LAUSR

To the Editor: Phospholipase A2 receptor 1 (PLA2R1) encodes a transmembrane receptor that plays a role in clearance of phospholipase A2, thereby inhibiting the action of phospholipaseA2 (1). Activation of PLAR1 leads to cellular senescence, apoptosis and inhibition of cell transformation, suggesting its tumor suppressor gene (TSG) activity in cells (1). Decreased PLA2R1 expression is found in many cancers, including colorectal cancer (CRC) (1). Hypermethylation of PLA2R1 promoter that may result in the decreased expression is identified in kidney cancers (1). SRSF protein kinase 1 (SRPK1) encodes a serine/arginine protein kinase specific for the SR (serine/arginine-rich domain) family of splicing factors (2). SRPK1 binds to AKT and either activates or inactivates AKT functions, thus performing dual opposing functions (oncogenic or TSG) depending on cellular context. For example, ablation of SRPK1 induces cellular transformation, confirming its TSG function (3). However, it remains unknown whether inactivating frameshift mutations of PLA2R1 and SRPK1 are common in gastric cancer (GC) and CRC. About one third of GC and CRC are classified as high microsatellite instability (MSI-H) cancers (4). Many TSGs harbor frameshift mutations at monocleotide repeats in MSI-H cancers (4). In the human genome database, we observed that both PLA2R1 and SRPK1 genes possess nucleotide repeats in coding sequences that might be mutated in MSI-H cancers. In this study, we analyzed an A8 repeat in PLA2R1 exon 28 and an A7 repeat in SRPK1 exon 7 by polymerase chain reaction (PCR)-based single strand conformation polymorphism (SSCP) assay. We did not analyze other possible inactivating variants such as nonsense or splicing variants in this study. Previously, the A8 of PLA2R1 and the A7 of SRPK1 harbored deletion mutations in 3 (one GC, one CRC and one melanoma) and one (one CRC) cases, respectively (COSMIC database). In this study, we used 79 GCs and 124 CRCs. The GCs were 34 GCs with MSI-H, 45 GCs with microsatellite stable/low MSI (MSS/MSI-L), 79 CRCs with MSI-H and 45 CRCs with MSS/MSI-L. In cancer tissues, malignant cells and normal cells were selectively procured by microdissection (5). Radioisotope ([P]dCTP) was incorporated into the PCR products, which were subsequently displayed in SSCP gels and analyzed with direct DNA sequencing (5). On SSCP, we observed aberrant bands of PLA2R1 in two cases of CRC and SRPK1 in one case of GC. Sanger sequencing of them revealed that they were frameshift mutations of PLA2R1 and SRPK1. DNA from the patients’ normal tissues showed no evidence of mutation in both SSCP and Sanger sequencing, indicating the mutations had risen somatically. These mutations were all deletions of one base in theA8 repeat of PLA2R1 (c.4113delA) and the A7 repeat of SRPK1 (c.574delA) that would result in frameshift mutations (p.Gly1372AlafsX32 of PLA2R1 and p.Ile192LeufsX32 of SRPK1). They were detected in two CRCs (2/79: 2.5%) and one GC (1/34: 2.9%) with MSI-H, but not in those with MSS (0/90). The frameshift mutation detected in the current study would result in premature stops of amino acid synthesis in PLA2R1 and SRPK1 proteins and hence resembles a typical inactivating mutation. Both of these would be expected to lead to premature stop codons in the mRNAs but not necessarily the production of a truncated protein as these transcripts are more likely to be degraded by nonsense mediated decay. Based on the previous reports that showed TSG activities of PLA2R1 and SRPK1 in cells (1,3), these frameshift mutations could contribute to cancer development by inhibiting their TSG activities. However, the incidence of the mutations is very low and identified only in MSI-H cancers. Our data may suggest that frameshift mutations of PLA2R1 and SRPK1 may not play a principal role in inhibiting their TSG functions in GC and CRC.