Reduced sister chromatid cohesion acts as a tumor penetrance modifier.

Sister chromatid cohesion (SCC) is an important process in chromosome segregation. ESCO2 is essential for establishment of SCC and is often deleted/altered in human cancers. We demonstrate that esco2 haploinsufficiency results in reduced SCC and accelerates the timing of tumor onset in both zebrafish and mouse p53 heterozygous null models, but not in p53 homozygous mutant or wild-type animals. These data indicate that esco2 haploinsufficiency accelerates tumor onset in a loss of heterozygosity (LOH) sensitive background. Analysis of The Cancer Genome Atlas (TCGA) confirmed ESCO2 deficient tumors have elevated number of LOH events throughout the genome. Further, we demonstrated heterozygous loss of sgo1, important in maintaining SCC, also results in reduced SCC and accelerated tumor formation in a p53 heterozygous background. Surprisingly, while we did observe elevated levels of chromosome missegregation and micronuclei formation in esco2 heterozygous mutant animals, this chromosomal instability did not contribute to the accelerated tumor onset in a p53 heterozygous background. Interestingly, SCC also plays a role in homologous recombination, and we did observe elevated levels of mitotic recombination derived p53 LOH in tumors from esco2 haploinsufficient animals; as well as elevated levels of mitotic recombination throughout the genome of human ESCO2 deficient tumors. Together these data suggest that reduced SCC contributes to accelerated tumor penetrance through elevated mitotic recombination.

Lack of nuclear localization of the Creb3l1 transcription factor causes defects in caudal fin bifurcation in zebrafish Danio rerio.

The formation of normal bone and bone healing require the cAMP-responsive element binding protein 3-like-1 (Creb3l1) transmembrane transcription factor, as deletion of the murine CREB3L1 results in osteopenic animals with limited capacity to repair bone after fracture. Creb3l1 undergoes regulated intra-membrane proteolysis (RIP) to release the N-terminal transcription activating (TA) fragment that enters the nucleus and regulates the expression of target genes. To expand our understanding of Creb3l1 role in skeletal development and skeletal patterning, we aimed to generate animals expressing only the TA fragment of Creb3l1 lacking the transmembrane domain and thereby not regulated through RIP. However, the CRISPR/Cas9-mediated genome editing in zebrafish D. rerio caused a frame-shift mutation that added 56 random amino acids at the C-terminus of the TA fragment (TA+), making it unable to enter the nucleus. Thus, TA+ doesn't regulate transcription, and the creb3l1TA+/TA+ fish animals are creb3l1 transcriptional nulls. We document that the creb3l1TA+/TA+ fish exhibit defects in the patterning of caudal fin lepidotrichia, with significantly distalized points of proximal bifurcation and decreased secondary bifurcations. Moreover, using the caudal fin amputation model, we show that creb3l1TA+/TA+ fish have decreased capacity for regeneration, and that their regenerates replicate the distalization and bifurcation defects observed in intact fins of creb3l1TA+/TA+ animals. These defects correlate with altered expression of the shha and ptch2 components of the Sonic Hedgehog signaling pathway in creb3l1TA+/TA+ regenerates. Together, our results uncover a previously unknown intersection between Creb3l1 and the Sonic Hedgehog pathway, and document a novel role of Creb3l1 in tissue patterning.

Truncating PKHD1 and PKD2 mutations alter energy metabolism.

Deficiency in polycystin 1 triggers specific changes in energy metabolism. To determine whether defects in other human cystoproteins have similar effects, we studied extracellular acidification and glucose metabolism in human embryonic kidney (HEK-293) cell lines with polycystic kidney and hepatic disease 1 ( PKHD1) and polycystic kidney disease (PKD) 2 ( PKD2) truncating defects along multiple sites of truncating mutations found in patients with autosomal recessive and dominant PKDs. While neither the PKHD1 or PKD2 gene mutations nor their position enhanced cell proliferation rate in our cell line models, truncating mutations in these genes progressively increased overall extracellular acidification over time ( P < 0.001 for PKHD1 and PKD2 mutations). PKHD1 mutations increased nonglycolytic acidification rate (1.19 vs. 1.03, P = 0.002), consistent with an increase in tricarboxylic acid cycle activity or breakdown of intracellular glycogen. In addition, they increased basal and ATP-linked oxygen consumption rates [7.59 vs. 5.42 ( P = 0.015) and 4.55 vs. 2.98 ( P = 0.004)]. The PKHD1 and PKD2 mutations also altered mitochondrial morphology, resembling the effects of polycystin 1 deficiency. Together, these data suggest that defects in major PKD genes trigger changes in mitochondrial energy metabolism. After validation in in vivo models, these initial observations would indicate potential benefits of targeting energy metabolism in the treatment of PKDs.

