Thoracic aortic dissection (TAD) associates with a high mortality rate. Treatment options are limited and mainly consist of surgical repair at critical aortic diameters as current pharmacological interventions are unable to stop disease progression. Despite the existence of different mouse models for thoracic aortic aneurysm (TAA) and TAD, the underlying disease mechanisms remain elusive. In humans, loss-of-function of SMAD3 or SMAD6 increases the risk for TAA. We therefore targeted both ohnologs of smad3 and smad6 in zebrafish in order to further investigate their contribution to aortic homeostasis. We found an increased diameter of the ventral aorta in smad3a−/−;smad3b−/− double knockout (smad3a/b DKO) zebrafish larvae, while smad6a−/−;smad6b−/− (smad6a/b DKO) zebrafish larvae have a reduced aortic diameter. Smad3a/b DKO survive normally to adulthood, but smad6a/b DKO die before the age of 8 months due to dissections and ruptures in the ventral aorta. Smad6a/b DKO zebrafish also show hypoplasia of the aortic arches and the distal part of the ventral aorta. Surprisingly, the smad3a−/−;smad3b−/−;smad6a−/−;smad6b−/− quadruple knockout (qKO) zebrafish model has normal survival and a milder vascular phenotype compared to the smad6a/b DKO. RNA sequencing of zebrafish larvae indicates upregulation of pathways related to melanogenesis, ribosome, blood vessel development and carboxylic acid transport, and downregulation of negative regulation of endopeptidase activity and immune system. Transcriptomic data of damaged aorta compared with healthy control aorta identifies significant differences in oxidative phosphorylation, mitochondrial function, extracellular matrix and the citrate cycle. In conclusion, data from our novel zebrafish models of thoracic aortic dissection and rupture indicate that SMAD3 function has an important modifying effect on the severe aortic manifestations induced by loss of SMAD6. Legend “TAA” thoracic aortic aneurysm, “TAD” thoracic aortic dissection, “BAV” bicuspid aortic valve, “LOF” loss-of-function. Figure created with BioRender.

Genomic variants of uncertain significance (VUS) impede clinical decision-making. In this study, we use a knock-in strategy in zebrafish to evaluate the COL1A2 c.2123G>A VUS, identified in a 59-year-old female with recurrent fractures. Using prime editing, we obtained different zebrafish lines respectively harboring the VUS, a known pathogenic variant, or a known benign variant. Comprehensive skeletal phenotyping revealed no significant abnormalities in the zebrafish modeling the benign variant and the VUS, while zebrafish modeling the pathogenic variant showed scoliosis of the vertebral column, vertebral fusions, vertebral compressions, fractures, and increased mineralization of the notochord and intervertebral ligament. Our findings demonstrate for the first time, that COL1A2 variant modeling in zebrafish models informs functional validation and shows potential for elucidating associated pathogenic mechanisms. This approach can be extended to study VUS in other genes. ### Competing Interest Statement The authors have declared no competing interest.

Zebrafish serve as a valuable model organism for studying human genetic diseases. While generating knockout lines is relatively straightforward, introducing precise disease-specific genetic variants by knock-in (KI) remains challenging. KI lines, however, enable more accurate studies of molecular and physiological consequences of genetic diseases. Their generation is often hampered by low editing efficiency (EE) and potential off-target effects. Here, we optimized conventional CRISPR-Cas9-mediated homology-directed repair (HDR) strategies for precise KI of genetic variants in zebrafish and compared their efficacy with prime editing, a recently developed technique that is not yet commonly used. Using next-generation sequencing, we determined KI EE by HDR for six unique base-pair substitutions in three different zebrafish genes. We assessed the effect of (1) varying Cas9 amounts, (2) HDR templates with chemical modifications to improve integration efficiency, (3) different microinjection procedures and (4) introduction of additional synonymous guide-blocking variants in the HDR template. Increasing Cas9 amounts augmented KI EE, with optimal injected amounts of Cas9 between 200 pg and 800 pg. The use of Alt-R HDR templates further increased KI EE, while guide-blocking modifications did not. Injecting components directly into the cell was not superior to injections into the yolk. Prime editing, however, increased EE up to fourfold and expanded the F0 founder pool for four targets compared with conventional HDR editing, with fewer off-target effects. Therefore, prime editing is a very promising methodology for improving the creation of precise genomic edits in zebrafish, facilitating the modeling of human diseases.

