Berdowski, W. M. | Search bioRxiv

bioRxiv2024

The non-canonical thioreductase TMX2 is essential for neuronal survival during embryonic brain development

Biallelic variants in thioredoxin-related transmembrane 2 protein (TMX2) can cause a brain malformation of cortical development (MCD), characterized by primary microcephaly, polymicrogyria and pachygyria by an unknown mechanism. To better understand and visualize how TMX2 loss disrupts brain development in vivo we investigated the function of TMX2, using the zebrafish embryo as a model system. We generated zebrafish deficient for TMX2 ortholog tmx2b, which during the first 2 days post fertilization (dpf) showed normal behavioral activity and brain developmental hallmarks. From 3 dpf onwards however, tmx2b mutants failed to exhibit locomotor activity, which was accompanied by cell death primarily in the brain, but not in other organs or in the spinal cord. Strikingly, cell death in tmx2b mutants occurs specifically in newborn neurons within a [~]1.5-hour timeframe, whereas neuronal progenitor and other glial cells are preserved, and could be suppressed by inhibiting neuronal activity. In vivo GCaMP6s calcium imaging showed a persistent [~]2-fold increase in calcium in neurons after the onset of cell death. This suggests that calcium homeostasis underlies the tmx2b mutant brain phenotype. Altogether, our results indicate that TMX2 is an evolutionary conserved, protective regulator essential specifically for newborn neurons to survive after their differentiation in the vertebrate embryonic brain.

developmental biology↗

bioRxiv2019

Loss of UGP2 in brain leads to a severe epileptic encephalopathy, emphasizing that bi-allelic isoform specific start-loss mutations of essential genes can cause genetic diseases

Developmental and/or epileptic encephalopathies (DEEs) are a group of devastating genetic disorders, resulting in early onset, therapy resistant seizures and developmental delay. Here we report on 12 individuals from 10 families presenting with a severe form of intractable epilepsy, severe developmental delay, progressive microcephaly and visual disturbance. Whole exome sequencing identified a recurrent, homozygous variant (chr2:64083454A>G) in the essential UDP-glucose pyrophosphorylase (UGP2) gene in all probands. This rare variant results in a tolerable Met12Val missense change of the longer UGP2 protein isoform but causes a disruption of the start codon of the shorter isoform. We show that the absence of the shorter isoform leads to a reduction of functional UGP2 enzyme in brain cell types, leading to altered glycogen metabolism, upregulated unfolded protein response and premature neuronal differentiation, as modelled during pluripotent stem cell differentiation in vitro. In contrast, the complete lack of all UGP2 isoforms leads to differentiation defects in multiple lineages in human cells. Reduced expression of Ugp2a/Ugp2b in vivo in zebrafish mimics visual disturbance and mutant animals show a behavioral phenotype. Our study identifies a recurrent start codon mutation in UGP2 as a cause of a novel autosomal recessive DEE. Importantly, it also shows that isoform specific start-loss mutations causing expression loss of a tissue relevant isoform of an essential protein can cause a genetic disease, even when an organism-wide protein absence is incompatible with life. We provide additional examples where a similar disease mechanism applies.

genetics↗