Unraveling Rare Diseases: Researching Hotspot Pathogenic Mutations in Humanized Models

Welcome to the 'Ten Deadly Sins of Rare Diseases' column, where we decode the pathogenic mechanisms of rare diseases, industry research progress (e.g. gene therapy), and innovative pre-clinical strategies to drive translational research results (such as model construction and drug screening, etc.).

Previous reviews:

1. Exploring RHO-Related Pathogenic Mechanisms and RHO Gene Therapy Research Progress [Ten Deadly Sins of Rare DiseasesⅠ]

2. Why are humanized mice more suitable for hemophilia research, which has a higher prevalence in men? [Ten Deadly Sins of Rare Diseases Ⅱ]

3. The TARDBP gene is closely associated with pathogenesis of amyotrophic lateral sclerosis (ALS), but is it an angel or a demon? [Ten Deadly Sins of Rare Diseases Ⅲ]

4. Reviewing Muscular Dystrophy: Types, Pathology, and Gene Therapy Research [Ten Deadly Sins of Rare Diseases Ⅳ]

5. Is Smn2 Gene Therapy the Key to Treating Spinal Muscular Atrophy? [Ten Deadly Sins of Rare Diseases Ⅴ]

6. Application of Humanized Mouse Models in DNA Repeat Expansion Diseases [Ten Deadly Sins of Rare Diseases Ⅵ]

7. Unraveling the Genetic Tapestry of Angelman Syndrome: Decoding Maternal Genetic Abnormalities Behind the 'Happy Puppet' Phenomenon. [Ten Deadly Sins of Rare Diseases Ⅶ]

Understanding Hotspot Pathogenic Mutations:

Gene mutations refer to changes in the structure or sequence order of genes, involving alterations in the arrangement or composition of base pairs. These mutations can occur within coding sequences as well as non-coding sequences such as promoters, introns, and splice sites. In other words, they can occur anywhere, potentially triggering a series of pathogenic risks. This article explores prevalent pathogenic mutations at various gene sequence sites, featuring:

• ELP1 intronic point mutation IVS20+6T>C leading to Familial Dysautonomia.
• Abnormal methylation near the DUX4 promoter region resulting in facioscapulohumeral muscular dystrophy.
• Abnormal expansion of CTG repeat segments in the 3' untranslated region of the DMPK gene causing myotonic dystrophies.

Familial Dysautonomia: ELP1 Intronic Point Mutation

Familial dysautonomia (FD) syndrome is a rare hereditary autosomal recessive genetic disorder characterized primarily by a group of clinical symptoms mainly characterized by neurological dysfunction, especially autonomic nervous system dysfunction. This condition is also known as Riley-Day syndrome, familial dysautonomia, and central autonomic dysfunction syndrome, among other names.

Pathogenesis of Familial Dysautonomia

More than 9% of individuals with FD carry a homozygous mutation in the ELP1 gene, specifically the IVS20+6T>C mutation in intron 20. This mutation disrupts the base pairing between U1 small nuclear ribonucleoprotein and the intron 20 donor splicing site, leading to skipping of the exon 20 coding sequence. This transcript mis-splicing introduces a frameshift, resulting in the translation of a truncated 79kDa protein ^[1-2].

Challenges in Familial Dysautonomia Research Models

Studying alterations in human cell splicing processes using mouse models is challenging because the splicing processes in the two species are different ^[3]. This challenge is even more pronounced when researching diseases caused by intronic point mutations. Existing literature has shown that homozygous knockout of this gene leads to embryonic lethality. Additionally, directly expressing the gene with the human mutation in wild-type mice may not result in an apparent disease phenotype, possibly due to the expression of normal levels of endogenous mouse ELP1 ^[4-6].

Humanized Models of Familial Dysautonomia

To address these challenges, Cyagen has independently developed whole-genome humanized ELP1 mice (hELP1 mice) and subsequently created humanized disease models with hotspot point mutations based on these mice. These targeted disease models can precisely replicate the genetic alterations found in patients, providing a more targeted and relevant platform for studying the pathogenesis of diseases associated with ELP1 mutations. Cyagen's humanized models with hotspot mutations enable preclinical research on FD and other diseases, overcoming challenges posed by species-specific splicing processes. This innovation allows precise investigation of diseases caused by intronic point mutations, where conventional models may fall short due to differences in splicing mechanisms.

