Modeling SMA Severity Through Humanized SMN2 Copy Number Mice

Introduction: Advancing SMA Research Through Precision Mouse Models

Spinal Muscular Atrophy (SMA) remains one of the most devastating genetic diseases affecting infants and children worldwide. As pharmaceutical companies race to develop effective therapies, researchers face a critical challenge: how to accurately model human SMA genetics in laboratory settings. The key lies in understanding and manipulating SMN2 copy numbers – nature's own disease modifier.

Cyagen's breakthrough humanized mouse models bridge this gap by incorporating varying human SMN2 copy numbers, creating a comprehensive spectrum of disease severity that mirrors human SMA types I, II, and III. These precision-engineered models provide an invaluable platform for investigating disease mechanisms and validating therapeutic approaches targeting SMN2, potentially accelerating the path from laboratory discovery to clinical application.

This article explores how these innovative mouse models recapitulate human SMA pathology and why they represent essential tools for both academic researchers and pharmaceutical companies developing the next generation of SMA treatments.

Understanding Spinal Muscular Atrophy: Disease Mechanism and Treatment Approaches

Spinal Muscular Atrophy (SMA) is a hereditary neuromuscular disorder caused by mutations in the SMN1 gene, leading to a deficiency of the SMN protein. This results in the degeneration of motor neurons in the anterior horn of the spinal cord, causing muscle weakness and atrophy. As the most common genetic cause of infant mortality, the earlier the onset of SMA, the more severe the condition tends to be—with a median survival of only about 10 months for children under the age of two.^[1]

The human genome contains the SMN2 gene, which is highly homologous to SMN1. Although SMN2 produces much less functional SMN protein, a higher copy number of SMN2 is generally associated with milder symptoms in patients. Consequently, regulating SMN2 expression has become a key therapeutic strategy for SMA, aiming to increase functional SMN protein levels to alleviate disease symptoms.^[2]

Biallelic Deletion of SMN1: The Root Cause of SMA

The Survival Motor Neuron 1 (SMN1) gene is broadly expressed throughout the human body, with particularly high levels in the spinal cord. It encodes the SMN protein, often referred to as a "housekeeping protein" essential for cellular survival. The SMN protein plays a pivotal role in the assembly of spliceosomal protein complexes, which directly influences the survival, function, and axonal development of motor neurons.^[3–4]

When both copies of SMN1 are deleted, the resulting severe deficiency of SMN protein leads to abnormal RNA splicing and functional impairment of spinal motor neurons. This triggers progressive motor neuron degeneration and denervation of skeletal muscles, ultimately causing muscle weakness and atrophy. Such degeneration significantly compromises a patient’s motor abilities and can be life-threatening.^[3–4]

Therefore, early intervention is critical—initiating treatment before irreversible motor neuron loss offers the best chance to preserve life and improve motor function recovery.

SMN2 Gene Copy Number: The "Regulator" of the Disease Severity and the "Key" to Treatment

In addition to SMN1, the human genome also contains the highly homologous SMN2 gene, which differs from SMN1 by only a few nucleotides.^[6] However, the SMN2 gene contains a c.840C>T point mutation in the splice enhancer of exon 7, which disrupts the splice enhancer or forms a splice silencer. This results in the majority of SMN2 precursor mRNA being spliced to exclude exon 7, leading to the production of truncated SMN protein that is nonfunctional and rapidly degraded.^[7] Only about 10% of SMN2 precursor mRNA produces full-length, functional SMN protein.^[8]

In approximately 95% of SMA patients, the biallelic deletion of SMN1 makes SMN2 the sole source of functional SMN protein. However, its output is insufficient to fully compensate for the loss of SMN1, leading to the development of the disease.^[6–8]

Based on the potential of SMN2, regulating its splicing or expression to increase functional SMN protein has become a major focus in SMA therapy. Among the three FDA-approved SMA drugs, two target SMN2: Biogen's antisense oligonucleotide drug Nusinersen and Roche's oral splicing modulator Risdiplam. Both drugs enhance the production of functional protein by modulating SMN2 splicing.^[12] In 2024, the sales of these two drugs reached $1.57 billion and nearly $1.8 billion, respectively, highlighting their clinical value and market potential.^[12]

Cyagen's SMA Mouse Models: Powerful Tools for Recapitulating Human Disease

Unlike humans, mice possess only a single Smn1 gene, and its knockout leads to embryonic lethality, making it difficult to directly model the pathogenic mechanism of SMA and the compensatory effect of SMN2.[13] Developing humanized mouse models that reflect the pathogenic mechanisms of SMA (especially the impact of SMN2 copy number) and mimic the human disease progression is crucial for the development and validation of SMN2-targeted therapies.

Cyagen has employed two strategies to establish SMN2 humanized foundational strains and has bred mice carrying different SMN2 copy numbers to create SMA humanized mouse models:

• Smn1^hSMN2/hSMN2 Mice: The mouse Smn1 gene's two copies are replaced in situ with two copies of the human SMN2 gene, simulating SMA patients carrying two copies of SMN2.
• ROSA26^hSMN2/hSMN2 Mice: Two copies of the human SMN2 gene are inserted at the mouse ROSA26 safe harbor locus.