Genomic variants of uncertain significance (VUS) impede clinical decision-making. In this study, we employ a knock-in strategy in zebrafish to evaluate the COL1A2 c.2123G>A VUS, identified in a 78-year-old female with atypical femoral fractures. Using prime editing, we generated different col1a2 zebrafish lines respectively harboring the VUS, a known pathogenic variant, and a known benign variant. Comprehensive skeletal phenotyping revealed no significant abnormalities in zebrafish harboring the VUS. In contrast, zebrafish with the pathogenic variant showed an increased eye diameter, scoliosis, vertebral fusions, vertebral compressions, fractures, and increased mineralization of the notochord and intervertebral ligament compared to wild type controls. Our findings represent the first demonstration that COL1A2 variant modeling via prime editing in zebrafish not only aids in functional validation, but also holds promise for uncovering the underlying pathogenic mechanisms. This approach can be applied to investigate VUS in other genes as well.

8q21.11 microdeletions involving ZFHX4 have previously been associated with a syndromic form of intellectual disability, hypotonia, and intellectual disability, distinctive facial characteristics, morphological abnormalities of the central nervous system, behavioral alterations, short stature, hypotonia, and occasionally cleft palate and anterior segment dysgenesis. The phenotypes associated with a mild common methylation profile for (micro)deletions. ZFHX4 shows increased expression during human brain development and transduction, cytosolic transport, and development. Additionally, using CUT&RUN, we observed that ZFHX4 binds the promoter of genes with crucial roles in embryonic, neuronal, and axonal development. Moreover, we investigated whether the disruption of zfhx4 have significantly shorter Meckel's cartilage and smaller ethmoid plates compared to control zebrafish. Behavioral assays showed a decreased movement frequency in the zfhx4 crispant zebrafish in comparison with controls. Furthermore, structural abnormalities were found in the zebrafish hindbrain. In conclusion, our findings delineate a ZFHX4-associated neurodevelopmental disorder and suggest a role for zfhx4 in facial skeleton patterning, palatal development, and behavior.

8q21.11 microdeletions involving ZFHX4 have previously been associated with a syndromic form of intellectual disability, hypotonia, unstable gait, and hearing loss. We report on 63 individuals-57 probands and 6 affected family members-with protein-truncating variants (n = 41), (micro)deletions (n = 21), or an inversion (n = 1) affecting ZFHX4. Probands display variable developmental delay and intellectual disability, distinctive facial characteristics, morphological abnormalities of the central nervous system, behavioral alterations, short stature, hypotonia, and occasionally cleft palate and anterior segment dysgenesis. The phenotypes associated with 8q21.11 microdeletions and ZFHX4 intragenic loss-of-function (LoF) variants largely overlap, although leukocyte-derived DNA shows a mild common methylation profile for (micro)deletions. ZFHX4 shows increased expression during human brain development and neuronal differentiation. Furthermore, ZFHX4-interacting factors identified via immunoprecipitation followed by mass spectrometry (IP-MS) suggest an important role for ZFHX4 in cellular pathways, especially during histone modifications, protein trafficking, signal transduction, cytosolic transport, and development. Additionally, using CUT&RUN, we observed that ZFHX4 binds the promoter of genes with crucial roles in embryonic, neuronal, and axonal development. Moreover, we investigated whether the disruption of zfhx4 causes craniofacial abnormalities in zebrafish. First-generation (F0) zfhx4 crispant zebrafish, a (mosaic) mutant for zfhx4 LoF variants, have significantly shorter Meckel's cartilage and smaller ethmoid plates compared to control zebrafish. Behavioral assays showed a decreased movement frequency in the zfhx4 crispant zebrafish in comparison with controls. Furthermore, structural abnormalities were found in the zebrafish hindbrain. In conclusion, our findings delineate a ZFHX4-associated neurodevelopmental disorder and suggest a role for zfhx4 in facial skeleton patterning, palatal development, and behavior.

