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Davidson JE, Russell JS, Martinez NN, Mowat DR, Jones KJ, Kirk EP, Kariyawasam D, Farrar M, D’Silva A. The Carrier Frequency of Two SMN1 Genes in Parents of Symptomatic Children with SMA and the Significance of SMN1 Exon 8 in Carriers. Genes (Basel) 2023; 14:1403. [PMID: 37510307 PMCID: PMC10379112 DOI: 10.3390/genes14071403] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2023] [Revised: 07/03/2023] [Accepted: 07/03/2023] [Indexed: 07/30/2023] Open
Abstract
BACKGROUND Current carrier screening methods do not identify a proportion of carriers that may have children affected by spinal muscular atrophy (SMA). Additional genetic data is essential to inform accurate risk assessment and genetic counselling of SMA carriers. This study aims to quantify the various genotypes among parents of children with SMA. METHOD A retrospective cohort study was undertaken at Sydney Children's Hospital Network, the major SMA referral centre for New South Wales, Australia. Participants included children with genetically confirmed SMA born between 2005 and 2021. Data was collected on parent genotype inclusive of copy number of SMN1 exons 7 and 8. The number of SMN2 exon 7 copies were recorded for the affected children. Descriptive statistics were used to determine the proportion of carriers of 2+0 genotype classified as silent carriers. Chi-square test was used to correlate the association between parents with a heterozygous SMN1 exon 7 deletion and two copies of exon 8 and ≥3 SMN2 copy number in the proband. RESULTS SMA carrier testing was performed in 118/154 (76.6%) parents, incorporating 59 probands with homozygous SMN1 deletions and one proband with compound heterozygote pathogenic variants. Among parents with a child with SMA, 7.6% had two copies of SMN1 exon 7. When only probands with a homozygous SMN1 exon 7 deletion were included, 6.9% of parents had two copies of SMN1 exon 7. An association was observed between heterozygous deletion of SMN1 exon 7 with two copies of exon 8 in a parent and ≥3 SMN2 copy number in the affected proband (p = 0.07). CONCLUSIONS This study confirmed a small but substantial proportion of silent carriers not identified by conventional screening within an Australian context. Accordingly, the effectiveness of carrier screening for SMA is linked with genetic counselling to enable health literacy regarding high and low risk results and is complemented by new-born screening and maintaining clinical awareness for SMA. Gene conversion events may underpin the associations between parent carrier status and proband SMN2 copy number.
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Affiliation(s)
- Joanne E Davidson
- Department of Neurology, Sydney Children’s Hospitals Network, Sydney, NSW 2031, Australia; (J.E.D.)
| | - Jacqueline S Russell
- Centre for Clinical Genetics, Sydney Children’s Hospital, Randwick, Sydney, NSW 2031, Australia; (J.S.R.)
| | - Noelia Nunez Martinez
- Centre for Clinical Genetics, Sydney Children’s Hospital, Randwick, Sydney, NSW 2031, Australia; (J.S.R.)
| | - David R Mowat
- Centre for Clinical Genetics, Sydney Children’s Hospital, Randwick, Sydney, NSW 2031, Australia; (J.S.R.)
| | - Kristi J Jones
- Department of Clinical Genetics, The Children’s Hospital at Westmead, and Discipline of Paediatrics and Child Health, University of Sydney, Sydney, NSW 2006, Australia
| | - Edwin P Kirk
- Centre for Clinical Genetics, Sydney Children’s Hospital, Randwick, Sydney, NSW 2031, Australia; (J.S.R.)
- Discipline of Paediatrics and Child Health, School of Clinical Medicine, University of New South Wales Sydney, Sydney, NSW 2052, Australia
| | - Didu Kariyawasam
- Department of Neurology, Sydney Children’s Hospitals Network, Sydney, NSW 2031, Australia; (J.E.D.)
- Discipline of Paediatrics and Child Health, School of Clinical Medicine, University of New South Wales Sydney, Sydney, NSW 2052, Australia
| | - Michelle Farrar
- Department of Neurology, Sydney Children’s Hospitals Network, Sydney, NSW 2031, Australia; (J.E.D.)
- Discipline of Paediatrics and Child Health, School of Clinical Medicine, University of New South Wales Sydney, Sydney, NSW 2052, Australia
| | - Arlene D’Silva
- Department of Neurology, Sydney Children’s Hospitals Network, Sydney, NSW 2031, Australia; (J.E.D.)
- Discipline of Paediatrics and Child Health, School of Clinical Medicine, University of New South Wales Sydney, Sydney, NSW 2052, Australia
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Butchbach MER. Genomic Variability in the Survival Motor Neuron Genes ( SMN1 and SMN2): Implications for Spinal Muscular Atrophy Phenotype and Therapeutics Development. Int J Mol Sci 2021; 22:ijms22157896. [PMID: 34360669 PMCID: PMC8348669 DOI: 10.3390/ijms22157896] [Citation(s) in RCA: 18] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2021] [Revised: 07/14/2021] [Accepted: 07/21/2021] [Indexed: 02/07/2023] Open
Abstract
Spinal muscular atrophy (SMA) is a leading genetic cause of infant death worldwide that is characterized by loss of spinal motor neurons leading to muscle weakness and atrophy. SMA results from the loss of survival motor neuron 1 (SMN1) gene but retention of its paralog SMN2. The copy numbers of SMN1 and SMN2 are variable within the human population with SMN2 copy number inversely correlating with SMA severity. Current therapeutic options for SMA focus on increasing SMN2 expression and alternative splicing so as to increase the amount of SMN protein. Recent work has demonstrated that not all SMN2, or SMN1, genes are equivalent and there is a high degree of genomic heterogeneity with respect to the SMN genes. Because SMA is now an actionable disease with SMN2 being the primary target, it is imperative to have a comprehensive understanding of this genomic heterogeneity with respect to hybrid SMN1–SMN2 genes generated by gene conversion events as well as partial deletions of the SMN genes. This review will describe this genetic heterogeneity in SMA and its impact on disease phenotype as well as therapeutic efficacy.
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Affiliation(s)
- Matthew E. R. Butchbach
- Center for Applied Clinical Genomics, Nemours Children’s Health Delaware, Wilmington, DE 19803, USA;
- Center for Pediatric Research, Nemours Children’s Health Delaware, Wilmington, DE 19803, USA
- Department of Biological Sciences, University of Delaware, Newark, DE 19716, USA
- Department of Pediatrics, Thomas Jefferson University, Philadelphia, PA 19107, USA
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