Dissertation
Genetic modifiers and treatments for striated muscle laminopathies
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2026
DOI: 10.25820/etd.008405
Abstract
The LMNA gene encodes the A-type lamins, lamins A and C, that are generated via alternative splicing. Lamins are intermediate filaments that form a meshwork underlying the inner nuclear membrane called the nuclear lamina. The nuclear lamina provides structural support to the nucleus, interacts with the LINC complex to mediate the nuclear response to mechanical stress, organizes chromatin to regulate gene expression, and stabilizes nuclear pore complexes to allow for the flow of macromolecules in and out of the nucleus. Mutations in LMNA cause a broad spectrum of human diseases known as laminopathies. The most common laminopathies are striated muscle diseases, which include three types of muscular dystrophy collectively known as LMNA-MD. These MDs are often accompanied by dilated cardiomyopathy with conduction defects, which can also occur in the absence of skeletal muscle disease, that leads to a shortened lifespan. LMNA-MD is highly phenotypically variable. The same primary LMNA mutation can cause different phenotypes in different individuals, including among siblings. The age of onset, rate of disease progression, and severity of LMNA-MD are variables that differ widely among affected individuals. This variability has long been attributed to genetic modifiers, which are differences in the genetic background among individuals that alter the effect of the primary mutation. To date, little progress has been made in identifying genetic modifiers of LMNA-MD. Furthermore, different LMNA mutations encode proteins with different molecular properties, complicating treatment development. There are currently no treatments for LMNA-MD beyond symptom management. The goal of my thesis research is to identify genetic contributors to the phenotypic variability associated with LMNA-MD and identify novel therapeutic targets for the disease.
To this end, families with siblings possessing the same LMNA mutation that present with variable phenotypes were identified. In the first family under study, affected individuals possess an LMNA mutation that alters pre-mRNA splicing of LMNA transcripts and causes LGMD1B. A subset of individuals in the family present with more severe muscle disease that includes skeletal muscle cores, a histopathologic phenotype not associated with LGMD. Through whole genome sequencing (WGS), a missense variant in CCDC78 was identified that co-segregates with the muscle core phenotype. While other variants in CCDC78 are known to cause core myopathy, the identified variant has a relatively high population allele frequency, suggesting that it is unlikely individually pathogenic. I discovered that this variant, in combination with the LMNA mutation, is associated with profound skeletal muscle disease. These finding suggest suggesting that myopathy caused by the rare LMNA mutation sensitizes muscles to the effects of the common CCDC78 variant. This represents a case of digenic inheritance, where variants in multiple genes associated with muscle disease can combine to create a composite phenotype more severe than what is caused by either mutation alone.
In the second family under study, four siblings possessing a LMNA mutation causing the amino acid substitution R527P exhibited muscular dystrophy of variable severity. Two of the siblings presented with childhood onset of muscle weakness that progressed to loss of ambulation in their early thirties. The other two siblings continue to have only mild muscle weakness past the age that the severely affected siblings lost ambulation. WGS was completed on the four siblings, and a variant in SMAD7, encoding a repressor of the SMAD signaling pathway, was identified only in the severely affected siblings. I performed functional analysis of this variant in Drosophila to demonstrate that it increases the severity of muscle defects caused by the mutant lamin. Furthermore, I showed that the mutant lamin alone activates SMAD signaling and this is enhanced by the SMAD7 variant, providing a mechanism for the enhancement of muscle defects. Consistent with these findings, six additional SMAD7 variants were identified in a larger cohort of individuals with LMNA mutations. Functional testing of two of these variants showed that they also enhance muscle defects in combination with multiple mutant lamins by the same mechanism of hyperactivation of SMAD signaling. To translate these findings to humans, I stained muscle biopsy tissue from individuals with LMNA-MD and discovered that multiple mutant lamins activate the SMAD signaling pathway. To demonstrate the therapeutic relevance of these findings, I show that targeting SMAD signaling either genetically through overexpression of SMAD7 or pharmacologically by treatment with SMAD pathway inhibitors partially rescues muscle and cardiac defects caused by mutant lamins. Collectively, these findings support SMAD7 as the first functionally tested modifier gene of LMNA-MD and demonstrate that genetic modifiers can lead to the discovery of therapeutic targets.
