library_construction_protocol: Total RNA was extracted from cardiac tissue samples ground in a cryomortar using the Monarch® Total RNA Miniprep Kit (T2010S; New England Biolabs) according to the manufacturers protocol. 40ng of total RNA per sample (differing for sample N2859 = 31ng, N2864 = 37ng, N2868 = 35ng, N2869 = 37ng, and N2872 = 32ng) were utilized as input for mRNA enrichment procedure with 'NEBNext® Poly(A) mRNA Magnetic Isolation Module' (E7490L; New England Biolabs) followed by stranded cDNA library generation using 'NEBNext® Ultra II Directional RNA Library Prep Kit for Illumina' (E7760L; New England Biolabs). All steps were performed as recommended in user manualE7760 (Version 1.0_02-2017; NEB) except that all reactions were downscaled to 2/3 of initial volumes. Furthermore, one additional purification step was introduced at the end of the standard procedure, using 1x 'Agencourt® AMPure® XP Beads' (#A63881; Beckman Coulter, Inc.).  cDNA libraries were barcoded by dual indexing approach, using 'NEBNext Multiplex Oligos for Illumina – 96 Unique Dual Index Primer Pairs' (6440S; New England Biolabs). All generated cDNA libraries were amplified with 12 cycles of final pcr.   Equal molar amounts of individually barcoded libraries were pooled for a common sequencing run. The library pools were denatured with NaOH and were finally diluted to 2pM according to the Denature and Dilute Libraries Guide (Document # 15048776 v02; Illumina). 1.3 ml of denatured pool was loaded on an Illumina NextSeq 550 sequencer using a High Output Flowcell for single reads (20024906; Illumina). Sequencing was performed with the following settings: Sequence reads 1 and 2 with 38 bases each; Index reads 1 and 2 with 8 bases each.