Resolving the chromatin impact of mosaic variants with targeted Fiber-seq

Accurately quantifying the functional consequences of noncoding mosaic variants requires the pairing of DNA sequences with both accessible and closed chromatin architectures along individual DNA molecules-a pairing that cannot be achieved using traditional fragmentation-based chromatin assays. We demonstrate that targeted single-molecule chromatin fiber sequencing (Fiber-seq) achieves this, permitting single-molecule, long-read genomic, and epigenomic profiling across targeted >100 kb loci with ∼10-fold enrichment over untargeted sequencing. Targeted Fiber-seq reveals that pathogenic expansions of the <i>DMPK</i> CTG repeat that underlie Myotonic Dystrophy 1 are characterized by somatic instability and disruption of multiple nearby regulatory elements, both of which are repeat length-dependent. Furthermore, we reveal that therapeutic adenine base editing of the segmentally duplicated γ-globin (<i>HBG1</i>/<i>HBG2</i>) promoters in primary human hematopoietic cells induced toward an erythroblast lineage increases the accessibility of the <i>HBG1</i> promoter as well as neighboring regulatory elements. Overall, we find that these non-protein coding mosaic variants can have complex impacts on chromatin architectures, including extending beyond the regulatory element harboring the variant.