Oncogenic mutations in GNAQ occur early in uveal melanoma

Abstract

Purpose: Early/initiating oncogenic mutations have been identified for many cancers, but such mutations remain unidentified in uveal melanoma (UM). An extensive search for such mutations was undertaken, focusing on the RAF/MEK/ERK pathway, which is often the target of initiating mutations in other types of cancer.

Methods: DNA samples from primary UMs were analyzed for mutations in 24 potential oncogenes that affect the RAF/MEK/ERK pathway. For GNAQ, a stimulatory alpha(q) G-protein subunit which was recently found to be mutated in UMs, resequencing was expanded to include 67 primary UMs and 22 peripheral blood samples. GNAQ status was analyzed for association with clinical, pathologic, chromosomal, immunohistochemical, and transcriptional features.

Results: Activating mutations at codon 209 were identified in GNAQ in 33 (49%) of 67 primary UMs, including 2 (22%) of 9 iris melanomas and 31 (54%) of 58 posterior UMs. No mutations were found in the other 23 potential oncogenes. GNAQ mutations were not found in normal blood DNA samples. Consistent with GNAQ mutation being an early or initiating event, this mutation was not associated with any clinical, pathologic, or molecular features associated with late tumor progression.

Conclusions: GNAQ mutations occur in about half of UMs, representing the most common known oncogenic mutation in this cancer. The presence of this mutation in tumors at all stages of malignant progression suggests that it is an early event in UM. Mutations in this G-protein-coupled receptor provide new insights into UM pathogenesis and could lead to new therapeutic possibilities.

Figure 1

Regions of chromosomal gain identified by CGH. (A) CGHminer result for sixteen class 1 and twelve class 2 tumors. DNA gains (indicated by orange and red vertical bars) with respect to chromosomal position (horizontal lines) on chromosomes 6p, 8q and, to a lesser extent, 20p and 20q. The p-arms are depicted to the left, and the q-arms to the right of the centromeres (vertical purple bars). The percent of samples showing DNA gain is indicated by the scale at the bottom. (B) CGH tracing of chromosome 5, showing two peaks with a mean log2ratio ≥ 3 standard deviations above the mean for the chromosomal arm. The larger peak at 5q13.1 corresponded to the location of PIK3R1. The other smaller peak did not correspond to a coding region. (C) Pathways that affect RAF/MEK/ERK activation. Arrows indicate stimulatory interactions, and T-bars indicate inhibitory interactions. Abbreviations: RTK, receptor tyrosine kinase; GPCR, G-protein coupled receptor. Other abbreviations are official gene symbols. Notations of specific genes analyzed in this study: ¹Ras superfamily of small GTPases: DIRAS2, REM1, GEM, RAB2A, RAB22A, RAB23 (HRAS, KRAS and NRAS were previously analyzed); ²PAK7; ³ARAF, RAF1 and RASIP1 (BRAF was previously analyzed); ⁴PTK2 and PTK6; ⁵PIK3R1 regulatory subunit; ⁶MAPK13 and MAPK14; ⁷GNAQ; ⁸GRM1. Red shapes indicate genes that were re-sequenced in this study.

Figure 2

Representative sequence tracings for GNAQ surrounding codon 209 (shaded). UM86, normal uveal melanocyte sample; MM31, uveal melanoma with wildtype sequence (CAA); MM37, MM18 and MM88, uveal melanomas with the three mutant sequences, as indicated.