Frequent mutation of BAP1 in metastasizing uveal melanomas

Abstract

Metastasis is a defining feature of malignant tumors and is the most common cause of cancer-related death, yet the genetics of metastasis are poorly understood. We used exome capture coupled with massively parallel sequencing to search for metastasis-related mutations in highly metastatic uveal melanomas of the eye. Inactivating somatic mutations were identified in the gene encoding BRCA1-associated protein 1 (BAP1) on chromosome 3p21.1 in 26 of 31 (84%) metastasizing tumors, including 15 mutations causing premature protein termination and 5 affecting its ubiquitin carboxyl-terminal hydrolase domain. One tumor harbored a frameshift mutation that was germline in origin, thus representing a susceptibility allele. These findings implicate loss of BAP1 in uveal melanoma metastasis and suggest that the BAP1 pathway may be a valuable therapeutic target.

Fig. 1

Inactivating mutations in BAP1 occur frequently in uveal melanomas. (A) Sanger sequence traces of MM 056 and MM 070 at the sites of the mutations. Location of mutated base in MM 056 and the start of the deletion of MM 070 are indicated (arrows). The non-coding BAP1 strand is shown for MM 070. (B) Map of BAP1 gene and location of BAP1 mutations. BAP1 contains 17 exons (shaded boxes) that encode a 728 amino acid protein. Introns are not to scale. Mutations are shown below the gene figure as indicated. The UCH domain (aa. 1-188) and UCH37-like domain (ULD) (aa. 635-693) are indicated (12, 13). The critical Q, C, H and D residues of the active site (Gln85, Cys91, His169 and Asp184) are indicated with asterisks. The catalytic cysteine is indicated with a circle. Also shown are: the NHNY consensus sequence for interaction with HCFC1 (aa. 363-365, exon 11), nuclear localization signals (NLS) at aa. 656-661 (exon 15) and aa. 717-722 (exon 17), the BARD1 binding domain within the region bounded by aa. 182-240 (13), and the BRCA1 binding domain within aa. 598-729 (11). (C) Location of BAP1 missense mutations in the UCH domain aligned to the crystal structure of UCH-L3 (21). Three-dimensional structure of UCH-L3 was visualized with MMDB software (22). The small molecule near C91G, H169Q and S172R represents a suicide inhibitor, illustrating the critical location of these mutations for catalytic activity. (D) BAP1 mRNA levels measured by quantitative RT-PCR in 9 non-metastasizing class 1 UMs and 28 metastasizing class 2 UMs. (E) Relationship between BAP1 mRNA levels (measured by quantitative RT-PCR) and type of BAP1 mutation in 9 UMs with nonsense mutations, 10 UMs with missense mutations (including small in-frame deletions, splice acceptor, and stop codon read-through mutations), and 4 class 2 UMs in which no BAP1 mutations were detected.

Fig. 2

UM cells depleted of BAP1 acquire properties that are typical of metastasizing class 2 tumor cells. (A) Phase contrast photomicrographs of 92.1 uveal melanoma cells transfected with BAP1 or control siRNA at the indicated days. Bottom panels show representative examples of class 1 and class 2 uveal melanoma cells obtained from patient biopsy samples (Papanicolaou stain). Scale bars, 10 microns. (B) 92.1 cells transfected with BAP1 siRNA and evaluated after five days. BAP1 protein levels were efficiently depleted to less than 95% of control levels (see western blot). Upper panel depicts principal component analysis to show effect of BAP1 knockdown on gene expression signature. The small spheres represent the training set of known class 1 (blue) and class 2 (red) tumors. Large spheres represent the control-transfected (gray) and BAP1 siRNA transfected (red) cells. Lower panel depicts mRNA levels measured by quantitative RT-PCR of a panel of melanocyte lineage genes, presented as fold change in BAP1 siRNA/control siRNA transfected cells. Results are representative of three independent experiments.