8q21.11 microdeletions encompassing the gene encoding transcription factor ZFHX4, have previously been associated by us with a syndromic form of intellectual disability, hypotonia, decreased balance and hearing loss. Here, we report on 57 individuals, 52 probands and 5 affected family members, with protein truncating variants (n=36), (micro)deletions (n=20) or an inversion (n=1) affecting ZFHX4 with variable developmental delay and intellectual disability, distinctive facial characteristics, morphological abnormalities of the central nervous system, behavioral alterations, short stature, hypotonia, and occasionally cleft palate and anterior segment dysgenesis. The phenotypes associated with 8q21.11 microdeletions and ZFHX4 intragenic loss-of-function variants largely overlap, identifying ZFHX4 as the main driver for the microdeletion syndrome, although leukocyte-derived DNA shows a mild common methylation profile for (micro)deletions only. We identify ZFHX4 as a transcription factor that is increasingly expressed during human brain development and neuronal differentiation. Furthermore, ZFHX4 interacting factors identified via IP-MS in neural progenitor cells, suggest an important role for ZFHX4 in cellular and developmental pathways, especially during histone modifications, cytosolic transport and development. Additionally, using CUT&RUN, we observed that ZFHX4 binds with the promoter regions of genes with crucial roles in embryonic, neuron and axon development. Since loss-of-function variants in ZFHX4 are found with consistent dysmorphic facial features, we investigated whether the disruption of zfhx4 causes craniofacial abnormalities in zebrafish. First-generation (F0) zfhx4 crispant zebrafish, (mosaic) mutant for zfhx4 loss-of-function variants, have significantly shorter Meckel's cartilages and smaller ethmoid plates compared to control zebrafish. Furthermore, behavioral assays show a decreased movement frequency in the zfhx4 crispant zebrafish in comparison with control zebrafish larvae. Although further research is needed, our in vivo work suggests a role for zfhx4 in facial skeleton patterning, palatal development and behavior.

ABSTRACT Thoracic aortic aneurysm and dissection (TAAD) associates with a high mortality rate. Despite the existence of different mouse models for TAAD, the underlying disease mechanisms remain elusive. Treatment options are limited and mainly consist of surgical repair at critical aortic diameters as current pharmacological interventions are unable to stop disease progression. In humans, loss of function (LOF) of SMAD3 and SMAD6 impairs vascular homeostasis, increasing the risk for TAAD. We developed a zebrafish model for thoracic aortic dissection/rupture by targeting both ohnologs of smad3 and smad6 . At 10 days post fertilization, we found an increased diameter of the ventral aorta in smad3a −/− ; smad3b −/− double knockout zebrafish, while smad6a −/− ; smad6b −/− double knockout zebrafish have a reduced aortic diameter associated with early mortality. We discovered that a smad3a −/− ; smad3b −/− ; smad6a −/− ; smad6b −/− quadruple knockout (qKO) zebrafish model is viable and survives to adulthood, although exposure to stress leads to sudden death. Histological analysis of the adult ventral aorta shows medial elastolysis, aortic dissections and ruptures at sites exposed to high biomechanical stress. RNA-sequencing of 5 days post fertilization qKO zebrafish indicates a profile of reduced negative regulation of proteolysis and upregulation of melanogenesis, a previously unaddressed pathway in this pathology. We confirm that pharmacological modulation of tyrosinase, the enzyme responsible for the production of melanin, influences aortic morphology. Overall, the qKO mutant, thus far the only known zebrafish model of thoracic aortic dissection and rupture, reveals novel SMAD3/6-dependent pathways that impact thoracic aortic homeostasis, in this way opening avenues for the development of novel treatments in TAAD.

