Asset Tag: Fragile X

June 1, 2021  |  

Sequencing of expanded CGG repeats in the FMR1 gene.

Alleles of the FMR1 gene with more than 200 CGG repeats generally undergo methylation-coupled gene silencing, resulting in fragile X syndrome, the leading heritable form of cognitive impairment. Smaller expansions (55-200 CGG repeats) result in elevated levels of FMR1 mRNA, which is directly responsible for the late-onset neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS). For mechanistic studies and genetic counseling, it is important to know with precision the number of CGG repeats; however, no existing DNA sequencing method is capable of sequencing through more than ~100 CGG repeats, thus limiting the ability to precisely characterize the disease-causing alleles. The recent development of single molecule, real-time sequencing represents a novel approach to DNA sequencing that couples the intrinsic processivity of DNA polymerase with the ability to read polymerase activity on a single-molecule basis. Further, the accuracy of the method is improved through the use of circular templates, such that each molecule can be read multiple times to produce a circular consensus sequence (CCS). We have succeeded in generating CCS reads representing multiple passes through both strands of repeat tracts exceeding 700 CGGs (>2 kb of 100 percent CG) flanked by native FMR1 sequence, with single-molecule readlengths exceeding 12 kb. This sequencing approach thus enables us to fully characterize the previously intractable CGG-repeat sequence, leading to a better understanding of the distinct associated molecular pathologies. Real-time kinetic data also provides insight into the activity of DNA polymerase inside this unique sequence. The methodology should be widely applicable for studies of the molecular pathogenesis of an increasing number of repeat expansion-associated neurodegenerative and neurodevelopmental disorders, and for the efficient identification of such disorders in the clinical setting.

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June 1, 2021  |  

Enrichment of unamplified DNA and long-read SMRT Sequencing in unlocking the underlying biological disease mechanisms of repeat expansion disorders

For many of the repeat expansion disorders, the disease gene has been discovered, however the underlying biological mechanisms have not yet been fully understood. This is mainly due to technological limitations that do not allow for the needed base-pair resolution of the long, repetitive genomic regions. We have developed a novel, amplification-free enrichment technique that uses the CRISPR/Cas9 system to target large repeat expansions. This method, in conjunction with PacBio’s long reads and uniform coverage, enables sequencing of these complex genomic regions. By using a PCR-free amplification method, we are able to access not only the repetitive elements and interruption sequences accurately, but also the epigenetic information.

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June 1, 2021  |  

Candidate gene screening using long-read sequencing

We have developed several candidate gene screening applications for both Neuromuscular and Neurological disorders. The power behind these applications comes from the use of long-read sequencing. It allows us to access previously unresolvable and even unsequencable genomic regions. SMRT Sequencing offers uniform coverage, a lack of sequence context bias, and very high accuracy. In addition, it is also possible to directly detect epigenetic signatures and characterize full-length gene transcripts through assembly-free isoform sequencing. In addition to calling the bases, SMRT Sequencing uses the kinetic information from each nucleotide to distinguish between modified and native bases.

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June 1, 2021  |  

Alternative splicing in FMR1 premutations carriers

Over 40% of males and ~16% of female carriers of a FMR1 premutation allele (55-200 CGG repeats) are at risk for developing Fragile X-associated Tremor/Ataxia Syndrome (FXTAS), an adult onset neurodegenerative disorder while, about 20% of female carriers will develop Fragile X-associated Primary Ovarian Insufficiency (FXPOI), in addition to a number of adult-onset clinical problems (FMR1 associated disorders). Marked elevation in FMR1 mRNA levels have been observed with premutation alleles and the resulting RNA toxicity is believed to be the leading molecular mechanism proposed for these disorders. The FMR1 gene, as many housekeeping genes, undergoes alternative splicing. Using long-read isoform sequencing (SMRT) and qRT-PCR we have recently reported that, although the relative abundance of all FMR1 mRNA isoforms is significantly increased in the premutation group compared to controls, there is a disproportionate increase, relative to the overall increase in mRNA, in the abundance of isoforms spliced at both exons 12 and 14. In total, we confirmed the existence of 16 out of 24 predicted isoforms in our samples. However, it is unknown, which isoforms, when overexpressed, may contribute to the premutation pathology. To address this question we have further defined the transcriptional FMR1 isoforms distribution pattern in different tissues, including heart, muscle, brain and testis derived from FXTAS premutation carriers and age-matched controls. Preliminary data indicates the presence of a transcriptional signature of the FMR1 gene, which clusters more by individual than by tissue type. We identified additional isoforms than the 16 reported in our previous study, including a group with particular splice patterns that were observed only in premutations but not in controls. Our findings suggest that the characterization of expression levels of the different FMR1 isoforms is fundamental for understanding the regulation of the FMR1 gene as well as for elucidating the mechanism(s) by which “toxic gain of function” of the FMR1 mRNA may play a role in FXTAS and/or in the other FMR1-associated conditions. In addition to the elevated levels of FMR1 isoforms, the altered abundance/ratio of the corresponding FMRP isomers may affect the overall function of FMRP in premutations.

