Nataliya Di Donato

Most primates have a large, highly folded (gyrencephalic) neocortex, a feature present in the primate common ancestor. In contrast, several New World monkey species display a comparatively small and largely smooth, unfolded (lissencephalic) neocortex. One prominent example is the common marmoset, an increasingly popular model in neuroscience. This phenotype likely reflects an evolutionary reduction from the ancestral primate condition, implying modifications in neurodevelopmental programs. One essential basis for neocortical development is the activity and behavior of neural progenitor cells (NPCs). Here, we identify coordinated adaptations in NPC biology that bias neurodevelopmental trajectories toward neocortical downscaling. By combining marmoset and human cerebral organoids with analyses of fetal marmoset neocortical tissue and previously published histological data, we uncover multiple adaptations in apical and basal progenitors that converge on reduced progenitor capacity, alter early progenitor dynamics, and likely constrain neuronal output, thereby limiting the size and folding of the marmoset neocortex.

Background: The transport of transfer RNAs (tRNAs) from the nucleus to the cytoplasm is a crucial step in the regulation of gene expression and protein synthesis. This process is mediated by specialized export molecules, among which XPOT (Exportin-t, XPO3) plays a central role by recognizing and transporting mature tRNAs through the nuclear pore complex. XPOT is not essential in RNA trafficking in the simple organisms, however the potential impact of XPOT deficiency in human health remains unresolved. Methods: We identified eight patients from five unrelated families with rare biallelic germline variants in XPOT resulting in putative loss-of-function. Functional analyses were carried out in patient-derived fibroblasts, lymphoblastoid cells and zebrafish models. Ex vivo immunohistochemical stainings for Xpot were performed in the mouse cochlea. xpot knockout zebrafish models were generated to assess the morphology and hearing ability. Results: All patients presented with a uniform clinical phenotype that included increased susceptibility to infection, bronchiectasis, severe sensorineural hearing loss, developmental delay, and growth retardation. We demonstrated a complete absence of XPOT protein expression in three patient-derived cell lines. XPOT deficiency leads to disruptions in protein synthesis of the cytokine TNFα pathway upon cellular stimulation. Additional XPO1 inhibition in XPOT deficient cells had little effect on cellular functions, suggesting alternative tRNA nuclear transporter pathways. Increased XPOT immunoreactivity was observed in type I spiral ganglion neurons and hair cells of the mouse cochlea, with enrichment in stereocilia. xpot knockout zebrafish model showed dysmorphic features, and reduced hearing, recapitulating key patient phenotypes. Conclusions: Our findings establish a direct connection between impaired XPOT-dependent tRNA export and human pathology. It illustrates that perturbations in nuclear export pathways lead to disease. It also raises the possibility that other nuclear transport receptors may play similarly underappreciated roles in human health and disease. The identification of XPOT as a disease-associated gene opens up new research directions and potential targets for therapeutic intervention.

Actins are cytoskeletal proteins that are essential for multiple cellular processes. Mutations in the ACTB and ACTG1 genes, encoding the ubiquitous beta- and gamma-cytoskeletal actin isoforms, respectively, cause a broad spectrum of neurodevelopmental disorders, with microcephaly as the most frequent one. To investigate the pathogenesis underlying this cortical malformation, we studied patient-derived cerebral organoids from induced pluripotent stem cells of individuals with the Baraitser–Winter-CerebroFrontoFacial syndrome (BWCFF-S) carrying an ACTB/ACTG1 missense mutation. These organoids were reduced in size, showing a thinner ventricular zone (VZ) due to reduced VZ progenitor abundance. Strikingly, VZ progenitors in BWCFF-S cerebral organoids displayed a shift in the orientation of their cleavage plane from a predominantly vertical to a majoritarian horizontal orientation. The latter cleavage plane orientation is incompatible with increasing VZ progenitor abundance and instead promotes basal progenitor generation. Various cytoskeletal and morphological irregularities of BWCFF-S VZ progenitors, notably in the apical region, seemingly contribute to this change in cleavage plane orientation. Our results provide insight into the cell biological basis of the microcephaly associated with BWCFF-S caused by actin mutations.

