THE1 repeats: Ancient endogenous retroviruses rampaging behind sarcoid myopathy

Transposon-like human element (THE1) repeats are classified as part of the ERVL (endogenous retrovirus-like) -MaLR family and were previously thought to be remnants derived from retroviral infections in the germ cells of ancient primates. At present, at least 60,561 THE1 repeats have been mapped in the human genome, and they are divided into four subfamilies: THE1A, THE1B, THE1C, and THE1D. Although these repeats are largely silenced in somatic cells through multiple mechanisms, some of them are known to influence gene functions and genomic structures, particularly under disease conditions. Recently, we happened to identify the CIR1 driven from THE1B in the individuals with sarcoid myopathy (SM), which is characterized by the non-caseating granulomatous inflammation recognized in skeletal muscles biopsy. Retrotrans-genomics based on the COFFEE method showed that various THE1B fusion transcripts (THE1B/FTs) and retrotransposon-associated genomic dynamics in muscle of SM, including (1) splicing of THE1B to neighborhood exons, (2) expression of THE1B associated long noncoding RNAs in the intergenic regions, (3) ectopic expression of tissue specific transcripts driven by THE1B in SM, and (4) identification of a read through transcript, SIRPB1-SIRPD. Seven of eight THE1B/FTs downregulated by tofacitinib in cutaneous sarcoidosis were confined to the granuloma associated macrophage in sarcoidosis. The remaining transcript, SMFT4 exhibited stronger expression in mesenchymal stem cells. In addition, ectopic co-expression of canonical and LINE1 and THE1A-exonizing IL23R transcripts were identified in SM. This directory of THE1B/FTs in SM may serve as a fundamental molecular basis for understanding the pathogenesis of sarcoidosis.