Mutation of Growth Arrest Specific 8 Reveals a Role in Motile Cilia Function and Human Disease.

Ciliopathies are genetic disorders arising from dysfunction of microtubule-based cellular appendages called cilia. Different cilia types possess distinct stereotypic microtubule doublet arrangements with non-motile or 'primary' cilia having a 9+0 and motile cilia have a 9+2 array of microtubule doublets. Primary cilia are critical sensory and signaling centers needed for normal mammalian development. Defects in their structure/function result in a spectrum of clinical and developmental pathologies including abnormal neural tube and limb patterning. Altered patterning phenotypes in the limb and neural tube are due to perturbations in the hedgehog (Hh) signaling pathway. Motile cilia are important in fluid movement and defects in motility result in chronic respiratory infections, altered left-right asymmetry, and infertility. These features are the hallmarks of Primary Ciliary Dyskinesia (PCD, OMIM 244400). While mutations in several genes are associated with PCD in patients and animal models, the genetic lesion in many cases is unknown. We assessed the in vivo functions of Growth Arrest Specific 8 (GAS8). GAS8 shares strong sequence similarity with the Chlamydomonas Nexin-Dynein Regulatory Complex (NDRC) protein 4 (DRC4) where it is needed for proper flagella motility. In mammalian cells, the GAS8 protein localizes not only to the microtubule axoneme of motile cilia, but also to the base of non-motile cilia. Gas8 was recently implicated in the Hh signaling pathway as a regulator of Smoothened trafficking into the cilium. Here, we generate the first mouse with a Gas8 mutation and show that it causes severe PCD phenotypes; however, there were no overt Hh pathway phenotypes. In addition, we identified two human patients with missense variants in Gas8. Rescue experiments in Chlamydomonas revealed a subtle defect in swim velocity compared to controls. Further experiments using CRISPR/Cas9 homology driven repair (HDR) to generate one of these human missense variants in mice demonstrated that this allele is likely pathogenic.

Approaches to Inactivate Genes in Zebrafish.

Animal models of tumor initiation and tumor progression are essential components toward understanding cancer and designing/validating future therapies. Zebrafish is a powerful model for studying tumorigenesis and has been successfully exploited in drug discovery. According to the zebrafish reference genome, 82 % of disease-associated genes in the Online Mendelian Inheritance in Man (OMIM) database have clear zebrafish orthologues. Using a variety of large-scale random mutagenesis methods developed to date, zebrafish can provide a unique opportunity to identify gene mutations that may be associated with cancer predisposition. On the other hand, newer technologies enabling targeted mutagenesis can facilitate reverse cancer genetic studies and open the door for complex genetic analysis of tumorigenesis. In this chapter, we will describe the various technologies for conducting genome editing in zebrafish with special emphasis on the approaches to inactivate genes.

Activity Suppression Behavior Phenotype in SULT4A1 Frameshift Mutant Zebrafish.

