Bart van der Sanden

Background: Diagnostic evaluation of rare genetic disorders continues to rely on multiple test modalities, despite the increasing use of short-read exome or genome sequencing as first-tier tests. Long-read genome sequencing (lrGS) has the potential to consolidate current standard-of-care (SoC) diagnostics into a single assay, but its accuracy and clinical utility in routine practice have not been established at large scale. Methods: We studied 1000 clinical samples, including 832 index cases, representative of one year of germline diagnostic testing at two tertiary centers. Samples underwent lrGS at approximately 30× coverage, followed by variant detection and interpretation. Diagnostic outcomes were compared with those obtained through SoC testing. Using these results, we modeled the effect of implementing lrGS as a first-tier test in a cohort of 15,150 index cases who underwent diagnostic testing during a single year. Results: The overall concordance between SoC and lrGS testing was 96.4%. lrGS identified clinically relevant findings that improved or refined genetic diagnoses in 3.4% of cases, largely through phasing for recessive disorders and the detection of novel disease-causing variants. In 0.2% of cases, SoC testing identified variants not detected by lrGS. Modeling of a generic lrGS-first diagnostic strategy demonstrated an estimated absolute increase in diagnostic yield of 2.5% across the full annual cohort. Conclusions: LrGS demonstrated high concordance with current diagnostic approaches and provided incremental diagnostic benefits. These findings support lrGS as a feasible and effective first-tier test for rare disease diagnostics.

Short-read sequencing (SRS) methods have improved the detection of small genetic variants but remain limited in highly homologous genomic regions, such as segmental duplications with gene-pseudogene pairs. These paralogous regions often require complex, locus-specific assays for accurate analysis. Long-read genome sequencing (lrGS) technologies, such as PacBio HiFi sequencing, can span these regions but still face challenges in variant calling due to alignment ambiguities. Here, we evaluated PacBio HiFi lrGS combined with Paraphase, a dedicated haplotype-based variant caller, in 86 individuals with 125 known clinically relevant variants across 11 paralogous loci. Standard HiFi variant callers detected 95/125 variants, while the remaining 30 variants were only identified by Paraphase. Together, the standard variant callers and Paraphase detected all known variants, including SNVs, InDels, CNVs, SVs, and gene conversions. In addition, lrGS allowed accurate phasing and gene-pseudogene copy number detection. We demonstrate that PacBio HiFi lrGS, particularly when integrated with Paraphase, enables comprehensive variant detection in previously difficult-to-assess genomic regions. These results also suggest that lrGS is ready for a wider implementation, possibly as a first-tier diagnostic approach for individuals with suspected variants in these paralogous regions.

Despite advances in genomic diagnostics, the majority of individuals with rare diseases remain without a confirmed genetic diagnosis. The rapid emergence of advanced omics technologies, such as long-read genome sequencing, optical genome mapping and multiomic profiling, has improved diagnostic yield but also substantially increased analytical and interpretational complexity. Addressing this complexity requires systematic multidisciplinary collaboration, as recently demonstrated by targeted diagnostic workshops. Here, we highlight the experience of the Solve-RD consortium, a pan-European initiative, in implementing four structured workshops, termed 'Solvathons', as a regular and effective component of its operational workflow. We provide actionable insights, best practices and lessons learned for successful data integration, expert training and scalable collaborative diagnostics within large research consortia.

