PGT-M for spinocerebellar ataxia type 1: development of a STR panel and a report of two clinical cases

PURPOSE

To present the developed preimplantation genetic testing (PGT) for spinocerebellar ataxia type 1 (SCA1) and the outcomes of IVF with PGT.

METHODS

PGT was performed for two unrelated couples from the Republic of Sakha (Yakutia) with the risk of SCA1 in one spouse. We have developed a system for PGT of a monogenic disease (PGT-M) for SCA1, which includes the analysis of a panel of 11 polymorphic STR markers linked to the ATXN1 gene and a pathogenic variant of the ATXN1 gene using nested PCR and fragment analysis. IVF/ICSI programs were performed according to standard protocols. Multiple displacement amplification (MDA) was used for whole genome amplification (WGA) and array comparative genomic hybridization (aCGH) for aneuploidy testing (PGT-A).

RESULTS

Eight STRs were informative for the first couple and ten for the second. Similarity of the haplotypes carrying pathogenic variants of the ATXN1 gene was noted. In the first case, during IVF/ICSI-PGT, three embryos reached the blastocyst stage and were biopsied. One embryo was diagnosed as normal by maternal STR haplotype and the ATXN1 allele. PGT-A revealed euploidy. The embryo transfer resulted in a singleton pregnancy, and a healthy boy was born. Postnatal diagnosis confirmed normal ATXN1. In the second case, two blastocysts were biopsied. Both were diagnosed as normal by PGT-M, but PGT-A revealed aneuploidy.

CONCLUSION

Birth of a healthy child after PGT for SCA1 was the first case of successful preimplantation prevention of SCA1 for the Yakut couple and the first case of successful PGT for SCA1 in Russia.

Antagonistic roles of canonical and Alternative-RPA in disease-associated tandem CAG repeat instability

Expansions of repeat DNA tracts cause >70 diseases, and ongoing expansions in brains exacerbate disease. During expansion mutations, single-stranded DNAs (ssDNAs) form slipped-DNAs. We find the ssDNA-binding complexes canonical replication protein A (RPA1, RPA2, and RPA3) and Alternative-RPA (RPA1, RPA3, and primate-specific RPA4) are upregulated in Huntington disease and spinocerebellar ataxia type 1 (SCA1) patient brains. Protein interactomes of RPA and Alt-RPA reveal unique and shared partners, including modifiers of CAG instability and disease presentation. RPA enhances in vitro melting, FAN1 excision, and repair of slipped-CAGs and protects against CAG expansions in human cells. RPA overexpression in SCA1 mouse brains ablates expansions, coincident with decreased ATXN1 aggregation, reduced brain DNA damage, improved neuron morphology, and rescued motor phenotypes. In contrast, Alt-RPA inhibits melting, FAN1 excision, and repair of slipped-CAGs and promotes CAG expansions. These findings suggest a functional interplay between the two RPAs where Alt-RPA may antagonistically offset RPA's suppression of disease-associated repeat expansions, which may extend to other DNA processes.

Identification of the ataxin-1 interaction network and its impact on spinocerebellar ataxia type 1

Background

Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disease caused by a polyglutamine expansion in the ataxin-1 protein. The pathogenic mechanism resulting in SCA1 is still unclear. Protein–protein interactions affect the function and stability of ataxin-1.

Methods

Wild-type and mutant ataxin-1 were expressed in HEK-293T cells. The levels of expression were assessed using real-time polymerase chain reaction (PCR) and Western blots. Co-immunoprecipitation was done in HEK-293T cells expressing exogenous wild-type and mutant ataxin-1 using anti-Flag antibody following by tandem affinity purification in order to study protein–protein interactions. The candidate interacting proteins were validated by immunoprecipitation. Chromatin immunoprecipitation and high-throughput sequencing and RNA immunoprecipitation and high-throughput sequencing were performed using HEK-293T cells expressing wild-type or mutant ataxin-1.

Results

In this study using HEK-293T cells, we found that wild-type ataxin-1 interacted with MCM2, GNAS, and TMEM206, while mutant ataxin-1 lost its interaction with MCM2, GNAS, and TMEM206. Two ataxin-1 binding targets containing the core GGAG or AAAT were identified in HEK-293T cells using ChIP-seq. Gene Ontology analysis of the top ataxin-1 binding genes identified SLC6A15, NTF3, KCNC3, and DNAJC6 as functional genes in neurons in vitro. Ataxin-1 also was identified as an RNA-binding protein in HEK-293T cells using RIP-seq, but the polyglutamine expansion in the ataxin-1 had no direct effects on the RNA-binding activity of ataxin-1.

Conclusions

An expanded polyglutamine tract in ataxin-1 might interfere with protein–protein or protein–DNA interactions but had little effect on protein–RNA interactions. This study suggested that the dysfunction of protein–protein or protein–DNA interactions is involved in the pathogenesis of SCA1.

A key lysine residue in the AXH domain of ataxin-1 is essential for its ubiquitylation

Spinocerebellar ataxia type 1 (SCA1), an autosomal-dominant neurodegenerative disorder, is caused by expansion of the polyglutamine tract within ataxin-1 (ATXN1). The AXH domain of ATXN1 can mediate neurodegeneration through its interaction with other proteins. We have previously showed that the ubiquitin-conjugating enzyme UbcH6 modulates the transcriptional repression activity of ATXN1 through ubiquitylation. In the present study, we sought to identify sites in the AXH domain that are ubiquitylated by UbcH6. Systematic replacement of each lysine residue in the AXH domain revealed that the lysine at 589 (K589) of ATXN1 is essential for its ubiquitylation by UbcH6. Mass spectrometry studies further confirmed the ubiquitylation site. Interestingly, protein aggregation was significantly enhanced in mutant AXH K589R, implying that the aggregation is strongly associated with the level of ATXN1 expression. Our study may suggest a therapeutic potential of UbcH6 in the treatment of SCA1.