Facioscapulohumeral muscular dystrophy: Abnormal DUX4 Expression

Facioscapulohumeral muscular dystrophy (FSHD) is inherited in an autosomal dominant manner, facilitating its prevalence as one of the most common muscular dystrophies. Clinical symptoms include progressive, asymmetric (or symmetric) muscle weakness and muscle atrophy. Although the progression of the disease is relatively slow, it can lead to significant disability, with approximately 20% of patients requiring a wheelchair and having severely impacted quality of life.

Pathogenesis of Facioscapulohumeral Muscular Dystrophy

In clinical practice, FSHD is divided into two types: FSHD1 and FSHD2. Although the underlying mechanisms of these two types are different, the ultimate result is a reduction in methylation levels, leading to abnormal expression of the DUX4 (double homeobox 4) gene in skeletal muscles or abnormal functionality of the DUX4 protein ^[7]. The abnormal DUX4 is a toxic transcription factor that activates hundreds of downstream genes, impairs muscle development, triggers immune responses, increases oxidative stress sensitivity, and ultimately activates cell death pathways.

Models of Facioscapulohumeral Muscular Dystrophy

In current preclinical pipeline research for this disease, the FLExDUX4 mouse model is frequently used ^[8]. This is an inducible model and is complex to construct and breed appropriately. Early models often used AAV delivery of DUX4 to create disease models ^[9]. Therefore, Cyagen has independently developed the next-generation AAV delivery of DUX4 to construct disease models, enabling rapid modeling within a few weeks for preclinical research on the disease.

Atrophic Muscular Dystrophy: CTG Repeats in Mutant DMPK

Atrophic muscular dystrophy, also known as Myotonic dystrophy (DM), is a rare autosomal dominant genetic disorder with a high mutation penetrance, often showing familial clustering. It exhibits high clinical heterogeneity, primarily characterized by muscle stiffness, muscle weakness, and muscle atrophy. DMs are divided into two subtypes: Myotonic dystrophy type 1 (DM1), also known as Dystrophia myotonica 1, and Myotonic dystrophy type 2 (DM2), also known as Dystrophia myotonica 2. DM1 is the most common and is caused by abnormal expansion of CTG repeat segments in the 3' untranslated region of the DMPK gene ^[10].

Pathogenesis of Atrophic Muscular Dystrophy

In normal individuals, there are typically 5-37 CTG repeats; in DM1 patients, the number of repeats can expand to anywhere from 50 to several thousand. Generally, when the CTG repeat number exceeds 37, it becomes unstable and prone to amplification during mitosis or meiosis. Increasing evidence suggests that mutated DMPK transcripts sequester in the cell nucleus, forming discrete foci, which exert toxic effects by disrupting the normal function of proteins required for cell development.

Research Models of Atrophic Muscular Dystrophy

Currently, disease models used in research primarily involve DMPK transgenic mice (carrying multiple CTG repeats) ^[11] and HSA^LR (human skeletal actin long repeat) transgenic mice ^[12]. Cyagen not only provides both of these transgenic models, but is also developing better models for preclinical research on rare diseases like DM1 using our full-length genomically humanized mouse model technology: HUGO-GT.

Next-Generation Humanized Mouse Models for Preclinical Gene Therapy Research: HUGO-GT^TM

To address research challenges of the above disorders – alongside various other conditions such as Retinitis Pigmentosa (RP), Age-related Macular Degeneration (AMD), and Parkinson's Disease (PD) – if one wants to delve deep into understanding their pathogenic mechanisms, long genomic segments or even whole-genome humanized mice are a preferable choice. However, the technical difficulty associated with whole-genome replacement is high, and the introduction of large-scale exogenous sequences may potentially affect the expression and regulation of native genes.

To overcome these obstacles, Cyagen has launched the groundbreaking Humanized Genomic Ortholog for Gene Therapy (HUGO-GT^TM) Program initiative - a remarkable leap in Next-Generation Humanized Mouse Model Development. Powered by our innovative TurboKnockout-Pro technology, HUGO-GT enables the seamless in situ replacement of mouse genes with fully-human genes. This breakthrough paves the way for the creation of fully humanized mice carrying entirely human genomic DNA segments, opening a realm of possibilities for diverse intervention targets and preclinical gene therapy research. 

