多瘤病毒是小型环状 dsDNA 病毒，可引起癌症。多瘤病毒早期转录本的选择性剪接产生大小肿瘤抗原（LT、ST），它们在病毒复制和肿瘤发生中发挥重要作用。一些多瘤病毒还表达中间肿瘤抗原 (MT) 或替代 LT 开放阅读框 (ALTO)，它们在进化上相关，但具有不同的基因结构。 MT 是早期转录本的剪接变体，而 ALTO 则以替代阅读框叠印在 LT 转录本的第二个外显子上，并通过替代起始密码子进行翻译。默克尔细胞多瘤病毒 (MCPyV) 是唯一一种导致癌症的人类多瘤病毒，编码 ALTO，但其在病毒生命周期和肿瘤发生中的作用仍然难以捉摸。在这里，我们展示了 MCPyV ALTO 作为肿瘤抑制因子，并且在默克尔细胞癌 (MCC) 中被沉默。拯救 MCC 细胞中的 ALTO 会诱导生长停滞并激活 NF-κB 信号传导。 ALTO 通过结合 SQSTM1 和 TRAF2 激活 NF-κB

Polyomaviruses are small, circular dsDNA viruses that can cause cancer. Alternative splicing of polyomavirus early transcripts generates large and small tumor antigens (LT, ST) that play essential roles in viral replication and tumorigenesis. Some polyomaviruses also express middle tumor antigens (MTs) or alternate LT open reading frames (ALTOs), which are evolutionarily related but have distinct gene structures. MTs are a splice variant of the early transcript whereas ALTOs are overprinted on the second exon of the LT transcript in an alternate reading frame and are translated via an alternative start codon. Merkel cell polyomavirus (MCPyV), the only human polyomavirus that causes cancer, encodes an ALTO but its role in the viral lifecycle and tumorigenesis has remained elusive. Here, we show MCPyV ALTO acts as a tumor suppressor and is silenced in Merkel cell carcinoma (MCC). Rescuing ALTO in MCC cells induces growth arrest and activates NF-κB signaling. ALTO activates NF-κB by binding SQSTM1 and TRAF2&3 via two N-Terminal Activating Regions (NTAR1+2), resembling Epstein-Barr virus (EBV) Latent Membrane Protein 1 (LMP1). Following activation, NF-κB dimers bind the MCPyV noncoding control region (NCCR) and downregulate early transcription. Beyond MCPyV, NTAR motifs are conserved in other polyomavirus ALTOs, which activate NF-κB signaling, but are lacking in MTs that do not. Furthermore, polyomavirus ALTOs downregulate their respective viral early transcription in an NF-κB- and NTAR-dependent manner. Our findings suggest that ALTOs evolved to suppress viral replication and promote viral latency and that MCPyV ALTO must be silenced for MCC to develop.