Dynamic interhemispheric desynchronization in marmosets and humans with disorders of the corpus callosum. Frontiers in Neural Circuits  14 (2020). doi:10.3389/fncir.2020.612595

The corpus callosum, the principal structural avenue for interhemispheric neuronal communication, controls the brain’s lateralization. Developmental malformations of the corpus callosum (CCD) can lead to learning and intellectual disabilities. Currently, there is no clear explanation for these symptoms. Here, we used resting-state functional MRI (rsfMRI) to evaluate the dynamic resting-state functional connectivity (rsFC) in both the cingulate cortex (CG) and the sensory areas (S1, S2, A1) in three marmosets (Callithrix jacchus) with spontaneous CCD. We also performed rsfMRI in 10 CCD human subjects (six hypoplasic and four agenesic). We observed no differences in the strength of rsFC between homotopic CG and sensory areas in both species when comparing them to healthy controls. However, in CCD marmosets, we found lower strength of quasi-periodic patterns (QPP) correlation in the posterior interhemispheric sensory areas. We also found a significant lag of interhemispheric communication in the medial CG, suggesting asynchrony between the two hemispheres. Correspondingly, in human subjects, we found that the CG of acallosal subjects had a higher QPP correlation than controls. In comparison, hypoplasic subjects had a lower QPP correlation and a delay of 1.6 s in the sensory regions. These results show that CCD affects the interhemispheric synchrony of both CG and sensory areas and that, in both species, its impact on cortical communication varies along the CC development gradient. Our study shines a light on how CCD misconnects homotopic regions and opens a line of research to explain the causes of the symptoms exhibited by CCD patients and how to mitigate them.

Authors:  Diego Szczupak, Cecil C.C. Yen, Cirong Liu,  Xiaoguang Tian, Roberto Lent, Fernanda Tovar-Moll, and Afonso C. Silva, in collaboration with the IRC5 Consortium

Long-distance aberrant heterotopic connectivity in a mouse strain with a high incidence of callosal anomalies. Neuroimage  217 (2020). doi:10.1016/j.neuroimage.2020.116875

Corpus callosum dysgenesis (CCD) is a developmental brain condition in which some white matter fibers fail to find their natural course across the midplane, reorganizing instead to form new aberrant pathways. This type of white matter reorganization is known as long-distance plasticity (LDP). The present work aimed to characterize the Balb/c mouse strain as a model of CCD. We employed high-resolution anatomical MRI in 81 Balb/c and 27 C57bl6 mice to show that the Balb/c mouse strain presents a variance in the size of the CC that is 3.9 times higher than the variance of normotypical C57bl6. We also performed high-resolution diffusion-weighted imaging (DWI) in 8 Balb/c and found that the Balb/c strain shows aberrant white matter bundles, such as the Probst (5/8 animals) and the Sigmoid bundles (7/8 animals), which are similar to those found in humans with CCD. Using a histological tracer technique, we confirmed the existence of these aberrant bundles in the Balb/c strain. Interestingly, we also identified sigmoid-like fibers in the C57bl6 strain, thought to a lesser degree. Next, we used a connectome approach and found widespread brain connectivity differences between Balb/c and C57bl6 strains. The Balb/c strain also exhibited increased variability of global connectivity. These findings suggest that the Balb/c strain presents local and global changes in brain structural connectivity. This strain often presents with callosal abnormalities, along with the Probst and the Sigmoid bundles, making it is an attractive animal model for CCD and LDP in general. Our results also show that even the C57bl6 strain, which typically serves as a normotypical control animal in a myriad of studies, presents sigmoid-fashion pattern fibers laid out in the brain. These results suggest that these aberrant fiber pathways may not necessarily be a pathological hallmark, but instead an alternative roadmap for misguided axons. Such findings offer new insights for interpreting the significance of CCD-associated LDP in humans.

Authors:  Diego Szczupak, Cirong Liu, Cecil C.C. Yen, Sang-Ho Choi, Fernanda Meireles, Caroline Victorino, Linda Richards, Roberto Lent, Afonso C. Silva, Fernanda Tovar-Moll, IRC5 Consortium

Altered structural connectivity networks in a mouse model of complete and partial dysgenesis of the corpus callosum. Neuroimage (2020). doi:10.1016/j.neuroimage.2020.116868

Corpus callosum dysgenesis (CCD) describes a collection of brain malformations in which the main fiber tract connecting the two hemispheres is either absent (complete CCD, or ‘agenesis of the corpus callosum’) or reduced in size (partial CCD). Humans with these neurodevelopmental disorders have a wide range of cognitive outcomes, including seemingly preserved features of interhemispheric communication in some cases. However, the structural substrates that could underlie this variability in outcome remain to be fully elucidated. Here, for the first time, we characterize the global brain connectivity of a mouse model of complete and partial CCD. We demonstrate features of structural brain connectivity that model those predicted in humans with CCD, including Probst bundles in complete CCD and heterotopic sigmoidal connections in partial CCD. Crucially, we also histologically validate the recently predicted ectopic sigmoid bundle present in humans with partial CCD, validating the utility of this mouse model for fine anatomical studies of this disorder. Taken together, this work describes a mouse model of altered structural connectivity in variable severity CCD and forms a foundation for future studies investigating the function and mechanisms of development of plastic tracts in developmental disorders of brain connectivity.

