clinical development process of the Raf kinase inhibitor BAY 43-9006 is discussed.

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Activating mutations in the gene for the alpha-chain of Gs, the stimulatory regulator of adenylyl cyclase, have been identified in human GH-secreting pituitary tumors. Using the polymerase chain reaction and allele-specific oligonucleotide hybridization, we screened 25 GH-secreting tumors for the presence of the activating mutations. We also reviewed the clinical charts of the patients from whom the tumors were removed. Of 25 tumors, 10 (40%) contained activating mutations. Patients in the mutation-positive group came to surgery with smaller tumors and had lower GH levels. The activating mutations identify a subgroup of GH-secreting pituitary tumors that probably arise from a shared oncogenic mechanism.