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June 1, 2021  |  

Targeted SMRT Sequencing of difficult regions of the genome using a Cas9, non-amplification based method

Targeted sequencing has proven to be an economical means of obtaining sequence information for one or more defined regions of a larger genome. However, most target enrichment methods are reliant upon some form of amplification. Amplification removes the epigenetic marks present in native DNA, and some genomic regions, such as those with extreme GC content and repetitive sequences, are recalcitrant to faithful amplification. Yet, a large number of genetic disorders are caused by expansions of repeat sequences. Furthermore, for some disorders, methylation status has been shown to be a key factor in the mechanism of disease. We have developed a novel, amplification-free enrichment technique that employs the CRISPR/Cas9 system for specific targeting of individual human genes. This method, in conjunction with SMRT Sequencing’s long reads, high consensus accuracy, and uniform coverage, allows the sequencing of complex genomic regions that cannot be investigated with other technologies.

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June 1, 2021  |  

Targeted enrichment without amplification and SMRT Sequencing of repeat-expansion disease causative genomic regions

Targeted sequencing has proven to be an economical means of obtaining sequence information for one or more defined regions of a larger genome. However, most target enrichment methods are reliant upon some form of amplification. Amplification removes the epigenetic marks present in native DNA, and some genomic regions, such as those with extreme GC content and repetitive sequences, are recalcitrant to faithful amplification. Yet, a large number of genetic disorders are caused by expansions of repeat sequences. Furthermore, for some disorders, methylation status has been shown to be a key factor in the mechanism of disease. We have developed a novel, amplification-free enrichment technique that employs the CRISPR/Cas9 system for specific targeting of individual human genes. This method, in conjunction with SMRT Sequencing’s long reads, high consensus accuracy, and uniform coverage, allows the sequencing of complex genomic regions that cannot be investigated with other technologies. Using human genomic DNA samples and this strategy, we have successfully targeted the loci of a number of repeat expansion disorders (HTT, FMR1, ATXN10, C9orf72). With this data, we demonstrate the ability to isolate hundreds of individual on-target molecules and accurately sequence through long repeat stretches, regardless of the extreme GC-content, followed by accurate sequencing on a single PacBio RS II SMRT Cell or Sequel SMRT Cell 1M. The method is compatible with multiplexing of multiple targets and multiple samples in a single reaction. Furthermore, this technique also preserves native DNA molecules for sequencing, allowing for the possibility of direct detection and characterization of epigenetic signatures. We demonstrate detection of 5-mC in human promoter sequences and CpG islands.

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June 1, 2021  |  

Amplification-free targeted enrichment and SMRT Sequencing of repeat-expansion genomic regions

Targeted sequencing has proven to be an economical means of obtaining sequence information for one or more defined regions of a larger genome. However, most target enrichment methods are reliant upon some form of amplification. Amplification removes the epigenetic marks present in native DNA, and some genomic regions, such as those with extreme GC content and repetitive sequences, are recalcitrant to faithful amplification. Yet, a large number of genetic disorders are caused by expansions of repeat sequences. Furthermore, for some disorders, methylation status has been shown to be a key factor in the mechanism of disease.