Authors:  Timothy J. Edwards, Laura R. Fenlon, Ryan J. Dean, Jens Bunt, IRC5 Consortium, Elliott H. Sherr, Linda J. Richards

Callosal agenesis and congenital mirror movements: outcomes associated with DCC mutations. Developmental Medicine and Child Neurology (2020). doi:10.1111/dmcn.14486

Pathogenic variants in the gene encoding deleted in colorectal cancer (DCC) are the first genetic cause of isolated agenesis of the corpus callosum (ACC). Here we present the detailed neurological, brain magnetic resonance imaging (MRI), and neuropsychological characteristics of 12 individuals from three families with pathogenic variants in DCC (aged 8–50y), who showed ACC and mirror movements (n=5), mirror movements only (n=2), ACC only (n=3), or neither ACC nor mirror movements (n=2). There was heterogeneity in the neurological and neuroimaging features on brain MRI, and performance across neuropsychological domains ranged from extremely low (impaired) to within normal limits (average). Our findings show that ACC and/or mirror movements are associated with low functioning in select neuropsychological domains and a DCC pathogenic variant alone is not sufficient to explain the disability.

Authors:  Megan Spencer‐Smith, Jacquelyn L Knight, Emmanuelle Lacaze, IRC5 Consortium, Christel Depienne, Paul J Lockhart, Linda J Richards, Delphine Heron, Richard J Leventer, Gail A Robinson

DCC mutation update: Congenital mirror movements, isolated agenesis of the corpus callosum, and developmental split brain syndrome. Human Mutations 39, 23-39 (2018). doi:10.1002/humu.23361

The deleted in colorectal cancer (DCC) gene encodes the netrin-1 (NTN1) receptor DCC, a transmembrane protein required for the guidance of commissural axons. Germline DCC mutations disrupt the development of predominantly commissural tracts in the central nervous system (CNS) and cause a spectrum of neurological disorders. Monoallelic, missense, and predicted loss-of-function DCC mutations cause congenital mirror movements, isolated agenesis of the corpus callosum (ACC), or both. Biallelic, predicted loss-of-function DCC mutations cause developmental split brain syndrome (DSBS). Although the underlying molecular mechanisms leading to disease remain poorly understood, they are thought to stem from reduced or perturbed NTN1 signaling. Here, we review the 26 reported DCC mutations associated with abnormal CNS development in humans, including 14 missense and 12 predicted loss-of-function mutations, and discuss their associated clinical characteristics and diagnostic features. We provide an update on the observed genotype-phenotype relationships of congenital mirror movements, isolated ACC and DSBS, and correlate this to our current understanding of the biological function of DCC in the development of the CNS. All mutations and their associated phenotypes were deposited into a locus-specific LOVD (https://databases.lovd.nl/shared/genes/DCC).

Authors:  Ashley P. L. Marsh, Timothy J. Edwards, Charles Galea, Helen M. Cooper, Elizabeth C. Engle, Saumya S. Jamuar, Aurélie Méneret, Marie‐Laure Moutard, Caroline Nava, Agnès Rastetter, Gail Robinson, Guy Rouleau, Emmanuel Roze, Megan Spencer‐Smith, Oriane Trouillard, Thierry Billette de Villemeur, Christopher A. Walsh, Timothy W. Yu, IRC5 Consortium, Delphine Heron, Elliott H. Sherr, Linda J. Richards, Christel Depienne, Richard J. Leventer, Paul J. Lockhart

Mutations in DCC cause isolated agenesis of the corpus callosum with incomplete penetrance.  Nature Genetics  49, 511–514

Brain malformations involving the corpus callosum are common in children with developmental disabilities. We identified DCC mutations in four families and five sporadic individuals with isolated agenesis of the corpus callosum (ACC) without intellectual disability. DCC mutations result in variable dominant phenotypes with decreased penetrance, including mirror movements and ACC associated with a favorable developmental prognosis. Possible phenotypic modifiers include the type and location of mutation and the sex of the individual.

Authors:  Ashley P L Marsh, Delphine Heron, Timothy J Edwards, Angélique Quartier, Charles Galea, Caroline Nava, Agnès Rastetter, Marie-Laure Moutard, Vicki Anderson, Pierre Bitoun, Jens Bunt, Anne Faudet, Catherine Garel, Greta Gillies, Ilan Gobius, Justine Guegan, Solveig Heide, Boris Keren, Fabien Lesne, Vesna Lukic, Simone A Mandelstam, George McGillivray, Alissandra McIlroy, Aurélie Méneret, Cyril Mignot, Laura R Morcom, Sylvie Odent, Annalisa Paolino, Kate Pope, Florence Riant, Gail A Robinson, Megan Spencer-Smith, Myriam Srour, Sarah E M Stephenson, Rick Tankard, Oriane Trouillard, Quentin Welniarz, Amanda Wood, Alexis Brice, Guy Rouleau, Tania Attié-Bitach, Martin B Delatycki, Jean-Louis Mandel, David J Amor, Emmanuel Roze, Amélie Piton, Melanie Bahlo, Thierry Billette de Villemeur, Elliott H Sherr, Richard J Leventer, Linda J Richards, Paul J Lockhart & Christel Depienne