Since its identification in 2000, sulfotransferase (SULT) 4A1 has presented an enigma to the field of cytosolic SULT biology. SULT4A1 is exclusively expressed in neural tissue, is highly conserved, and has been identified in every vertebrate studied to date. Despite this singular level of conservation, no substrate or function for SULT4A1 has been identified. Previous studies demonstrated that SULT4A1 does not bind the obligate sulfate donor, 3'-phosphoadenosine-5'-phosphosulfate, yet SULT4A1 is classified as a SULT superfamily member based on sequence and structural similarities to the other SULTs. In this study, transcription activator-like effector nucleases were used to generate heritable mutations in the SULT4A1 gene of zebrafish. The mutation (SULT4A1(Δ8)) consists of an 8-nucleotide deletion within the second exon of the gene, resulting in a frameshift mutation and premature stop codon after 132 AA. During early adulthood, casual observations were made that mutant zebrafish were exhibiting excessively sedentary behavior during the day. These observations were inconsistent with published reports on activity in zebrafish that are largely diurnal organisms and are highly active during the day. Thus, a decrease in activity during the day represents an abnormal behavior and warranted further systematic analysis. EthoVision video tracking software was used to monitor activity levels in wild-type (WT) and SULT4A1(Δ8/Δ8) fish over 48 hours of a normal light/dark cycle. SULT4A1(Δ8/Δ8) fish were shown to exhibit increased inactivity bout length and frequency as well as a general decrease in daytime activity levels when compared with their WT counterparts.

Inhibition of SULT4A1 expression induces up-regulation of phototransduction gene expression in 72-hour postfertilization zebrafish larvae.

Sulfotransferase (SULT) 4A1 is an orphan enzyme that shares distinct structure and sequence similarities with other cytosolic SULTs. SULT4A1 is primarily expressed in neuronal tissue and is also the most conserved SULT, having been identified in every vertebrate investigated to date. Certain haplotypes of the SULT4A1 gene are correlated with higher baseline psychopathology in schizophrenic patients, but no substrate or function for SULT4A1 has yet been identified despite its high level of sequence conservation. In this study, deep RNA sequencing was used to search for alterations in gene expression in 72-hour postfertilization zebrafish larvae following transient SULT4A1 knockdown (KD) utilizing splice blocking morpholino oligonucleotides. This study demonstrates that transient inhibition of SULT4A1 expression in developing zebrafish larvae results in the up-regulation of several genes involved in phototransduction. SULT4A1 KD was verified by immunoblot analysis and quantitative real-time polymerase chain reaction (qPCR). Gene regulation changes identified by deep RNA sequencing were validated by qPCR. This study is the first identification of a cellular process whose regulation appears to be associated with SULT4A1 expression.

Standardization of zebrafish drug testing parameters for muscle diseases.

Skeletal muscular diseases predominantly affect skeletal and cardiac muscle, resulting in muscle weakness, impaired respiratory function and decreased lifespan. These harmful outcomes lead to poor health-related quality of life and carry a high healthcare economic burden. The absence of promising treatments and new therapies for muscular disorders requires new methods for candidate drug identification and advancement in animal models. Consequently, the rapid screening of drug compounds in an animal model that mimics features of human muscle disease is warranted. Zebrafish are a versatile model in preclinical studies that support developmental biology and drug discovery programs for novel chemical entities and repurposing of established drugs. Due to several advantages, there is an increasing number of applications of the zebrafish model for high-throughput drug screening for human disorders and developmental studies. Consequently, standardization of key drug screening parameters, such as animal husbandry protocols, drug compound administration and outcome measures, is paramount for the continued advancement of the model and field. Here, we seek to summarize and explore critical drug treatment and drug screening parameters in the zebrafish-based modeling of human muscle diseases. Through improved standardization and harmonization of drug screening parameters and protocols, we aim to promote more effective drug discovery programs.

Chromosome stability, in the absence of apoptosis, is critical for suppression of tumorigenesis in Trp53 mutant mice.

The p53 protein integrates multiple upstream signals and functions as a tumor suppressor by activating distinct downstream genes. At the cellular level, p53 induces apoptosis, cell cycle arrest and senescence. A rare mutant form of p53 with the amino acid substitution R175P, found in human tumors, is completely defective in initiating apoptosis but still induces cell cycle arrest. To decipher the functional importance of these pathways in spontaneous tumorigenesis, we used homologous recombination to generate mice with mutant p53-R172P (the mouse equivalent of R175P in humans). Mice inheriting two copies of this mutation (Trp53(515C/515C)) escape the early onset of thymic lymphomas that characterize Trp53-null mice. At 7 months of age, 90% of Trp53-null mice had died, but 85% of Trp53(515C/515C) mice were alive and tumor-free, indicating that p53-dependent apoptosis was not required for suppression of early onset of spontaneous tumors. The lymphomas and sarcomas that eventually developed in Trp53(515C/515C) mice retained a diploid chromosome number, in sharp contrast to aneuploidy observed in tumors and cells from Trp53-null mice. The ability of mutant p53-R172P to induce a partial cell cycle arrest and retain chromosome stability are crucial for suppression of early onset tumorigenesis.

