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Compound heterozygosity for an expanded (GAA) and a (GAAGGA) repeat at FXN locus: from a diagnostic pitfall to potential clues to the pathogenesis of Friedreich ataxia

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Abstract

Friedreich’s ataxia (FRDA) is usually due to a homozygous GAA expansion in intron 1 of the frataxin (FXN) gene. Rarely, uncommon molecular rearrangements at the FXN locus can cause pitfalls in the molecular diagnosis of FRDA. Here we describe a family whose proband was affected by late-onset Friedreich’s ataxia (LOFA); long-range PCR (LR-PCR) documented two small expanded GAA alleles both in the proband and in her unaffected younger sister, who therefore received a diagnosis of pre-symptomatic LOFA. Later studies, however, revealed that the proband’s unaffected sister, as well as their healthy mother, were both carriers of an expanded GAA allele and an uncommon (GAAGGA)66–67 repeat mimicking a GAA expansion at the LR-PCR that was the cause of the wrong initial diagnosis of pre-symptomatic LOFA. Extensive studies in tissues from all the family members, including LR-PCR, assessment of methylation status of FXN locus, MboII restriction analysis and direct sequencing of LR-PCR products, analysis of FXN mRNA, and frataxin protein expression, support the virtual lack of pathogenicity of the rare (GAAGGA)66–67 repeat, also providing significant data about the modulation of epigenetic modifications at the FXN locus. Overall, this report highlights a rare but possible pitfall in FRDA molecular diagnosis, emphasizing the need of further analysis in case of discrepancy between clinical and molecular data.

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Data availability

The data that support the findings of this study are openly available in ClinVar-NCBI at the following URL: https.//www.ncbi.nlm.nih.gov/clinvar/variation/ VCV000804267.1.

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Acknowledgments

We thank Manuela Papacci and Andrea Sabino for the technical assistance and Roisin Sullivan for revising the English form of the manuscript.

Funding

This work was partially supported by grants to G.S. (Italian Ministry of University and research/M.I.U.R./Linea D1).

Author information

Authors and Affiliations

  1. IRCCS Fondazione Don Carlo Gnocchi, Piazzale Morandi, 6, 20121, Milan, Italy

    Massimo Santoro

  2. Dept of Neuroscience, Faculty of Medicine and Surgery, Università Cattolica del Scaro Cuore, L.go F. Vito 1, 000168, Rome, Italy

    Alessia Perna, Salvatore Rossi, Vittorio Riso, Tommaso Filippo Nicoletti & Gabriella Silvestri

  3. Unit of Muscular and Neurodegenerative Diseases, Ospedale Pediatrico Bambino Gesù, IRCCS, Viale San Paolo, 15, 00146, Rome, Italy

    Piergiorgio La Rosa, Sara Petrillo & Fiorella Piemonte

  4. Institute of Neurology, Neuroscience Area, Neurology Unit, Fondazione Policlinico Universitario A. Gemelli IRCCS, Largo Agostino Gemelli, 8, 00168, Rome, Italy

    Vittorio Riso, Tommaso Filippo Nicoletti, Anna Modoni & Gabriella Silvestri

  5. Institute of Genomic Medicine, Università Cattolica del Sacro Cuore, Fondazione Policlinico Universitario A. Gemelli IRCCS, Largo Agostino Gemelli, 8, 00168, Rome, Italy

    Maria Grazia Pomponi & Pietro Chiurazzi

Authors
  1. Massimo Santoro
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  2. Alessia Perna
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Contributions

Study conception and design, critical revision for intellectual content, manuscript draft: G. Silvestri, M. Santoro, A. Perna, S. Petrillo, F. Piemonte, and P. Chiurazzi.

Clinical data assessment and tissue collection: V. Riso, T. Nicoletti, A. Modoni.

Material preparation, experimental plan, and data analysis were performed by M. Santoro, A. Perna, S. Petrillo, La Rosa, Rossi, and Pomponi.

First draft and manuscript revised: G. Silvestri, M. Santoro, A. Perna, F. Piemonte, and P. Chiurazzi. All authors gave their contribution to critical revision and discussion of the study results, read, and approved the final form of the manuscript.

Corresponding author

Correspondence to Gabriella Silvestri.

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Conflict of interest

The authors declare that they have no conflicts of interest.

Ethics approval

All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, and its later amendments or comparable ethical standards. Biological samples were obtained by all participants primarily for diagnostic purposes following appropriate written informed consent in accordance with the guidelines and with approval of the Local Ethical Committee.

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A written informed consent was obtained from all the subjects included in this study.

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In the written informed consent, all subjects gave their consent to participate to this study.

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All the participants gave their consent to publish data regarding the results obtained in this study.

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Massimo Santoro and Alessia Perna wish it to be known that, in their opinion, the first two authors should be regarded as joint First Authors.

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Santoro, M., Perna, A., La Rosa, P. et al. Compound heterozygosity for an expanded (GAA) and a (GAAGGA) repeat at FXN locus: from a diagnostic pitfall to potential clues to the pathogenesis of Friedreich ataxia. Neurogenetics 21, 279–287 (2020). https://doi.org/10.1007/s10048-020-00620-7

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  • DOI: https://doi.org/10.1007/s10048-020-00620-7

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