MotorPlex provides accurate variant detection across large muscle genes both in single myopathic patients and in pools of DNA samples

Abstract

Mutations in ~100 genes cause muscle diseases with complex and often unexplained genotype/phenotype correlations. Next-generation sequencing studies identify a greater-than-expected number of genetic variations in the human genome. This suggests that existing clinical monogenic testing systematically miss very relevant information.We have created a core panel of genes that cause all known forms of nonsyndromic muscle disorders (MotorPlex). It comprises 93 loci, among which are the largest and most complex human genes, such as TTN, RYR1, NEB and DMD. MotorPlex captures at least 99.2% of 2,544 exons with a very accurate and uniform coverage. This quality is highlighted by the discovery of 20-30% more variations in comparison with whole exome sequencing. The coverage homogeneity has also made feasible to apply a cost-effective pooled sequencing strategy while maintaining optimal sensitivity and specificity.We studied 177 unresolved cases of myopathies for which the best candidate genes were previously excluded. We have identified known pathogenic variants in 52 patients and potential causative ones in further 56 patients. We have also discovered 23 patients showing multiple true disease-associated variants suggesting complex inheritance. Moreover, we frequently detected other nonsynonymous variants of unknown significance in the largest muscle genes. Cost-effective combinatorial pools of DNA samples were similarly accurate (97-99%). MotorPlex is a very robust platform that overcomes for power, costs, speed, sensitivity and specificity the gene-by-gene strategy. The applicability of pooling makes this tool affordable for the screening of genetic variability of muscle genes also in a larger population. We consider that our strategy can have much broader applications.

Figure 1

A comparison between MotorPlex and a Whole Exome strategy (WES) demonstrates the better performance of the targeted strategy. (a) 97.75% of reads generated in a MotorPlex experiment fall in the regions of interest and only 0.67% of targeted regions are not sequenced. On the contrary, for WES 88.66% of reads are on target and 14.89% of targeted exons are not effectively covered. (b) The percentage of targeted regions covered at high depth by MotorPlex is higher than that obtained by WES. In particular, 96.01% and 81.6% of regions are, respectively, covered at 100x and 200x by using MotorPlex versus 35.49% and 1.90% by WES.

Figure 2

NGS targeting workflow. Ninety-three disease genes causing a muscular phenotype were selected. To cover all their exons and the ten flanking bases, an enrichment strategy, based on HaloPlex system, was designed. DNA samples of 80 patients were analyzed twice in an independent manner, using a combinatorial pooling scheme. As requested by HaloPlex protocol, DNA samples were digested, barcoded and amplified. The 80 samples were run at the same time in a single lane of the flow cell of HiSeq 1000. The following data analysis allowed us to detect putative causative variants validated by Sanger sequencing.