Mosaic deletions in X-linked dystonia-parkinsonism influence repeat stability and disease onset

Background: While multiple genetic causes of movement disorders have been identified in the past decade, modifying factors of disease expression are still largely unknown for most conditions. X-linked dystonia-parkinsonism (XDP) is an inherited neurodegenerative disease caused by a SINE-VNTR-Alu (SVA)-type retrotransposon insertion that contains a hexanucleotide repeat within an intron of the TAF1 gene. To date, four putative genetic modifiers explain about 65% of variance in age at onset in XDP. However, additional genetic modifiers are conceivably at play in XDP and may include mismatches of the SVA hexanucleotide repeat motif. We aim to identify additional genetic modifiers of XDP expressivity and age at onset (AAO). Methods: Third-generation sequencing of PCR amplicons from XDP patients (n=202) was performed to assess potential repeat interruption and instability. Repeat-primed PCR and Cas9-mediated targeted enrichment were used to confirm the presence of identified repeat mismatches. Results: An increased frequency of deletions at the beginning of the hexanucleotide repeat (CCCTCT)n domain was found. Specifically, three deletions at positions 11, 14, and 17 of the TAF1 SVA repeat motif of somatic mosaic origins were detected in different combinations. The most common one was three deletions (1-2-3) at a median frequency 0.425 (IQR:0.42-0.43) and deletions within positions 11 and 14 (1-2-wt) at a median frequency 0.128 (IQR:0.12-0.13). The frequency of deletions at positions 11 and 14 correlated with repeat number (r=-0.48, p=9.5x10-13) and AAO (r=0.34, p=9.5x10-7). The association with AAO still stands when including other modifier genotypes (MSH3 and PMS2) in a regression model. However, the association dissipates when including repeat numbers. Conclusion: We present a novel mosaic repeat motif deletion within the hexanucleotide repeat (CCCTCT)n domain of TAF1 SVA. Our study illustrates: 1) the importance of somatic mosaic genotypes; 2) the biological plausibility of multiple modifiers (both germline and somatic) that can have additive effects on repeat instability; 3) that these variations may remain undetected without assessment of single molecules.

Elucidating Hexanucleotide Repeat Number and Methylation within the X-Linked Dystonia-Parkinsonism (XDP)-Related SVA Retrotransposon in TAF1 with Nanopore Sequencing

Background: X-linked dystonia-parkinsonism (XDP) is an adult-onset neurodegenerative disorder characterized by progressive dystonia and parkinsonism. It is caused by a SINE-VNTR-Alu (SVA) retrotransposon insertion in the TAF1 gene with a polymorphic (CCCTCT)n domain that acts as a genetic modifier of disease onset and expressivity. Methods: Herein, we used Nanopore sequencing to investigate SVA genetic variability and methylation. We used blood-derived DNA from 96 XDP patients for amplicon-based deep Nanopore sequencing and validated it with fragment analysis which was performed using fluorescence-based PCR. To detect methylation from blood- and brain-derived DNA, we used a Cas9-targeted approach. Results: High concordance was observed for hexanucleotide repeat numbers detected with Nanopore sequencing and fragment analysis. Within the SVA locus, there was no difference in genetic variability other than variations of the repeat motif between patients. We detected high CpG methylation frequency (MF) of the SVA and flanking regions (mean MF = 0.94, SD = ±0.12). Our preliminary results suggest only subtle differences between the XDP patient and the control in predicted enhancer sites directly flanking the SVA locus. Conclusions: Nanopore sequencing can reliably detect SVA hexanucleotide repeat numbers, methylation and, lastly, variation in the repeat motif.

Advancing genomic technologies and clinical awareness accelerates discovery of disease-associated tandem repeat sequences

Expansions of gene-specific DNA tandem repeats (TRs), first described in 1991 as a disease-causing mutation in humans, are now known to cause >60 phenotypes, not just disease, and not only in humans. TRs are a common form of genetic variation with biological consequences, observed, so far, in humans, dogs, plants, oysters, and yeast. Repeat diseases show atypical clinical features, genetic anticipation, and multiple and partially penetrant phenotypes among family members. Discovery of disease-causing repeat expansion loci accelerated through technological advances in DNA sequencing and computational analyses. Between 2019 and 2021, 17 new disease-causing TR expansions were reported, totaling 63 TR loci (>69 diseases), with a likelihood of more discoveries, and in more organisms. Recent and historical lessons reveal that properly assessed clinical presentations, coupled with genetic and biological awareness, can guide discovery of disease-causing unstable TRs. We highlight critical but underrecognized aspects of TR mutations. Repeat motifs may not be present in current reference genomes but will be in forthcoming gapless long-read references. Repeat motif size can be a single nucleotide to kilobases/unit. At a given locus, repeat motif sequence purity can vary with consequence. Pathogenic repeats can be “insertions” within nonpathogenic TRs. Expansions, contractions, and somatic length variations of TRs can have clinical/biological consequences. TR instabilities occur in humans and other organisms. TRs can be epigenetically modified and/or chromosomal fragile sites. We discuss the expanding field of disease-associated TR instabilities, highlighting prospects, clinical and genetic clues, tools, and challenges for further discoveries of disease-causing TR instabilities and understanding their biological and pathological impacts—a vista that is about to expand.