The size of non-pathogenic CTG repeats in the 3’UTR of the DMPK gene varies from 5–35, whereas repeats over 50 units are pathogenic. The Intermediate repeats of 36–50 are considered ‘premutation’, as they are present in individuals unaffected by myotonic dystrophy, but are prone to further enlargement into the pathogenic range upon transmission to offspring. In this study, we showed that CCGCTG interrupted intermediate repeats, in the repeat size of 37–43 units, have been detected in multiple families with a history of myotonic dystrophy. However, segregation and microsatellite marker analysis of these interrupted intermediate alleles revealed that these alleles are not the same alleles (haplotypes) that were found expanded in affected family members. In contrast to the pure intermediate alleles, the CCGCTG intermediate repeats within families did not show intergenerational variability in size. Furthermore, we showed that the CCGCTG interrupted intermediate alleles have an allele frequency of approximately 0.35% in the general population, while CCGCTG interruptions were not detected in pathogenic repeat expansions over 50 repeat units in our control cohort. We postulate that intermediate repeats of size 37–43 having CCGCTG interruptions are not prone to further expansion, and therefore not act as premutations, which has great relevance for individuals with these alleles and has implications for genetic counseling and testing.

Short tandem repeats (STRs) are common variations in human genomes that frequently expand or contract, causing genetic disorders, mainly when expanded. Traditional diagnostic methods for identifying these expansions, such as repeat-primed PCR and Southern blotting, are often labor-intensive, locus-specific, and are unable to precisely determine long repeat expansions. Sequencing-based methods, although capable of genome-wide detection, are limited by inaccuracy (short-read technologies) and high associated costs (long-read technologies). This study evaluated optical genome mapping (OGM) as an efficient, accurate approach for measuring STR lengths and assessing somatic stability in 85 samples with known pathogenic repeat expansions in DMPK , CNBP , and RFC1 , causing myotonic dystrophy types 1 and 2 and cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS), respectively. Three workflows—manual de novo assembly, local guided assembly (local-GA), and a molecule distance script—were applied, of which the latter two were developed as part of this study to assess the repeat sizes and somatic repeat stability. OGM successfully identified 84/85 (98.8%) of the pathogenic expansions, distinguishing between wild-type and expanded alleles or between two expanded alleles in recessive cases, with greater accuracy than standard of care (SOC) for long repeats and no apparent upper size limit. Notably, OGM detected somatic instability in a subset of DMPK , CNBP , and RFC1 samples. These findings suggest OGM could advance diagnostic accuracy for large repeat expansions, providing a more comprehensive genome-wide assay for repeat expansion disorders by measuring exact repeat lengths and somatic instability across multiple loci simultaneously.

Short tandem repeat (STR) expansions are associated with more than 60 genetic disorders. The size and stability of these expansions correlate with the severity and age of onset of the disease. Therefore, being able to accurately detect the absolute length of STRs is important. Current diagnostic assays include laborious lab experiments, including repeat‐primed PCR and Southern blotting, that still cannot precisely determine the exact length of very long repeat expansions. Optical genome mapping (OGM) is a cost‐effective and easy‐to‐use alternative to traditional cytogenetic techniques and allows the comprehensive detection of chromosomal aberrations and structural variants >500 bp in length, including insertions, deletions, duplications, inversions, translocations, and copy number variants. Here, we provide methodological guidance for preparing samples and performing OGM as well as running the analysis pipelines and using the specific repeat expansion workflows to determine the exact repeat length of repeat expansions expanded beyond 500 bp. Together these protocols provide all details needed to analyze the length and stability of any repeat expansion with an expected repeat size difference from the expected wild‐type allele of >500 bp. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC.Basic Protocol 1: Genomic ultra‐high‐molecular‐weight DNA isolation, labeling, and stainingBasic Protocol 2: Data generation and genome mapping using the Bionano Saphyr® SystemBasic Protocol 3: Manual De Novo Assembly workflowBasic Protocol 4: Local guided assembly workflowBasic Protocol 5: EnFocus Fragile X workflowBasic Protocol 6: Molecule distance script workflow