In addition to targeting the SMAD signaling pathway, an unbiased drug screen was performed to identify drugs that rescue lethality in a Drosophila model of LMNA-MD expressing the equivalent of lamin A/C R249Q. This screen identified L-type voltage gated calcium channel (LTCC) blockers as potential drug repurposing candidates. I show that multiple dihydropyridines (DHPs) and non-DHPs, which target LTCCs by different mechanisms, partially rescue lethality caused by mutant lamin expression. These results support the LTCC as a therapeutic target. I discovered that the LTCC blockers were successful at rescuing lethality caused by only three out of seven tested lamin substitutions, suggesting that personalized treatments are likely required for the wide spectrum of LMNA-MD mutations.
Collectively, my research demonstrates the power of WGS to identify genetic contributors to phenotypic variability in monogenic disease and the power of model organisms as tools for functionally testing identified variants. Furthermore, my research shows how identifying genetic modifiers can inform on disease biology and translate to novel therapeutic targets. My findings have the potential to improve prognosis predictions and lead to new treatments for LMNA-MD. The impact of my studies extends beyond LMNA-MD, as I demonstrate a workflow that can be used to identify and test modifier variants for other monogenic diseases. In addition, the therapeutic targets I identified might be relevant to other skeletal muscle and cardiac diseases.
Details
- Title: Subtitle
- Genetic modifiers and treatments for striated muscle laminopathies
- Creators
- Nathaniel Peter Mohar
- Contributors
- Lori L Wallrath (Advisor)Barry London (Committee Member)Katherine D Mathews (Committee Member)Todd E Scheetz (Committee Member)Michael J Schnieders (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Genetics
- Date degree season
- Spring 2026
- DOI
- 10.25820/etd.008405
- Publisher
- University of Iowa
- Number of pages
- xxiv, 242 pages
- Copyright
- Copyright 2026 Nathaniel Peter Mohar
- Language
- English
- Date submitted
- 04/16/2026
- Description illustrations
- illustrations, tables, graphs
- Description bibliographic
- Includes bibliographical references (pages 205-242).
- Public Abstract (ETD)
Muscular dystrophies are diseases that cause muscle weakness and wasting that often gets worse with age. These diseases are caused by changes (mutations) in genes needed for muscle function. My thesis project focused on rare types of muscular dystrophy caused by mutations in the LMNA gene, which makes lamins. Lamins form a meshwork that supports the nucleus, a circular structure in cells that contains the genomic DNA. Individuals with the exact same change in the LMNA gene can have extreme differences in the severity of their muscular dystrophy. The goal of my thesis was to understand what causes this variability in disease symptoms. Additionally, I sought to identify treatments for the disease, as there currently are none. This information will help doctors better predict the course of each patient’s disease and provide improved care.
My project began by analyzing the genomic DNA sequences of family members with mild and severe muscular dystrophy. I identified mutations in additional genes that were present only in the family members with severe disease. The effects of these additional mutations were tested in fruit flies that have muscle problems like humans with muscular dystrophy. The additional DNA mutations made the muscle problems more severe, just like in the severely affected people. Reversing the effects of these additional mutations improved muscle function, suggesting treatments for the disease.
To identify additional potential treatments for patients with muscular dystrophy, I performed a drug screen. Fruit flies with muscle defects caused by mutant lamins were fed food containing drugs. I identified several drugs that improved their muscle function. These drugs are already FDA approved for other types of diseases and shown to be safe, making them attractive treatment options. In conclusion, my thesis research identified mutations that make muscle disease more severe. This can improve the ability of doctors to predict disease severity. In addition, I identified multiple possible treatments s [sic] for muscular dystrophy. These studies have the potential to improve patient quality of life.
- Academic Unit
- Interdisciplinary Graduate Program in Genetics
- Record Identifier
- 9985177376502771
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