A kidney resident macrophage subset is a candidate biomarker for renal cystic disease in preclinical models.

Although renal macrophages have been shown to contribute to cyst development in polycystic kidney disease (PKD) animal models, it remains unclear whether there is a specific macrophage subpopulation involved. Here, we analyzed changes in macrophage populations during renal maturation in association with cystogenesis rates in conditional Pkd2 mutant mice. We observed that CD206+ resident macrophages were minimal in a normal adult kidney but accumulated in cystic areas in adult-induced Pkd2 mutants. Using Cx3cr1 null mice, we reduced macrophage number, including CD206+ macrophages, and showed that this significantly reduced cyst severity in adult-induced Pkd2 mutant kidneys. We also found that the number of CD206+ resident macrophage-like cells increased in kidneys and in the urine from autosomal-dominant PKD (ADPKD) patients relative to the rate of renal functional decline. These data indicate a direct correlation between CD206+ resident macrophages and cyst formation, and reveal that the CD206+ resident macrophages in urine could serve as a biomarker for renal cystic disease activity in preclinical models and ADPKD patients. This article has an associated First Person interview with the first author of the paper.

Evolutionarily conserved genetic interactions between nphp-4 and bbs-5 mutations exacerbate ciliopathy phenotypes.

Primary cilia are sensory and signaling hubs with a protein composition that is distinct from the rest of the cell due to the barrier function of the transition zone (TZ) at the base of the cilium. Protein transport across the TZ is mediated in part by the BBSome, and mutations disrupting TZ and BBSome proteins cause human ciliopathy syndromes. Ciliopathies have phenotypic variability even among patients with identical genetic variants, suggesting a role for modifier loci. To identify potential ciliopathy modifiers, we performed a mutagenesis screen on nphp-4 mutant Caenorhabditis elegans and uncovered a novel allele of bbs-5. Nphp-4;bbs-5 double mutant worms have phenotypes not observed in either individual mutant strain. To test whether this genetic interaction is conserved, we also analyzed zebrafish and mouse mutants. While Nphp4 mutant zebrafish appeared overtly normal, Bbs5 mutants exhibited scoliosis. When combined, Nphp4;Bbs5 double mutant zebrafish did not exhibit synergistic effects, but the lack of a phenotype in Nphp4 mutants makes interpreting these data difficult. In contrast, Nphp4;Bbs5 double mutant mice were not viable and there were fewer mice than expected carrying three mutant alleles. In addition, postnatal loss of Bbs5 in mice using a conditional allele compromised survival when combined with an Nphp4 allele. As cilia are still formed in the double mutant mice, the exacerbated phenotype is likely a consequence of disrupted ciliary signaling. Collectively, these data support an evolutionarily conserved genetic interaction between Bbs5 and Nphp4 alleles that may contribute to the variability in ciliopathy phenotypes.

Variable phenotypes and penetrance between and within different zebrafish ciliary transition zone mutants.

