An Eye on Metastasis: Despite the considerable progress being made in elucidating the cell biology of metastasis, little is known about the genetic alterations that promote metastasis of human tumors, the cause of most cancer deaths. A potentially important clue now emerges from the work of Harbour et al. (p. 1410 , published online 4 November), who used an exome-sequencing approach to search for genetic mutations in uveal melanomas, an eye cancer associated with a high rate of fatal metastasis. Remarkably, over 80% of tumor samples with a high metastatic risk had inactivating somatic mutations in the gene encoding BAP1 (BRCA1-associated protein 1), a nuclear protein involved in controlling protein degradation. Thus, in this tumor type, mutational inactivation of BAP1 may be a key event in the acquisition of metastatic competence.

William Harbour, Anne Bowcock and colleagues identify recurrent mutations at codon 625 of SF3B1 in uveal melanomas. These mutations occur in low-grade tumors and are associated with favorable prognosis. Uveal melanoma is the most common primary cancer of the eye and often results in fatal metastasis. Here, we describe mutations occurring exclusively at codon 625 of the SF3B1 gene, encoding splicing factor 3B subunit 1, in low-grade uveal melanomas with good prognosis. Thus, uveal melanoma is among a small group of cancers associated with SF3B1 mutations, and these mutations denote a distinct molecular subset of uveal melanomas.

Uveal melanoma (UM) is a highly metastatic cancer that, in contrast to cutaneous melanoma, is largely unresponsive to checkpoint immunotherapy. Here, we interrogate the tumor microenvironment at single-cell resolution using scRNA-seq of 59,915 tumor and non-neoplastic cells from 8 primary and 3 metastatic samples. Tumor cells reveal novel subclonal genomic complexity and transcriptional states. Tumor-infiltrating immune cells comprise a previously unrecognized diversity of cell types, including CD8⁺ T cells predominantly expressing the checkpoint marker LAG3, rather than PD1 or CTLA4. V(D)J analysis shows clonally expanded T cells, indicating that they are capable of mounting an immune response. An indolent liver metastasis from a class 1B UM is infiltrated with clonally expanded plasma cells, indicative of antibody-mediated immunity. This complex ecosystem of tumor and immune cells provides new insights into UM biology, and LAG3 is identified as a potential candidate for immune checkpoint blockade in patients with high risk UM.

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 α_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.