High-frequency cardiac ultrasound is the only well-established method to characterize in vivo cardiovascular function in adult zebrafish noninvasively. Pulsed-wave Doppler imaging allows measurements of blood flow velocities at well-defined anatomical positions, but the measurements and results obtained using this technique need to be analyzed carefully, taking into account the substantial baseline variability within one recording and the possibility for operator bias. To address these issues and to increase throughput by limiting hands-on analysis time, we have developed a fully automated processing pipeline. This framework enables the fast, unbiased analysis of all cardiac cycles in a zebrafish pulsed-wave Doppler recording of both atrioventricular valve flow as well as aortic valve flow without operator-dependent inputs. Applying this automated pipeline to a large number of recordings from wild-type zebrafish shows a strong agreement between the automated results and manual annotations performed by an experienced operator. The reference data obtained from this analysis showed that the early wave peak during ventricular inflow is lower for female compared with male zebrafish. We also found that the peaks of the ventricular inflow and outflow waves as well as the peaks of the regurgitation waves are all correlated positively with body surface area. In general, the presented reference data, as well as the automated Doppler measurement processing tools developed and validated in this study will facilitate future (high-throughput) cardiovascular phenotyping studies in adult zebrafish ultimately leading to a more comprehensive understanding of human (genetic) cardiovascular diseases.

Hereditary disorders of connective tissue (HDCT) compromise a heterogeneous group of diseases caused by pathogenic variants in genes encoding different components of the extracellular matrix and characterized by pleiotropic manifestations, mainly affecting the cutaneous, cardiovascular, and musculoskeletal systems. We report the case of a 9-year-old boy with a discernible connective tissue disorder characterized by cutis laxa (CL) and multiple herniations and caused by biallelic loss-of-function variants in EFEMP1. Hence, we identified EFEMP1 as a novel disease-causing gene in the CL spectrum, differentiating it from other HDCT.

The inability to maintain a strictly regulated endo(lyso)somal acidic pH through the proton-pumping action of the vacuolar-ATPases (v-ATPases) has been associated with various human diseases including heritable connective tissue disorders. Autosomal recessive (AR) cutis laxa (CL) type 2C syndrome is associated with genetic defects in the ATP6V1E1 gene and is characterized by skin wrinkles or loose redundant skin folds with pleiotropic systemic manifestations. The underlying pathological mechanisms leading to the clinical presentations remain largely unknown. Here, we show that loss of atp6v1e1b in zebrafish leads to early mortality, associated with craniofacial dysmorphisms, vascular anomalies, cardiac dysfunction, N-glycosylation defects, hypotonia, and epidermal structural defects. These features are reminiscent of the phenotypic manifestations in ARCL type 2C patients. Our data demonstrates that loss of atp6v1e1b alters endo(lyso)somal protein levels, and interferes with non-canonical v-ATPase pathways in vivo. In order to gain further insights into the processes affected by loss of atp6v1e1b, we performed an untargeted analysis of the transcriptome, metabolome, and lipidome in early atp6v1e1b-deficient larvae. We report multiple affected pathways including but not limited to oxidative phosphorylation, sphingolipid, fatty acid, and energy metabolism together with profound defects on mitochondrial respiration. Taken together, our results identify complex pathobiological effects due to loss of atp6v1e1b in vivo.

The inability to maintain a strictly regulated endo(lyso)somal acidic pH through the proton-pumping action of the vacuolar-ATPases has been associated with various human diseases including heritable connective tissue disorders, neurodegenerative diseases and cancer. Multiple studies have investigated the pleiotropic effects of reduced acidification in vitro , but the mechanisms elicited by impaired endo(lyso)somal acidification in vivo remain poorly understood. Here, we show that loss of atp6v1e1b in zebrafish leads to early mortality, associated with craniofacial dysmorphisms, vascular anomalies, cardiac dysfunction, hypotonia and epidermal structural defects, reminiscent of the phenotypic manifestations in cutis laxa patients carrying a defect in the ATP6V1E1 gene. Mechanistically, we found that in vivo genetic depletion of atp6v1e1b leads to N-glycosylation defects and reduced maturation of endosomal and lysosomal vesicles, but retains the hypoxia-mediated response. In order to gain further insights into the processes affected by aberrant organelle acidification, we performed an untargeted analysis of the transcriptome and metabolome in early atp6v1e1b -deficient larvae. We report multiple affected pathways including but not limited to oxidative phosphorylation, sphingolipid and fatty acid metabolism with profound defects in mitochondrial respiration. Taken together, our results identify new complex biological effects of reduced organelle acidification in vivo , which likely contribute to the multisystemic manifestations observed in disorders caused by v-ATPase deficiency.