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June 1, 2021  |  

Amplification-free, CRISPR-Cas9 targeted enrichment and SMRT Sequencing of repeat-expansion disease causative genomic regions

Targeted sequencing has proven to be economical for obtaining sequence information for defined regions of the genome. However, most target enrichment methods are reliant upon some form of amplification which can negatively impact downstream analysis. For example, amplification removes epigenetic marks present in native DNA, including nucleotide methylation, which are hypothesized to contribute to disease mechanisms in some disorders. In addition, some genomic regions known to be causative of many genetic disorders have extreme GC content and/or repetitive sequences that tend to be recalcitrant to faithful amplification. We have developed a novel, amplification-free enrichment technique that employs the CRISPR/Cas9 system to target individual genes. This method, in conjunction with the long reads, high consensus accuracy, and uniform coverage of SMRT Sequencing, allows accurate sequence analysis of complex genomic regions that cannot be investigated with other technologies. Using this strategy, we have successfully targeted a number of repeat expansion disorder loci (HTT, FMR1, ATXN10, C9orf72).With this data, we demonstrate the ability to isolate thousands of individual on-target molecules and, using the Sequel System, accurately sequence through long repeats regardless of the extreme GC-content. The method is compatible with multiplexing of multiple target loci and multiple samples in a single reaction. Furthermore, because there is no amplification step, this technique also preserves native DNA molecules for sequencing, allowing for the direct detection and characterization of epigenetic signatures. To this end, we demonstrate the detection of 5-mC in the CGG repeat of the FMR1 gene that is responsible for Fragile X syndrome.

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June 1, 2021  |  

Amplification-free targeted enrichment powered by CRISPR-Cas9 and long-read Single Molecule Real-Time (SMRT) Sequencing can efficiently and accurately sequence challenging repeat expansion disorders

Genomic regions with extreme base composition bias and repetitive sequences have long proven challenging for targeted enrichment methods, as they rely upon some form of amplification. Similarly, most DNA sequencing technologies struggle to faithfully sequence regions of low complexity. This has been especially trying for repeat expansion disorders such as Fragile-X disease, Huntington disease and various Ataxias, where the repetitive elements range from several hundreds of bases to tens of kilobases. We have developed a robust, amplification-free targeted enrichment technique, called No-Amp Targeted Sequencing, that employs the CRISPR-Cas9 system. In conjunction with SMRT Sequencing, which delivers long reads spanning the entire repeat expansion, high consensus accuracy, and uniform coverage, these previously inaccessible regions are now accessible. This method is completely amplification-free, therefore removing any PCR errors and biases from the experiment. Furthermore, this technique also preserves native DNA molecules, allowing for direct detection and characterization of epigenetic signatures. The No-Amp method is a two-day protocol that is compatible with multiplexing of multiple targets and multiple samples in a single reaction, using as little as 1 µg of genomic DNA input per sample. We have successfully targeted a number of repeat expansion disorder loci including HTT, FMR1, C9orf7,2 as well as built an Ataxia panel which consists of 15 different disease-causing repeat expansion regions. Using the No-Amp method we have isolated hundreds of individual on-target molecules, allowing for reliable repeat size estimation, mosaicism detection and identification of interruption sequences with alleles as long as >2700 repeat unites ( >13 kb). In addition to multiplexing several targets, we have also multiplexed at least 20 samples in one experiment making the No-Amp Targeted Sequencing method a cost-effective option. Combining the CRISPR-Cas9 enrichment method with Single Molecule, Real-Time Sequencing provided us with base-level resolution of previously inaccessible regions of the genome, like disease-causing repeat expansions. No-Amp Targeted Sequencing captures, in one experiment, many aspects of repeat expansion disorders which are important for better understanding the underlying disease mechanisms.

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September 22, 2019  |  

Differential increases of specific FMR1 mRNA isoforms in premutation carriers.

Over 40% of male and ~16% of female carriers of a premutation FMR1 allele (55-200 CGG repeats) will develop fragile X-associated tremor/ataxia syndrome, an adult onset neurodegenerative disorder, while about 20% of female carriers will develop fragile X-associated primary ovarian insufficiency. Marked elevation in FMR1 mRNA transcript levels has been observed with premutation alleles, and RNA toxicity due to increased mRNA levels is the leading molecular mechanism proposed for these disorders. However, although the FMR1 gene undergoes alternative splicing, it is unknown whether all or only some of the isoforms are overexpressed in premutation carriers and which isoforms may contribute to the premutation pathology.To address this question, we have applied a long-read sequencing approach using single-molecule real-time (SMRT) sequencing and qRT-PCR. Our SMRT sequencing analysis performed on peripheral blood mononuclear cells, fibroblasts and brain tissue samples derived from premutation carriers and controls revealed the existence of 16 isoforms of 24 predicted variants. Although the relative abundance of all mRNA isoforms was significantly increased in the premutation group, as expected based on the bulk increase in mRNA levels, there was a disproportionate (fourfold to sixfold) increase, relative to the overall increase in mRNA, in the abundance of isoforms spliced at both exons 12 and 14, specifically Iso10 and Iso10b, containing the complete exon 15 and differing only in splicing in exon 17.These findings suggest that RNA toxicity may arise from a relative increase of all FMR1 mRNA isoforms. Interestingly, the Iso10 and Iso10b mRNA isoforms, lacking the C-terminal functional sites for fragile X mental retardation protein function, are the most increased in premutation carriers relative to normal, suggesting a functional relevance in the pathology of FMR1-associated disorders. Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.