Short tandem repeats (STRs) are amongst the most abundant class of variations in human genomes and are meiotically and mitotically unstable which leads to expansions and contractions. STR expansions are frequently associated with genetic disorders, with the size of expansions often correlating with the severity and age of onset. Therefore, being able to accurately detect the total repeat expansion length and to identify potential somatic repeat instability is important. Current standard of care (SOC) diagnostic assays include laborious repeat-primed PCR-based tests as well as Southern blotting, which are unable to precisely determine long repeat expansions and/or require a separate set-up for each locus. Sequencing-based assays have proven their potential for the genome-wide detection of repeat expansions but have not yet replaced these diagnostic assays due to their inaccuracy to detect long repeat expansions (short-read sequencing) and their costs (long-read sequencing). Here, we tested whether optical genome mapping (OGM) can efficiently and accurately identify the STR length and assess the stability of known repeat expansions. We performed OGM for 85 samples with known clinically relevant repeat expansions in DMPK , CNBP and RFC1 , causing myotonic dystrophy type 1 and 2 and cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS), respectively. After performing OGM, we applied three different repeat expansion detection workflows, i.e. manual de novo assembly, local guided assembly (local-GA) and molecule distance script of which the latter two were developed as part of this study. The first two workflows estimated the repeat size for each of the two alleles, while the third workflow was used to detect potential somatic instability. The estimated repeat sizes were compared to the repeat sizes reported after the SOC and concordance between the results was determined. All except one known repeat expansions above the pathogenic repeat size threshold were detected by OGM, and allelic differences were distinguishable, either between wildtype and expanded alleles, or two expanded alleles for recessive cases. An apparent strength of OGM over current SOC methods was the more accurate length measurement, especially for very long repeat expansion alleles, with no upper size limit. In addition, OGM enabled the detection of somatic repeat instability, which was detected in 9/30 DMPK , 23/25 CNBP and 4/30 RFC1 samples, leveraging the analysis of intact, native DNA molecules. In conclusion, for tandem repeat expansions larger than ∼300 bp, OGM provides an efficient method to identify exact repeat lengths and somatic repeat instability with high confidence across multiple loci simultaneously, enabling the potential to provide a significantly improved and generic genome-wide assay for repeat expansion disorders.

Background: Long-read sequencing (LRS) techniques have been very successful in identifying structural variants (SVs). However, the high error rate of LRS made the detection of small variants (substitutions and short indels < 20 bp) more challenging. The introduction of PacBio HiFi sequencing makes LRS also suited for detecting small variation. Here we evaluate the ability of HiFi reads to detect de novo mutations (DNMs) of all types, which are technically challenging variant types and a major cause of sporadic, severe, early-onset disease. Methods: We sequenced the genomes of eight parent–child trios using high coverage PacBio HiFi LRS (~ 30-fold coverage) and Illumina short-read sequencing (SRS) (~ 50-fold coverage). De novo substitutions, small indels, short tandem repeats (STRs) and SVs were called in both datasets and compared to each other to assess the accuracy of HiFi LRS. In addition, we determined the parent-of-origin of the small DNMs using phasing. Results: We identified a total of 672 and 859 de novo substitutions/indels, 28 and 126 de novo STRs, and 24 and 1 de novo SVs in LRS and SRS respectively. For the small variants, there was a 92 and 85% concordance between the platforms. For the STRs and SVs, the concordance was 3.6 and 0.8%, and 4 and 100% respectively. We successfully validated 27/54 LRS-unique small variants, of which 11 (41%) were confirmed as true de novo events. For the SRS-unique small variants, we validated 42/133 DNMs and 8 (19%) were confirmed as true de novo event. Validation of 18 LRS-unique de novo STR calls confirmed none of the repeat expansions as true DNM. Confirmation of the 23 LRS-unique SVs was possible for 19 candidate SVs of which 10 (52.6%) were true de novo events. Furthermore, we were able to assign 96% of DNMs to their parental allele with LRS data, as opposed to just 20% with SRS data. Conclusions: HiFi LRS can now produce the most comprehensive variant dataset obtainable by a single technology in a single laboratory, allowing accurate calling of substitutions, indels, STRs and SVs. The accuracy even allows sensitive calling of DNMs on all variant levels, and also allows for phasing, which helps to distinguish true positive from false positive DNMs.