DNA double strand break repair (DSBR) represents a fundamental process required to maintain genome stability and prevent the onset of disease. Whilst cell cycle phase and the chromatin context largely dictate which repair pathway is utilised to restore damaged DNA, it has been recently shown that nuclear actin filaments play a major role in clustering DNA breaks to facilitate DSBR by homologous recombination (HR). However, the mechanism with which nuclear actin and the different actin nucleating factors regulate HR is unclear. Interestingly, patients with biallelic mutations in the actin nucleating factor DIAPH1 exhibit a striking overlap of clinical features with the HR deficiency disorders, Nijmegen Breakage Syndrome (NBS) and Warsaw Breakage Syndrome (WABS). This suggests that DIAPH1 may play a role in regulating HR and that some of the clinical deficits associated with DIAPH1 mutations may be caused by an underlying DSBR defect. In keeping with this clinical similarity, we demonstrate that cells from DIAL (DIAPH1 Loss-of-function) Syndrome patients display an HR repair defect comparable to loss of NBS1. Moreover, we show that this DSBR defect is also observed in a subset of patients with Baraitser-Winter Cerebrofrontofacial (BWCFF) syndrome associated with mutations in ACTG1 (γ-actin) but not ACTB (β-actin). Lastly, we demonstrate that DIAPH1 and γ-actin promote HR-dependent repair by facilitating the relocalisation of the MRE11/RAD50/NBS1 complex to sites of DNA breaks to initiate end-resection. Taken together, these data provide a mechanistic explanation for the overlapping clinical symptoms exhibited by patients with DIAL syndrome, BWCFF syndrome and NBS.

Background: Cytoskeletal non-muscle actin isoforms are the most abundant intracellular proteins and extensively interact with other molecules. Biological consequences and genotype-phenotype correlations of the variants in genes encoding these isoforms, ACTB and ACTG1, are not delineated. Methods: Clinical data analysis from 290 individuals with pathogenic ACTB/ACTG1 variants; characterization of patient cells, mutant proteins, patient-derived iPSC-based models and mutant mice. Results: We show that ACTB and ACTG1 variants have distinct clinical profiles. ACTB nonsense, frameshift and missense variants that lead to rapid protein degradation result in milder phenotypes. Heterozygous Actb knockout causes altered neuronal cell morphology and abnormal expression of actin-related genes in newborn mouse brains. Truncating ACTG1 variants are likely to be non-pathogenic, but chromosomal deletions encompassing ACTG1 and flanking genes may result in susceptibility to neurodevelopmental phenotypes. Subsets of disease-causing ACTB missense variants (MVs) result in more severe Type 1 Baraitser-Winter Cerebrofrontofacial (BWCFF1) or Deafness Dystonia syndromes. Pathogenic ACTG1 MVs cause BWCFF2 or isolated hearing loss. These amino acid substitutions are associated with dramatically dysregulated actin polymerization and depolymerization dynamics and, in induced pluripotent stem cells (iPSC) derived models, lead to neuronal migration defects. A significant subset of MVs result in disorders that cannot yet be classified into recognizable groups. Conclusions: ACTB or ACTG1 variants and result in minimum eight mechanistically diverse non-muscle actinopathies. These results will improve their diagnosis and management, and pave the way for new treatment strategies. This study reflects the scale of collaborative clinical studies and multi-modal mechanistic studies required to dissect rare allelic disorders.

Individuals with ultrarare disorders pose a structural challenge for healthcare systems since expert clinical knowledge is required to establish diagnoses. In TRANSLATE NAMSE, a 3-year prospective study, we evaluated a novel diagnostic concept based on multidisciplinary expertise in Germany. Here we present the systematic investigation of the phenotypic and molecular genetic data of 1,577 patients who had undergone exome sequencing and were partially analyzed with next-generation phenotyping approaches. Molecular genetic diagnoses were established in 32% of the patients totaling 370 distinct molecular genetic causes, most with prevalence below 1:50,000. During the diagnostic process, 34 novel and 23 candidate genotype–phenotype associations were identified, mainly in individuals with neurodevelopmental disorders. Sequencing data of the subcohort that consented to computer-assisted analysis of their facial images with GestaltMatcher could be prioritized more efficiently compared with approaches based solely on clinical features and molecular scores. Our study demonstrates the synergy of using next-generation sequencing and phenotyping for diagnosing ultrarare diseases in routine healthcare and discovering novel etiologies by multidisciplinary teams.