Linked-Read Whole Genome Sequencing Solves a Double DMD Gene Rearrangement

Next generation sequencing (NGS) has changed our approach to diagnosis of genetic disorders. Nowadays, the most comprehensive application of NGS is whole genome sequencing (WGS) that is able to detect virtually all DNA variations. However, even after accurate WGS, many genetic conditions remain unsolved. This may be due to the current NGS protocols, based on DNA fragmentation and short reads. To overcome these limitations, we applied a linked-read sequencing technology that combines single-molecule barcoding with short-read WGS. We were able to assemble haplotypes and distinguish between alleles along the genome. As an exemplary case, we studied the case of a female carrier of X-linked muscular dystrophy with an unsolved genetic status. A deletion of exons 16-29 in DMD gene was responsible for the disease in her family, but she showed a normal dosage of these exons by Multiplex Ligation-dependent Probe Amplification (MLPA) and array CGH. This situation is usually considered compatible with a "non-carrier" status. Unexpectedly, the girl also showed an increased dosage of flanking exons 1-15 and 30-34. Using linked-read WGS, we were able to distinguish between the two X chromosomes. In the first allele, we found the 16-29 deletion, while the second allele showed a 1-34 duplication: in both cases, linked-read WGS correctly mapped the borders at single-nucleotide resolution. This duplication in trans apparently restored the normal dosage of exons 16-29 seen by quantitative assays. This had a dramatic impact in genetic counselling, by converting a non-carrier into a double carrier status prediction. We conclude that linked-read WGS should be considered as a valuable option to improve our understanding of unsolved genetic conditions.