Meckel syndrome, nephronophthisis, Joubert syndrome and Bardet-Biedl syndrome are caused by mutations in proteins that localize to the ciliary transition zone (TZ). The phenotypically distinct syndromes suggest that these TZ proteins have differing functions. However, mutations in a single TZ gene can result in multiple syndromes, suggesting that the phenotype is influenced by modifier genes. We performed a comprehensive analysis of ten zebrafish TZ mutants, including mks1, tmem216, tmem67, rpgrip1l, cc2d2a, b9d2, cep290, tctn1, nphp1 and nphp4, as well as mutants in ift88 and ift172. Our data indicate that variations in phenotypes exist between different TZ mutants, supporting different tissue-specific functions of these TZ genes. Further, we observed phenotypic variations within progeny of a single TZ mutant, reminiscent of multiple disease syndromes being associated with mutations in one gene. In some mutants, the dynamics of the phenotype became complex with transitory phenotypes that are corrected over time. We also demonstrated that multiple-guide-derived CRISPR/Cas9 F0 'crispant' embryos recapitulate zygotic null phenotypes, and rapidly identified ciliary phenotypes in 11 cilia-associated gene candidates (ankfn1, ccdc65, cfap57, fhad1, nme7, pacrg, saxo2, c1orf194, ttc26, zmynd12 and cfap52).

Puma, noxa, p53, and p63 differentially mediate stress pathway induced apoptosis.

Cellular stress can lead to several human disease pathologies due to aberrant cell death. The p53 family (tp53, tp63, and tp73) and downstream transcriptional apoptotic target genes (PUMA/BBC3 and NOXA/PMAIP1) have been implicated as mediators of stress signals. To evaluate the importance of key stress response components in vivo, we have generated zebrafish null alleles in puma, noxa, p53, p63, and p73. Utilizing these genetic mutants, we have deciphered that the apoptotic response to genotoxic stress requires p53 and puma, but not p63, p73, or noxa. We also identified a delayed secondary wave of genotoxic stress-induced apoptosis that is p53/puma independent. Contrary to genotoxic stress, ER stress-induced apoptosis requires p63 and puma, but not p53, p73, or noxa. Lastly, the oxidative stress-induced apoptotic response requires p63, and both noxa and puma. Our data also indicate that while the neural tube is poised for apoptosis due to genotoxic stress, the epidermis is poised for apoptosis due to ER and oxidative stress. These data indicate there are convergent as well as unique molecular pathways involved in the different stress responses. The commonality of puma in these stress pathways, and the lack of gross or tumorigenic phenotypes with puma loss suggest that a inhibitor of Puma may have therapeutic application. In addition, we have also generated a knockout of the negative regulator of p53, mdm2 to further evaluate the p53-induced apoptosis. Our data indicate that the p53 null allele completely rescues the mdm2 null lethality, while the puma null completely rescues the mdm2 null apoptosis but only partially rescues the phenotype. Indicating Puma is the key mediator of p53-dependent apoptosis. Interestingly the p53 homozygous null zebrafish develop tumors faster than the previously described p53 homozygous missense mutant zebrafish, suggesting the missense allele may be hypomorphic allele.

Genotype-phenotype correlations and novel molecular insights into the DHX30-associated neurodevelopmental disorders.

BACKGROUND: We aimed to define the clinical and variant spectrum and to provide novel molecular insights into the DHX30-associated neurodevelopmental disorder. METHODS: Clinical and genetic data from affected individuals were collected through Facebook-based family support group, GeneMatcher, and our network of collaborators. We investigated the impact of novel missense variants with respect to ATPase and helicase activity, stress granule (SG) formation, global translation, and their effect on embryonic development in zebrafish. SG formation was additionally analyzed in CRISPR/Cas9-mediated DHX30-deficient HEK293T and zebrafish models, along with in vivo behavioral assays. RESULTS: We identified 25 previously unreported individuals, ten of whom carry novel variants, two of which are recurrent, and provide evidence of gonadal mosaicism in one family. All 19 individuals harboring heterozygous missense variants within helicase core motifs (HCMs) have global developmental delay, intellectual disability, severe speech impairment, and gait abnormalities. These variants impair the ATPase and helicase activity of DHX30, trigger SG formation, interfere with global translation, and cause developmental defects in a zebrafish model. Notably, 4 individuals harboring heterozygous variants resulting either in haploinsufficiency or truncated proteins presented with a milder clinical course, similar to an individual harboring a de novo mosaic HCM missense variant. Functionally, we established DHX30 as an ATP-dependent RNA helicase and as an evolutionary conserved factor in SG assembly. Based on the clinical course, the variant location, and type we establish two distinct clinical subtypes. DHX30 loss-of-function variants cause a milder phenotype whereas a severe phenotype is caused by HCM missense variants that, in addition to the loss of ATPase and helicase activity, lead to a detrimental gain-of-function with respect to SG formation. Behavioral characterization of dhx30-deficient zebrafish revealed altered sleep-wake activity and social interaction, partially resembling the human phenotype. CONCLUSIONS: Our study highlights the usefulness of social media to define novel Mendelian disorders and exemplifies how functional analyses accompanied by clinical and genetic findings can define clinically distinct subtypes for ultra-rare disorders. Such approaches require close interdisciplinary collaboration between families/legal representatives of the affected individuals, clinicians, molecular genetics diagnostic laboratories, and research laboratories.

