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Evidence for a modifying pathway in SMA discordant families: reduced SMN level decreases the amount of its interacting partners and Htra2-beta1

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Abstract

Proximal spinal muscular atrophy (SMA) is a neuromuscular disorder caused by homozygous mutations of the SMN1 gene. SMN1 interacts with multiple proteins with functions in snRNP biogenesis, pre-mRNA splicing and presumably neural transport. SMN2, a nearly identical copy of SMN1, produces predominantly exon 7-skipped transcripts, whereas SMN1 mainly produces full-length transcripts. The SR-like splicing factor Htra2-beta1 facilitates correct splicing of SMN2 exon 7 through direct interaction with an exonic splicing enhancer within exon 7. In rare cases, siblings with identical 5q13-homologues and homozygous absence of SMN1 show variable phenotypes, suggesting that SMA is modified by other factors. By analysing nine SMA discordant families, we demonstrate that in all families unaffected siblings produce significantly higher amounts of SMN, Gemin2, Gemin3, ZPR1 and hnRNP-Q protein in lymphoblastoid cell lines, but not in primary fibroblasts, compared with their affected siblings. Protein p53, an additional SMN-interacting protein, is not subject to an SMN-dependent regulation. Surprisingly, Htra2-beta1 is also regulated by this tissue-specific mechanism. A similar regulation was found in all type I–III SMA patients, although at a different protein level than in discordant families. Thus, our data show that reduced SMN protein levels cause a reduction in the amount of its interacting proteins and of Htra2-beta1 in both discordant and non-discordant SMA families. We provide evidence that an intrinsic SMA modifying factor acts directly on the expression of SMN, thus influencing the SMA phenotype. Further insights into the molecular pathway and the identification of SMA modifying gene(s) may help to find additional targets for a therapy approach.

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Acknowledgements

We are grateful to all the families with SMA and clinicians who helped with this study. We are grateful to Utz Fischer for providing an antibody against Gemin2, Michael Sendtner and W. Rossol for providing an antibody against hnRNP-Q, F.A. Grässer for an antibody against Gemin3, L. Gangwani and R. Davis for antibodies against ZPR1. This work was generously supported by grants from the Deutsche Forschungsgemeinschaft (Grant SFB400-A6, Graduiertenkolleg 246), Families of SMA and BONFOR.

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Authors and Affiliations

  1. Institute of Human Genetics, University of Bonn, Wilhelmstrasse 31, 53111, Bonn, Germany

    Claudia Helmken, Yvonne Hofmann, Frank Schoenen, Gabriela Oprea, Heidrun Raschke & Brunhilde Wirth

  2. Institute of Human Genetics, University of Cologne, Kerpener Strasse 34, 50931, Cologne, Germany

    Gabriela Oprea & Brunhilde Wirth

  3. Institute of Human Genetics, RWTH, Aachen, Germany

    Sabine Rudnik-Schöneborn & Klaus Zerres

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  1. Claudia Helmken
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  2. Yvonne Hofmann
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Correspondence to Brunhilde Wirth.

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Helmken, C., Hofmann, Y., Schoenen, F. et al. Evidence for a modifying pathway in SMA discordant families: reduced SMN level decreases the amount of its interacting partners and Htra2-beta1. Hum Genet 114, 11–21 (2003). https://doi.org/10.1007/s00439-003-1025-2

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  • DOI: https://doi.org/10.1007/s00439-003-1025-2

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