Abnormal subcortical activity in congenital mirror movement disorder with RAD51 mutation

Defining the Genetic Landscape of Congenital Mirror Movements in 80 Affected Individuals

BACKGROUND

Congenital mirror movements (CMM) is a rare neurodevelopmental disorder characterized by involuntary movements from one side of the body that mirror voluntary movements on the opposite side. To date, five genes have been associated with CMM, namely DCC, RAD51, NTN1, ARHGEF7, and DNAL4.

OBJECTIVE

The aim of this study is to characterize the genetic landscape of CMM in a large group of 80 affected individuals.

METHODS

We screened 80 individuals with CMM from 43 families for pathogenic variants in CMM genes. In large CMM families, we tested for presence of pathogenic variants in multiple affected and unaffected individuals. In addition, we evaluated the impact of three missense DCC variants on binding between DCC and Netrin-1 in vitro.

RESULTS

Causal pathogenic/likely pathogenic variants were found in 35% of probands overall, and 70% with familial CMM. The most common causal gene was DCC, responsible for 28% of CMM probands and 80% of solved cases. RAD51, NTN1, and ARHGEF7 were rare causes of CMM, responsible for 2% each. Penetrance of CMM in DCC pathogenic variant carriers was 68% and higher in males than females (74% vs. 54%). The three tested missense variants (p.Ile164Thr; p.Asn176Ser; and p.Arg1343His) bind Netrin-1 similarly to wild type DCC.

CONCLUSIONS

A genetic etiology can be identified in one third of CMM individuals, with DCC being the most common gene involved. Two thirds of CMM individuals were unsolved, highlighting that CMM is genetically heterogeneous and other CMM genes are yet to be discovered. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

New candidate region for mirror hand movements: two patients with terminal 9p deletion and 20p duplication

The 9p deletion syndrome, which was defined in a detailed way in the previous studies, was characterized by various clinical features such as psychomotor retardation, dysmorphic features and genital anomalies. In contrast to 9p deletion syndrome, 20p duplication was rarely reported in the literature with only a few case reports. Regarding the combination of 9p deletion syndrome and 20p duplication, we found that it was reported in only four patients. In the current study, we aimed to investigate a rare chromosomal rearrangement, partial monosomy 9p and trisomy 20p which was observed in two patients with mirror hand movements. The mirror hand movements was influenced by the combination of genetic and environmental factors. While some cases have been associated with mutations in the DCC, NTN1, RAD51, and DNAL4, there were many cases where the genetic basis of mirror hand movements remained unexplained. There was no alteration detected in genes that were previously known as a cause of mirror hand movement in our patients. This new finding could potentially be attributed to the dosage effect of genes within the 9p deletion or 20p duplication regions or to the genes disrupted within the breakpoint region. Future research focusing on the genes within this genomic locus may hold the potential to uncover novel etiologic reasons for mirror hand movements.

Congenital mirror movements are associated with defective polymerisation of RAD51

BACKGROUND

Mirror movements are involuntary movements of one hand that mirror intentional movements of the other hand. Congenital mirror movements (CMM) is a rare genetic disorder with autosomal dominant inheritance, in which mirror movements are the main neurological manifestation. CMM is associated with an abnormal decussation of the corticospinal tract, a major motor tract for voluntary movements. RAD51 is known to play a key role in homologous recombination with a critical function in DNA repair. While RAD51 haploinsufficiency was first proposed to explain CMM, other mechanisms could be involved.

METHODS

We performed Sanger sequencing of RAD51 in five newly identified CMM families to identify new pathogenic variants. We further investigated the expression of wild-type and mutant RAD51 in the patients' lymphoblasts at mRNA and protein levels. We then characterised the functions of RAD51 altered by non-truncating variants using biochemical approaches.

RESULTS

The level of wild-type RAD51 protein was lower in the cells of all patients with CMM compared with their non-carrier relatives. The reduction was less pronounced in asymptomatic carriers. In vitro, mutant RAD51 proteins showed loss-of-function for polymerisation, DNA binding and strand exchange activity.

CONCLUSION

Our study demonstrates that RAD51 haploinsufficiency, including loss-of-function of non-truncating variants, results in CMM. The incomplete penetrance likely results from post-transcriptional compensation. Changes in RAD51 levels and/or polymerisation properties could influence guidance of the corticospinal axons during development. Our findings open up new perspectives to understand the role of RAD51 in neurodevelopment.

Noncanonical Roles of RAD51

Homologous recombination (HR), an evolutionary conserved pathway, plays a paramount role(s) in genome plasticity. The pivotal HR step is the strand invasion/exchange of double-stranded DNA by a homologous single-stranded DNA (ssDNA) covered by RAD51. Thus, RAD51 plays a prime role in HR through this canonical catalytic strand invasion/exchange activity. The mutations in many HR genes cause oncogenesis. Surprisingly, despite its central role in HR, the invalidation of RAD51 is not classified as being cancer prone, constituting the "RAD51 paradox". This suggests that RAD51 exercises other noncanonical roles that are independent of its catalytic strand invasion/exchange function. For example, the binding of RAD51 on ssDNA prevents nonconservative mutagenic DNA repair, which is independent of its strand exchange activity but relies on its ssDNA occupancy. At the arrested replication forks, RAD51 plays several noncanonical roles in the formation, protection, and management of fork reversal, allowing for the resumption of replication. RAD51 also exhibits noncanonical roles in RNA-mediated processes. Finally, RAD51 pathogenic variants have been described in the congenital mirror movement syndrome, revealing an unexpected role in brain development. In this review, we present and discuss the different noncanonical roles of RAD51, whose presence does not automatically result in an HR event, revealing the multiple faces of this prominent actor in genomic plasticity.

Facilitative effects of use-dependent learning on interlimb transfer of visuomotor adaptation in a person with congenital mirror movements

It has been shown that use-dependent learning can facilitate interlimb transfer of motor learning in neurologically intact individuals. However, it is unknown whether it can also facilitate interlimb transfer in individuals with neurological impairment. In this case study, we examined the effect of use-dependent learning on interlimb transfer of visuomotor adaptation in a person with congenital mirror movements, DB, who showed no interlimb transfer in our previous studies (Bao, Morgan, Lei, & Wang, 2020; Javidialsaadi, & Wang, 2021). DB first performed reaching movements with the right arm repeatedly while adapting to a visuomotor rotation condition with the left arm (training session), and then adapted to the same rotation condition with the right arm (transfer session). DB's right arm performance in the transfer session was significantly better than that observed in our previous studies, indicating interlimb transfer of visuomotor adaptation. The percentage of transfer was over 90%, which is similar to that observed in healthy young adults previously. These findings suggest that interlimb transfer of visuomotor adaptation can occur by involving model-based learning, which is effector independent, and/or use-dependent (or model-free) learning, which is effector specific; and also that the relative contribution of use-dependent learning to interlimb transfer of visuomotor adaptation can be as large as that of model-based learning.