A TREM2-derived circRNA is upregulated in the entorhinal cortex of Alzheimer's disease patients

Circular RNAs (circRNAs) are a novel class of noncoding RNAs characterized by a covalent and stable closed loop structure. circRNAs are enriched in neural tissues, particularly at synapses, where they are involved in synaptic plasticity. Alzheimers disease (AD) is considered a synaptopathy since neurodegeneration causes loss or dysfunction of synapses. Microglia participate in synaptic pruning and also play a crucial role in developing AD. For instance, genetic variants in TREM2, a microglia-related gene, are risk factors for AD. Alterations in circRNAs expression have been described in different neurological diseases, including AD. However, no TREM2-derived circRNAs have been described so far. TREM2 has 3 linear RNA variants due to alternative splicing. We hypothesized that alternative splicing of exon 4 might be favoring circRNAs originating from TREM2 (circTREM2s), which in turn might be involved in AD pathogenesis. First, divergent primers (overlapping exons 3-4 and 4-5) were designed to amplify circRNAs by RT-PCR, which were confirmed by Sanger sequencing. Three candidate TREM2-derived circRNAs were identified on control human entorhinal samples. Then, additional primer sets were used to confirm back-splicing junctions. One of the circRNAs, circTREM2_1, was consistently amplified with all primer sets. In addition, circTREM2_1 was also present in AD entorhinal cortex samples and in HMC3 cells. We observed that circTREM2_1 is up-regulated in AD entorhinal cortex samples compared to controls, particularly at early stages of the disease, when we performed RT-qPCR. In conclusion, we have identified a novel circRNA derived from the TREM2 gene that could play a role in AD pathogenesis.