A First Genome Survey and Genomic SSR Marker Analysis of Trematomus loennbergii Regan, 1913

Trematomus loennbergii Regan, 1913, is an evolutionarily important marine fish species distributed in the Antarctic Ocean. However, its genome has not been studied to date. In the present study, whole genome sequencing was performed using next-generation sequencing (NGS) technology to characterize its genome and develop genomic microsatellite markers. The 25-mer frequency distribution was estimated to be the best, and the genome size was predicted to be 815,042,992 bp. The heterozygosity, average rate of read duplication, and sequencing error rates were 0.536%, 0.724%, and 0.292%, respectively. These data were used to analyze microsatellite markers, and a total of 2,264,647 repeat motifs were identified. The most frequent repeat motif was di-nucleotide with 87.00% frequency, followed by tri-nucleotide (10.45%), tetra-nucleotide (1.94%), penta-nucleotide (0.34%), and hexa-nucleotide (0.27%). The AC repeat motif was the most abundant motif among di-nucleotides and among all repeat motifs. Among microsatellite markers, 181 markers were selected and PCR technology was used to validate several markers. A total of 15 markers produced only one band. In summary, these results provide a good basis for further studies, including evolutionary biology studies and population genetics of Antarctic fish species.

Targeting the ALS/FTD-associated A-DNA kink with anthracene-based metal complex causes DNA backbone straightening and groove contraction

The use of a small molecule compound to reduce toxic repeat RNA transcripts or their translated aberrant proteins to target repeat-expanded RNA/DNA with a G4C2 motif is a promising strategy to treat C9orf72-linked disorders. In this study, the crystal structures of DNA and RNA–DNA hybrid duplexes with the -GGGCCG- region as a G4C2 repeat motif were solved. Unusual groove widening and sharper bending of the G4C2 DNA duplex A-DNA conformation with B-form characteristics inside was observed. The G4C2 RNA–DNA hybrid duplex adopts a more typical rigid A form structure. Detailed structural analysis revealed that the G4C2 repeat motif of the DNA duplex exhibits a hydration shell and greater flexibility and serves as a ‘hot-spot’ for binding of the anthracene-based nickel complex, NiII(Chro)2 (Chro = Chromomycin A3). In addition to the original GGCC recognition site, NiII(Chro)2 has extended specificity and binds the flanked G:C base pairs of the GGCC core, resulting in minor groove contraction and straightening of the DNA backbone. We have also shown that Chro-metal complexes inhibit neuronal toxicity and suppresses locomotor deficits in a Drosophila model of C9orf72-associated ALS. The approach represents a new direction for drug discovery against ALS and FTD diseases by targeting G4C2 repeat motif DNA.

Characterizing nucleotide variation and expansion dynamics in human-specific variable number tandem repeats

There are over 55,000 variable number tandem repeats (VNTRs) in the human genome, notable for both their striking polymorphism and mutability. Despite their role in human evolution and genomic variation, they have yet to be studied collectively and in detail, partially due to their large size, variability, and predominant location in non-coding regions. Here, we examine 467 VNTRs that are human-specific expansions, unique to one location in the genome, and not associated with retrotransposons. We leverage publicly available long-read genomes – including from the Human Genome Structural Variant Consortium – to ascertain the exact nucleotide composition of these VNTRs, and compare their composition of alleles. We then confirm repeat unit composition in over 3000 short-read samples from the 1000 Genomes Project. Our analysis reveals that these VNTRs contain remarkably structured repeat motif organization, modified by frequent deletion and duplication events. While overall VNTR compositions tend to remain similar between 1000 Genomes Project super-populations, we describe a notable exception with substantial differences in repeat composition (in PCBP3), as well as several VNTRs that are significantly different in length between super-populations (in ART1, PROP1, WDR60, and LOC102723906). We also observe that most of these VNTRs are expanded in archaic human genomes, yet remain stable in length between single generations. Collectively, our findings indicate that repeat motif variability, repeat composition, and repeat length are all informative modalities to consider when characterizing VNTRs and their contribution to genomic variation.

Stability of the pH-Dependent Parallel-Stranded d(CGA) Motif

ABSTRACTNon-canonical DNA structures that retain programmability and structural predictability are increasingly being used in DNA nanotechnology applications, where they offer versatility beyond traditional Watson-Crick interactions. The d(CGA) triplet repeat motif is structurally dynamic and can transition between parallel-stranded homo-base paired duplex and anti-parallel unimolecular hairpin in a pH-dependent manner. Here, we evaluate the thermodynamic stability and nuclease sensitivity of oligonucleotides composed of the d(CGA) motif and several structurally related sequence variants. These results show that the structural transition resulting from decreasing the pH is accompanied by both a significant energetic stabilization and decreased nuclease sensitivity as unimolecular hairpin structures are converted to parallel-stranded homo-base paired duplexes. Furthermore, the stability of the parallel-stranded duplex form can be altered by changing the 5′-nucleobase of the d(CGA) triplet and the frequency and position of the altered triplets within long stretches of d(CGA) triplets. This work offers insight into the stability and versatility of the d(CGA) triplet repeat motif and provides constraints for using this pH-adaptive structural motif for creating DNA-based nanomaterials.STATEMENT OF SIGNIFICANCEThis article addresses the stability of the d(CGA) triplet motif and variants in solution. Our study reveals changes in thermodynamic stability and nuclease resistance in response to pH. The identity of the 5′-nucleobase within each triplet and the position and frequency of different triplets within stretches of d(CGA) triplets can tune parallel-stranded duplex stability. This tunability can be used for nanotechnological applications where the specificity of the 5′-nucleobase pairing interaction is used to order of long stretches of d(CGA) triplets. These results can inform the rational design of pH-sensitive structurally switchable DNA-based nanomaterials.