A mouse model of MEIS1-associated restless legs syndrome: insights and challenges

Abstract

Restless legs syndrome (RLS) remains a clinically enigmatic condition, characterized by an irresistible urge to move the legs, often accompanied by unpleasant sensations during periods of rest [1]. The assistance of genetics and its various branches, coupled with the investigation of cardiovascular pathology using animal models, holds the potential to unveil novel insights into the pathogenesis of the disorder. The ultimate goal is to provide new therapeutic avenues. A recent systematic review examining potential animal models of RLS has yielded evidence of construct validity, elucidating the underlying pathophysiological mechanisms contributing to or causing RLS [2]. The study adhered to established guidelines for developing rodent models of RLS [3]. Genome-wide association studies, as underscored in a recent meta-analysis, have confirmed the MEIS1 gene’s status as the most robust genetic risk factor for RLS in humans, as corroborated by mouse models [4, 5]. Additionally, periodic leg movements during sleep (PLMS), frequently observed in RLS [1], have also been linked to polymorphisms in the MEIS1 gene [6]. MEIS1 is part of the homeobox-containing transcriptional regulatory network crucial during development. Animal model studies have demonstrated an association between this gene and iron homeostasis [7], aligning with contemporary concepts of RLS pathophysiology [1]. Furthermore, reducing MEIS1 expression in simulations resulted in circadian hyperactivity, a phenotype compatible with RLS [7]. Exploring the role of MEIS1 in RLS is particularly intriguing, given its implication in various other functions, including cell proliferation and differentiation. This involvement has subsequent implications in carcinogenesis and neurodegeneration processes [8]. Recently, MEIS1 has been proposed to play a key role in regulating cardiomyocyte metabolism, with implications for cardiovascular function [9]. In an effort to shed light on the pathogenesis of RLS, the study by Leu et al. [10] published in this issue sought to establish a mouse model based on a discrete missense variant within the MEIS1 gene, a gene that has been implicated in patients with RLS [4, 11, 12]. While the study has notable strengths, particularly its clinically relevant approach and phenotypic validation, it is essential to consider its limitations and the complex nature of RLS. One of the commendable aspects of this study is its choice to focus on a missense variant within the MEIS1 gene, mirroring the genetic makeup of patients with RLS [13, 14]. By creating a mouse model with the R272H mutation, the researchers aimed to replicate the clinical features of RLS more faithfully than models based on null alleles, which often exhibit extreme and less specific phenotypes [3]. This targeted approach represents a significant strength, offering a way to understand the role of MEIS1 in RLS pathogenesis. The decision to concentrate on homozygote female mice is also rationalized based on the higher prevalence of RLS in females [1]. Despite the absence of homozygous carriers in the original human cohort, the study’s focus on female homozygote carriers aligns with the expectation that any resulting phenotype would be more pronounced, aiding in the detection of specific disease features. This sex-specific analysis is a noteworthy strength, recognizing the importance of considering sex differences in disease manifestation. The study’s meticulous examination of various phenotypic features, such as locomotion, balance, sensory processing, and sleep behaviors, reflects a comprehensive, and clinically relevant approach. Patients with RLS commonly exhibit restlessness and frequent arousals during rest, particularly before bedtime [15], and the study appropriately targeted these behaviors. The analyses revealed mixed responses in MEIS1 R272H/R272H mice, emphasizing the complexity of RLS-related phenotypes. However, certain limitations merit consideration. The absence of significant alterations in locomotion in most phenotypic tests could be attributed to the young age of the mice during testing, as RLS incidence increases with age [1], as well as the prevalence of PLMS, frequently associated with this disorder [16]. The study acknowledges this limitation and suggests that future investigations should involve aged cohorts to better capture age-related changes in locomotion, which is crucial for understanding the progressive nature of RLS. Additionally, the study highlights subtle alterations in sensory feedback, mirroring observations in patients with RLS [1]. Yet, the lack of statistical significance in some assessments raises questions about the robustness of these findings. Further studies, perhaps employing fiber-selective assessments, could elucidate whether the MEIS1 R272H mutation influences descending inhibition or spinal excitability, aligning the mouse model more closely with RLS symptoms [15]. The piezo sleep–wake screen, despite offering valuable insights into wake activity, presents challenges in deciphering the underlying causes of increased arousal in MEIS1 R272H/R272H mice. The study rightly acknowledges the ambiguity surrounding whether hindlimb discomfort, akin to that experienced by patients with RLS, is driving the observed arousal. This uncertainty underscores the complexity of translating clinical symptoms into observable behaviors in animal models. While the study did not find changes in iron-related measures in young mice, the potential impact of age and an iron-deficient diet on the MEIS1 R272H/R272H phenotype remains an open question. Investigating these factors in aged cohorts, as well as employing an iron challenge, could unveil subtle phenotypic changes and provide further insights into the relationship between MEIS1, iron metabolism, and RLS. The associations with iron and the enhancer activity of highly conserved noncoding regions of MEIS1 suggest promising connections to RLS pathways. Additionally, this gene exhibits lower mRNA expression in the blood and thalamus of individuals with the MEIS1 RLS risk haplotype [7], aligning with the involvement of this structure and the basal ganglia in RLS in humans [17]. A recent postulation proposes a role for noncoding RNA in the association between carcinogenesis and neurodegeneration [18], consistent with recent functions attributed to the MEIS1 gene [8]. It would be highly interesting to direct research towards understanding the role of MEIS1 in the cognitive sphere in RLS, considering the aforementioned studies on mouse models and emerging research indicating possible impairments in cognitive functions, especially attention, in RLS patients [19]. Another intriguing area of research involves evaluating the connections between MEIS1, cardiovascular factors, and RLS, as well as leg movements during sleep. This consideration stems from reports of an association between these movement disorders and certain cardiovascular disorders in humans [20–22], although the pathogenetic link remains unclear. A future challenge could be to enhance genetic studies related to epigenetics to better define environmental factors that can modify the functions of this gene. This would help us understand the biological pathways associated with MEIS1 that are compromised in RLS. Preliminary evidence supports the role of MEIS1 modulation in the pharmacological response to dopamine agonists in RLS mouse models [23]. Further investigations in this regard should be implemented, considering the intriguing perspectives demonstrated in the oncology field, where therapeutic outcomes have recently been demonstrated following epigenetic modifications of factors connected to MEIS1 [24, 25]. In conclusion, the study on the MEIS1 R272H/R272H mouse model offers valuable contributions to our understanding of RLS pathogenesis. The choice of a clinically relevant missense variant, consideration of gender-specific effects, and a comprehensive phenotypic analysis showcase the strengths of the study. However, the limitations, including the age of mice during testing and the complexity of interpreting certain behaviors, emphasize the need for continued research to unravel the intricate mechanisms underlying RLS. As the field progresses, the MEIS1 R272H/R272H mouse model stands as a valuable tool, albeit one that requires further exploration and refinement to fully unlock its potential for advancing RLS research.