Antisense transcription of the myotonic dystrophy locus yields low-abundant RNAs with and without (CAG)n repeat

The unstable (CTG·CAG)n trinucleotide repeat in the myotonic dystrophy type 1 (DM1) locus is bidirectionally transcribed from genes with terminal overlap. By transcription in the sense direction, the <i>DMPK</i> gene produces various alternatively spliced mRNAs with a (CUG)n repeat in their 3′ UTR. Expression in opposite orientation reportedly yields (CAG)n-repeat containing RNA, but both structure and biologic significance of this antisense gene (<i>DM1-AS</i>) are largely unknown. Via a combinatorial approach of computational and experimental analyses of RNA from unaffected individuals and DM1 patients we discovered that <i>DM1-AS</i> spans &gt;6 kb, contains alternative transcription start sites and uses alternative polyadenylation sites up- and downstream of the (CAG)n repeat. Moreover, its primary transcripts undergo alternative splicing, whereby the (CAG)n segment is removed as part of an intron. Thus, in patients a mixture of <i>DM1-AS</i> RNAs with and without expanded (CAG)n repeat are produced. <i>DM1-AS</i> expression appears upregulated in patients, but transcript abundance remains very low in all tissues analyzed. Our data suggest that <i>DM1-AS</i> transcripts belong to the class of long non-coding RNAs. These and other biologically relevant implications for how (CAG)n-expanded transcripts may contribute to DM1 pathology can now be explored experimentally.

1型肌强直性营养不良（myotonic dystrophy type 1, DM1）基因座内的不稳定(CTG·CAG)n三核苷酸重复序列，可从存在末端重叠的基因位点双向转录。正向转录时，肌强直性营养不良蛋白激酶（<i>DMPK</i>）基因可生成多种在3'非翻译区（3' untranslated region, 3' UTR）携带(CUG)n重复序列的可变剪接mRNA。据报道，反向转录可产生含有(CAG)n重复序列的RNA，但该反义基因（<i>DM1-AS</i>）的结构及生物学意义目前仍未明确。
本研究通过对健康个体与DM1患者的RNA开展计算与实验分析的联合策略，发现<i>DM1-AS</i>基因跨度超过6 kb，拥有可变转录起始位点，并在(CAG)n重复序列的上下游区域使用可变多聚腺苷酸化位点。此外，其初级转录本可发生可变剪接，使(CAG)n区段作为内含子的一部分被移除。因此，DM1患者体内可同时产生携带与不携带扩增型(CAG)n重复序列的<i>DM1-AS</i> RNA。
<i>DM1-AS</i>的表达水平似乎在患者体内有所上调，但在所分析的所有组织中，其转录本丰度仍极低。本研究结果提示，<i>DM1-AS</i>转录本属于长链非编码RNA（long non-coding RNAs, lncRNA）一类。目前可通过实验进一步探究(CAG)n扩增型转录本如何参与DM1病理过程，以及其他相关生物学意义。