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September 22, 2019  |  

Single molecule real-time (SMRT) sequencing comes of age: applications and utilities for medical diagnostics.

Short read massive parallel sequencing has emerged as a standard diagnostic tool in the medical setting. However, short read technologies have inherent limitations such as GC bias, difficulties mapping to repetitive elements, trouble discriminating paralogous sequences, and difficulties in phasing alleles. Long read single molecule sequencers resolve these obstacles. Moreover, they offer higher consensus accuracies and can detect epigenetic modifications from native DNA. The first commercially available long read single molecule platform was the RS system based on PacBio’s single molecule real-time (SMRT) sequencing technology, which has since evolved into their RSII and Sequel systems. Here we capsulize how SMRT sequencing is revolutionizing constitutional, reproductive, cancer, microbial and viral genetic testing.© The Author(s) 2018. Published by Oxford University Press on behalf of Nucleic Acids Research.

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September 22, 2019  |  

Altered expression of the FMR1 splicing variants landscape in premutation carriers.

FMR1 premutation carriers (55-200 CGG repeats) are at risk for developing Fragile X-associated Tremor/Ataxia Syndrome (FXTAS), an adult onset neurodegenerative disorder. Approximately 20% of female carriers will develop Fragile X-associated Primary Ovarian Insufficiency (FXPOI), in addition to a number of clinical problems affecting premutation carriers throughout their life span. Marked elevation in FMR1 mRNA levels have been observed with premutation alleles resulting in RNA toxicity, the leading molecular mechanism proposed for the FMR1 associated disorders observed in premutation carriers. The FMR1 gene undergoes alternative splicing and we have recently reported that the relative abundance of all FMR1 mRNA isoforms is significantly increased in premutation carriers. In this study, we characterized the transcriptional FMR1 isoforms distribution pattern in different tissues and identified a total of 49 isoforms, some of which observed only in premutation carriers and which might play a role in the pathogenesis of FXTAS. Further, we investigated the distribution pattern and expression levels of the FMR1 isoforms in asymptomatic premutation carriers and in those with FXTAS and found no significant differences between the two groups. Our findings suggest that the characterization of the expression levels of the different FMR1 isoforms is fundamental for understanding the regulation of the FMR1 gene as imbalance in their expression could lead to an altered functional diversity with neurotoxic consequences. Their characterization will also help to elucidating the mechanism(s) by which “toxic gain of function” of the FMR1 mRNA may play a role in FXTAS and/or in the other FMR1-associated conditions. Copyright © 2017. Published by Elsevier B.V.

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September 21, 2019  |  

Detecting AGG interruptions in females with a FMR1 premutation by long-read Single-Molecule Sequencing: A 1 year clinical experience.

The fragile X syndrome arises from the FMR1 CGG expansion of a premutation (55-200 repeats) to a full mutation allele (>200 repeats) and is the most frequent cause of inherited X-linked intellectual disability. The risk for a premutation to expand to a full mutation allele depends on the repeat length and AGG triplets interrupting this repeat. In genetic counseling it is important to have information on both these parameters to provide an accurate risk estimate to women carrying a premutation allele and weighing up having children. For example, in case of a small risk a woman might opt for a natural pregnancy followed up by prenatal diagnosis while she might choose for preimplantation genetic diagnosis (PGD) if the risk is high. Unfortunately, the detection of AGG interruptions was previously hampered by technical difficulties complicating their use in diagnostics. Therefore we recently developed, validated and implemented a new methodology which uses long-read single-molecule sequencing to identify AGG interruptions in females with a FMR1 premutation. Here we report on the assets of AGG interruption detection by sequencing and the impact of implementing the assay on genetic counseling.

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