A rapid and efficient method of genotyping zebrafish mutants.

In order to facilitate high throughput genotyping of zebrafish, we have developed a novel technique that uses High Resolution Melting Analysis (HRMA) to distinguish wild-type, heterozygous mutants and homogyzous mutants. This one hour technique removes the need for restriction enzymes and agarose gels. The generated melting curve profiles are sensitive enough to detect non-specific PCR products. We have been able to reliably genotype three classes of mutations in zebrafish, including point mutants, apc(hu745) (apc(mcr)), and p53(zy7) (p53(I166T)), a small deletion mutant (bap28(y75)) and a retroviral insertion mutant (wdr43(hi821a)). This technique can genotype individual zebrafish embryos and adults (by tail-clip) and is applicable to other model organisms.

Human transcription factors responsive to initial reprogramming predominantly undergo legitimate reprogramming during fibroblast conversion to iPSCs.

The four transcription factors OCT4, SOX2, KLF4, and MYC (OSKM) together can convert human fibroblasts to induced pluripotent stem cells (iPSCs). It is, however, perplexing that they can do so only for a rare population of the starting cells with a long latency. Transcription factors (TFs) define identities of both the starting fibroblasts and the end product, iPSCs, and are also of paramount importance for the reprogramming process. It is critical to upregulate or activate the iPSC-enriched TFs while downregulate or silence the fibroblast-enriched TFs. This report explores the initial TF responses to OSKM as the molecular underpinnings for both the potency aspects and the limitation sides of the OSKM reprogramming. The authors first defined the TF reprogramome, i.e., the full complement of TFs to be reprogrammed. Most TFs were resistant to OSKM reprogramming at the initial stages, an observation consistent with the inefficiency and long latency of iPSC reprogramming. Surprisingly, the current analyses also revealed that most of the TFs (at least 83 genes) that did respond to OSKM induction underwent legitimate reprogramming. The initial legitimate transcriptional responses of TFs to OSKM reprogramming were also observed in the reprogramming fibroblasts from a different individual. Such early biased legitimate reprogramming of the responsive TFs aligns well with the robustness aspect of the otherwise inefficient and stochastic OSKM reprogramming.

Tissue-specific differences of p53 inhibition by Mdm2 and Mdm4.

The function of the p53 tumor suppressor to inhibit proliferation or initiate apoptosis is often abrogated in tumor cells. Mdm2 and its homolog, Mdm4, are critical inhibitors of p53 that are often overexpressed in human tumors. In mice, loss of Mdm2 or Mdm4 leads to embryonic lethal phenotypes that are completely rescued by concomitant loss of p53. To examine the role of Mdm2 and Mdm4 in a temporal and tissue-specific manner and to determine the relationships of these inhibitors to each other, we generated conditional alleles. We deleted Mdm2 and Mdm4 in cardiomyocytes, since proliferation and apoptosis are important processes in heart development. Mice lacking Mdm2 in the heart were embryonic lethal and showed defects at the time recombination occurred. A critical number of cardiomyocytes were lost by embryonic day 13.5, resulting in heart failure. This phenotype was completely rescued by deletion of p53. Mice lacking Mdm4 in the heart were born at the correct ratio and appeared to be normal. Our studies provide the first direct evidence that Mdm2 can function in the absence of Mdm4 to regulate p53 activity in a tissue-specific manner. Moreover, Mdm4 cannot compensate for the loss of Mdm2 in heart development.