Below are tables with the currently available HUGO-GT^TM mouse models and other humanized mouse models of disease:

• Selected Humanized Model Recommendations
  
  

• More Recommendations
  
  

Conclusion

In unraveling the mysteries of rare diseases, our exploration of pathogenic mutations at diverse gene sequence sites highlights the pivotal role of humanized models. These models, akin to adaptable bricks, pave the way for groundbreaking insights into the mechanisms of ailments like familial dysautonomia, facioscapulohumeral muscular dystrophy, and myotonic dystrophies.

Our commitment to precision extends beyond these specific diseases. The initiation of the HUGO-GT program underscores our dedication to addressing a myriad of conditions, including retinitis pigmentosa, macular degeneration, and Parkinson's disease. The TurboKnockout-Pro technology empowers us to construct fully humanized mice, offering a more nuanced and realistic representation for preclinical drug research.

As we present our curated models and recommendations, we invite you to explore the vast potential for tailored, disease-specific research. Our mouse models cover a spectrum of diseases, ensuring precise representation for in-depth research. Explore our recommended models, references, and our expert team can answer any questions you have regarding development of your animal model study.

Can't find what you are looking for? We can develop a custom model for your study - contact us for a free consultation.

Together, let's advance the frontier of rare disease research, bringing us one step closer to transformative breakthroughs and enhanced patient outcomes.

References:

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[2]Anderson S L , Coli R , Daly I W ,et al.Familial Dysautonomia Is Caused by Mutations of the IKAP Gene[J].The American Journal of Human Genetics, 2001, 68(3):753-758.DOI:10.1086/318808.

[3]Pan Q , Bakowski M A , Morris Q ,et al.Alternative splicing of conserved exons is frequently species-specific in human and mouse.[J].Trends in Genetics, 2005, 21(2):73-77.DOI:10.1016/j.tig.2004.12.004.

[4]Hims M M , Shetty R S , Pickel J ,et al.A humanized IKBKAP transgenic mouse models a tissue-specific human splicing defect.[J].Genomics, 2007, 90(3):389-396.DOI:10.1016/j.ygeno.2007.05.012.

[5]Chen Y T , Hims M M , Shetty R S ,et al.Loss of Mouse Ikbkap, a Subunit of Elongator, Leads to Transcriptional Deficits and Embryonic Lethality That Can Be Rescued by Human IKBKAP[J].Molecular and Cellular Biology, 2009, 29(3):736-744.DOI:10.1128/MCB.01313-08.

[6]Dietrich P , Yue J , Shuyu E ,et al.Deletion of Exon 20 of the Familial Dysautonomia Gene Ikbkap in Mice Causes Developmental Delay, Cardiovascular Defects, and Early Embryonic Lethality[J].Plos One, 2011, 6(10):e27015.DOI:10.1371/journal.pone.0027015.

[7]Ansseau Eugénie,Vanderplanck Céline, Armelle W ,et al.Antisense Oligonucleotides Used to Target the DUX4 mRNA as Therapeutic Approaches in FaciosScapuloHumeral Muscular Dystrophy (FSHD)[J].Genes, 2017, 8(3):93.DOI:10.3390/genes8030093.

[8]Lim K R Q , Maruyama R , Echigoya Y ,et al.Inhibition of DUX4 expression with antisense LNA gapmers as a therapy for facioscapulohumeral muscular dystrophy (vol 117, pg 16509, 2020)[J].Proceedings of the National Academy of Sciences of the United States of America.  2020(35):117.

[9]Ba L M W , Garwick S E , Mei W ,et al.DUX4, a candidate gene for facioscapulohumeral muscular dystrophy, causes p53-dependent myopathy in vivo.[J].Annals of Neurology, 2011, 69(3):540-552.DOI:10.1002/ana.22275.

[10]Yin Q , Wang H , Li N ,et al.Dosage effect of multiple genes accounts for multisystem disorder of myotonic dystrophy type 1[J].Cell Research, 2019, 30(2):1-13.DOI:10.1038/s41422-019-0264-2.

[11]Mahadevan M S , Yadava R S , Yu Q ,et al.Reversible model of RNA toxicity and cardiac conduction defects in myotonic dystrophy.[J].Nature Genetics, 2006, 38(9):1066.DOI:10.1038/ng1857.

[12]Mankodi A, Logigian E, Callahan L, McClain C, White R, Henderson D, Krym M, Thornton CA. Myotonic dystrophy in transgenic mice expressing an expanded CUG repeat. Science. 2000 Sep 8;289(5485):1769-73. doi: 10.1126/science.289.5485.1769. PMID: 10976074.