Through multiple generations of breeding, SMA mouse models carrying 2, 3, and 4 copies of SMN2 on a Smn1 knockout background were developed, namely B6-2*hSMN2 mice (Product ID: C001504), B6-3*hSMN2 mice (Product ID: C001681), and B6-4*hSMN2 mice (Product ID: C001682).

Validation Data for SMA Mouse Models

The following validation data demonstrates the characteristics of these mouse models. For more comprehensive information, please refer to the specific strain datasheet.

Expression of Human SMN2 Gene and Mouse Smn1 Gene

Full-length human SMN2 transcripts were detected in the B6-2*hSMN2, B6-3*hSMN2, and B6-4*hSMN2 mice, while the expression of the mouse Smn1 gene was completely absent. As the SMN2 copy number increased, the expression level of full-length SMN2 transcripts progressively rose.

SMN Protein Expression

The SMN protein levels followed the same trend as the expression of full-length SMN2 transcripts, gradually increasing with the number of SMN2 copies.

Growth Curves

The growth curves below demonstrate differences in development between wild-type control mice and SMA model mice with varying SMN2 copy numbers:

Tail Length

Most B6-3*hSMN2 mice experience complete tail loss around 10 weeks of age. B6-4*hSMN2 mice also exhibit tail loss, but the tail length remains relatively constant starting from around 8 weeks of age.

Occurrence of Common Disease Symptoms

B6-3*hSMN2 mice exhibit more severe disease symptoms compared to B6-4*hSMN2 mice, indicating that an increased SMN2 copy number can alleviate disease progression.

Summary of Disease Phenotype Progression in Different SMA Mouse Models

In summary, B6-2*hSMN2 (Product ID: C001504), B6-3*hSMN2 (Product ID: C001681), and B6-4*hSMN2 (Product ID: C001682) mice, which completely lack mouse Smn1 gene expression, carry 2, 3, and 4 copies of the human SMN2 gene, respectively, simulating the genetic characteristics of human type I, II, and III SMA.

Data show that these mice, in terms of full-length SMN2 transcript and functional SMN protein expression, growth development, and survival, generally match the disease progression of human SMA patients carrying the corresponding SMN2 copy number. Therefore, these humanized mouse models not only effectively simulate the etiology of SMA, particularly the impact of SMN2 copy number on disease progression, but also provide valuable tools for studying SMA pathogenesis and developing therapies targeting human SMN2.

Cyagen's Neurological Disease CRO Services Platform

Cyagen has established a neurological disease CRO services platform specializing in neurodegenerative diseases, offering comprehensive services for researchers, including neurological disease model construction, behavioral analysis, and efficacy evaluation.

Rich Collection of Neurological Disease Models

Leveraging our advanced animal model development technology, we offer over 2,000 ready-to-use KO/CKO neurological mice, as well as more than 20 types of gene-edited and drug-induced rodent models for neurological diseases. These models encompass various targeting methods such as:

• Gene knockout
• Conditional knockout
• Point mutation
• Transgenics
• Humanization

Our HUGO-GT™ (Humanized Genomic Ortholog for Gene Therapy) mouse models, developed specifically for neuroscience research, cover a broader range of intervention targets and provide precise tools for studying disease mechanisms and developing targeted therapies. In addition to off-the-shelf models, we also offer custom mouse model development or collaborative projects tailored to researchers' needs.

References

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[6] Kolb SJ, Kissel JT. Spinal Muscular Atrophy. Neurol Clin. 2015 Nov;33(4):831-46.

[7] Day JW, Howell K, Place A, Long K, Rossello J, Kertesz N, Nomikos G. Advances and limitations for the treatment of spinal muscular atrophy. BMC Pediatr. 2022 Nov 3;22(1):632.

[8] 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 Jul 23;22(15):7896.

[9] Haque US, Yokota T. Recent Progress in Gene-Targeting Therapies for Spinal Muscular Atrophy: Promises and Challenges. Genes (Basel). 2024 Jul 30;15(8):999.

[10] Cuscó I, Bernal S, Blasco-Pérez L, Calucho M, Alias L, Fuentes-Prior P, Tizzano EF. Practical guidelines to manage discordant situations of SMN2 copy number in patients with spinal muscular atrophy. Neurol Genet. 2020 Nov 18;6(6):e530.

[11] Calucho M, Bernal S, Alías L, March F, Venceslá A, Rodríguez-Álvarez FJ, Aller E, Fernández RM, Borrego S, Millán JM, Hernández-Chico C, Cuscó I, Fuentes-Prior P, Tizzano EF. Correlation between SMA type and SMN2 copy number revisited: An analysis of 625 unrelated Spanish patients and a compilation of 2834 reported cases. Neuromuscul Disord. 2018 Mar;28(3):208-215.

[12] FiercePharma. (2025, February 12). Roche nabs FDA nod: Evrysdi tablets gaining potential convenience edge over SMA meds Biogen. Retrieved March 20, 2025, from https://www.fiercepharma.com/pharma/roche-nabs-fda-nod-evrysdi-tablets-gaining-potential-convenience-edge-over-sma-meds-biogen

[13] Edens BM, Ajroud-Driss S, Ma L, Ma YC. Molecular mechanisms and animal models of spinal muscular atrophy. Biochim Biophys Acta. 2015 Apr;1852(4):685-92.