Xenobiotic stress induces hepatomegaly and liver tumors via the nuclear receptor constitutive androstane receptor.

The constitutive androstane receptor (CAR, NR1I3) is a central regulator of xenobiotic metabolism. CAR activation induces hepatic expression of detoxification enzymes and transporters and increases liver size. Here we show that CAR-mediated hepatomegaly is a transient, adaptive response to acute xenobiotic stress. In contrast, chronic CAR activation results in hepatocarcinogenesis. In both acute and chronic xenobiotic responses, hepatocyte DNA replication is increased and apoptosis is decreased. These effects are absent in CAR null mice, which are completely resistant to tumorigenic effects of chronic xenobiotic stress. In the acute response, direct up-regulation of Mdm2 expression by CAR contributes to both increased DNA replication and inhibition of p53-mediated apoptosis. These results demonstrate an essential role for CAR in regulating both liver homeostasis and tumorigenesis in response to xenobiotic stresses, and they also identify a specific molecular mechanism linking chronic environmental stress and tumor formation.

Switching mechanisms of cell death in mdm2- and mdm4-null mice by deletion of p53 downstream targets.

The p53 tumor suppressor ensures maintenance of genome integrity by initiating either apoptosis or cell cycle arrest in response to DNA damage. Deletion of either mdm2 or mdm4 genes, which encode p53 inhibitors, results in embryonic lethality. The lethal phenotypes are rescued in the absence of p53, which indicates that increased activity of p53 is the cause of lethality in the mdm2- and mdm4-null embryos. Here we show that mdm2-null embryos die because of apoptosis initiated at 3.5 days postcoitum (dpc). Partial rescue of mdm2-null embryos by deletion of bax allows survival to 6.5 dpc and alters the mechanism of death from apoptosis to cell cycle arrest, indicating that bax is a critical component of the p53 pathway in early embryogenesis. The death of mdm4-null embryos is due to p53-initiated cell cycle arrest at 7.5 dpc. Deletion of p21(p21(waf1/cip1)), a p53 downstream target partially responsible for cell cycle arrest, does not rescue this phenotype; however, deletion of p21 alters the mechanism of cell death from lack of proliferation to apoptosis. Thus, in both examples, deletion of a p53 downstream target gene allows p53 to redirect its efforts, highlighting a high degree of plasticity in p53 function.

Observing Mitotic Division and Dynamics in a Live Zebrafish Embryo.

Mitosis is critical for organismal growth and differentiation. The process is highly dynamic and requires ordered events to accomplish proper chromatin condensation, microtubule-kinetochore attachment, chromosome segregation, and cytokinesis in a small time frame. Errors in the delicate process can result in human disease, including birth defects and cancer. Traditional approaches investigating human mitotic disease states often rely on cell culture systems, which lack the natural physiology and developmental/tissue-specific context advantageous when studying human disease. This protocol overcomes many obstacles by providing a way to visualize, with high resolution, chromosome dynamics in a vertebrate system, the zebrafish. This protocol will detail an approach that can be used to obtain dynamic images of dividing cells, which include: in vitro transcription, zebrafish breeding/collecting, embryo embedding, and time-lapse imaging. Optimization and modifications of this protocol are also explored. Using H2A.F/Z-EGFP (labels chromatin) and mCherry-CAAX (labels cell membrane) mRNA-injected embryos, mitosis in AB wild-type, auroraB(hi1045) (,) and esco2(hi2865) mutant zebrafish is visualized. High resolution live imaging in zebrafish allows one to observe multiple mitoses to statistically quantify mitotic defects and timing of mitotic progression. In addition, observation of qualitative aspects that define improper mitotic processes (i.e., congression defects, missegregation of chromosomes, etc.) and improper chromosomal outcomes (i.e., aneuploidy, polyploidy, micronuclei, etc.) are observed. This assay can be applied to the observation of tissue differentiation/development and is amenable to the use of mutant zebrafish and pharmacological agents. Visualization of how defects in mitosis lead to cancer and developmental disorders will greatly enhance understanding of the pathogenesis of disease.