Early phase trials targeting the T-cell inhibitory molecule PD-L1 have shown clinical efficacy in cancer. This study was undertaken to determine whether PD-L1 is overexpressed in triple-negative breast cancer (TNBC) and to investigate the loss of the phosphatase and tensin homolog (PTEN) as a mechanism of PD-L1 regulation. The Cancer Genome Atlas (TCGA) RNA sequencing data showed significantly greater expression of the PD-L1 gene in TNBC (n=120) compared to non-TNBC (n=716) (P<0.001). Breast tumor tissue microarrays were evaluated for PD-L1 expression which was present in 19% (20 of 105) TNBC specimens. PD-L1+ tumors had greater CD8+ T-cell infiltrate than PD-L1− tumors (688 cells/mm versus 263 cells/mm; P<0.0001). To determine the effect of PTEN loss on PD-L1 expression, stable cell lines were generated using PTEN shRNA. PTEN knockdown led to significantly higher cell-surface PD-L1 expression and PD-L1 transcripts, suggesting transcriptional regulation. Moreover, PI3K pathway inhibition using the AKT inhibitor MK-2206 or rapamycin resulted in decreased PD-L1 expression, further linking PTEN and PI3K signaling to PD-L1 regulation. Co-culture experiments were performed to determine the functional effect of altered PD-L1 expression. Increased PD-L1 cell surface expression by tumor cells induced by PTEN loss led to decreased T cell proliferation and increased apoptosis. PD-L1 is expressed in 20% of TNBC, suggesting PD-L1 as a therapeutic target in TNBC. Since PTEN loss is one mechanism regulating PD-L1 expression, agents targeting the PI3K pathway may increase the antitumor adaptive immune responses.

Myotonic dystrophy type 1 (DM1) is caused by a CTG trinucleotide expansion in the 3' untranslated region of the DM protein kinase gene. People with DM1 have an unusual form of insulin resistance caused by a defect in skeletal muscle. Here we demonstrate that alternative splicing of the insulin receptor (IR) pre-mRNA is aberrantly regulated in DM1 skeletal muscle tissue, resulting in predominant expression of the lower-signaling nonmuscle isoform (IR-A). IR-A also predominates in DM1 skeletal muscle cultures, which exhibit a decreased metabolic response to insulin relative to cultures from normal controls. Steady-state levels of CUG-BP, a regulator of pre-mRNA splicing proposed to mediate some aspects of DM1 pathogenesis, are increased in DM1 skeletal muscle; overexpression of CUG-BP in normal cells induces a switch to IR-A. The CUG-BP protein mediates this switch through an intronic element located upstream of the alternatively spliced exon 11, and specifically binds within this element in vitro. These results support a model in which increased expression of a splicing regulator contributes to insulin resistance in DM1 by affecting IR alternative splicing.

Myotonic dystrophy (DM) is caused by a CTG expansion in the 3' untranslated region of the DM gene. One model of DM pathogenesis suggests that RNAs from the expanded allele create a gain-of-function mutation by the inappropriate binding of proteins to the CUG repeats. Data presented here indicate that the conserved heterogeneous nuclear ribonucleoprotein, CUG-binding protein (CUG-BP), may mediate the trans-dominant effect of the RNA. CUG-BP was found to bind to the human cardiac troponin T (cTNT) pre-messenger RNA and regulate its alternative splicing. Splicing of cTNT was disrupted in DM striated muscle and in normal cells expressing transcripts that contain CUG repeats. Altered expression of genes regulated posttranscriptionally by CUG-BP therefore may contribute to DM pathogenesis.