ZNF432 stimulates PARylation and inhibits DNA resection to balance PARPi sensitivity and resistance.

Zinc finger (ZNF) motifs are some of the most frequently occurring domains in the human genome. It was only recently that ZNF proteins emerged as key regulators of genome integrity in mammalian cells. In this study, we report a new role for the Krüppel-type ZNF-containing protein ZNF432 as a novel poly(ADP-ribose) (PAR) reader that regulates the DNA damage response. We show that ZNF432 is recruited to DNA lesions via DNA- and PAR-dependent mechanisms. Remarkably, ZNF432 stimulates PARP-1 activity in vitro and in cellulo. Knockdown of ZNF432 inhibits phospho-DNA-PKcs and increases RAD51 foci formation following irradiation. Moreover, purified ZNF432 preferentially binds single-stranded DNA and impairs EXO1-mediated DNA resection. Consequently, the loss of ZNF432 in a cellular system leads to resistance to PARP inhibitors while its overexpression results in sensitivity. Taken together, our results support the emerging concept that ZNF-containing proteins can modulate PARylation, which can be embodied by the pivotal role of ZNF432 to finely balance the outcome of PARPi response by regulating homologous recombination.

Assessment of Global DNA Double-Strand End Resection using BrdU-DNA Labeling coupled with Cell Cycle Discrimination Imaging.

The study of the DNA damage response (DDR) is a complex and essential field, which has only become more important due to the use of DDR-targeting drugs for cancer treatment. These targets are poly(ADP-ribose) polymerases (PARPs), which initiate various forms of DNA repair. Inhibiting these enzymes using PARP inhibitors (PARPi) achieves synthetic lethality by conferring a therapeutic vulnerability in homologous recombination (HR)-deficient cells due to mutations in breast cancer type 1 (BRCA1), BRCA2, or partner and localizer of BRCA2 (PALB2). Cells treated with PARPi accumulate DNA double-strand breaks (DSBs). These breaks are processed by the DNA end resection machinery, leading to the formation of single-stranded (ss) DNA and subsequent DNA repair. In a BRCA1-deficient context, reinvigorating DNA resection through mutations in DNA resection inhibitors, such as 53BP1 and DYNLL1, causes PARPi resistance. Therefore, being able to monitor DNA resection in cellulo is critical for a clearer understanding of the DNA repair pathways and the development of new strategies to overcome PARPi resistance. Immunofluorescence (IF)-based techniques allow for monitoring of global DNA resection after DNA damage. This strategy requires long-pulse genomic DNA labeling with 5-bromo-2'-deoxyuridine (BrdU). Following DNA damage and DNA end resection, the resulting single-stranded DNA is specifically detected by an anti-BrdU antibody under native conditions. Moreover, DNA resection can also be studied using cell cycle markers to differentiate between various phases of the cell cycle. Cells in the S/G2 phase allow the study of end resection within HR, whereas G1 cells can be used to study non-homologous end joining (NHEJ). A detailed protocol for this IF method coupled to cell cycle discrimination is described in this paper.

Poly(ADP-ribose) polymerase-1 antagonizes DNA resection at double-strand breaks.

PARP-1 is rapidly recruited and activated by DNA double-strand breaks (DSBs). Upon activation, PARP-1 synthesizes a structurally complex polymer composed of ADP-ribose units that facilitates local chromatin relaxation and the recruitment of DNA repair factors. Here, we identify a function for PARP-1 in DNA DSB resection. Remarkably, inhibition of PARP-1 leads to hyperresected DNA DSBs. We show that loss of PARP-1 and hyperresection are associated with loss of Ku, 53BP1 and RIF1 resection inhibitors from the break site. DNA curtains analysis show that EXO1-mediated resection is blocked by PARP-1. Furthermore, PARP-1 abrogation leads to increased DNA resection tracks and an increase of homologous recombination in cellulo. Our results, therefore, place PARP-1 activation as a critical early event for DNA DSB repair activation and regulation of resection. Hence, our work has direct implications for the clinical use and effectiveness of PARP inhibition, which is prescribed for the treatment of various malignancies.

Emerging roles of eraser enzymes in the dynamic control of protein ADP-ribosylation.

Protein ADP-ribosylation is essential for the regulation of several cellular pathways, enabling dynamic responses to diverse pathophysiological conditions. It is modulated through a dynamic interplay between ADP-ribose readers, writers and erasers. While ADP-ribose synthesis has been studied and reviewed extensively, ADP-ribose processing by erasing enzymes has received comparably less attention. However, major progress in the mass spectrometric identification of ADP-ribosylated residues and the biochemical characterization of ADP-ribose erasers has substantially expanded our knowledge of ADP-ribosylation dynamics. Herein, we describe recent insights into the biology of ADP-ribose erasers and discuss the intricately orchestrated cellular processes to switch off ADP-ribose-dependent mechanisms.

SAM68 interaction with U1A modulates U1 snRNP recruitment and regulates mTor pre-mRNA splicing.

Src associated in mitosis (SAM68) plays major roles in regulating RNA processing events, such as alternative splicing and mRNA translation, implicated in several developmental processes. It was previously shown that SAM68 regulates the alternative splicing of the mechanistic target of rapamycin (mTor), but the mechanism regulating this process remains elusive. Here, we report that SAM68 interacts with U1 small nuclear ribonucleoprotein (U1 snRNP) to promote splicing at the 5' splice site in intron 5 of mTor. We also show that this direct interaction is mediated through U1A, a core-component of U1snRNP. SAM68 was found to bind the RRM1 domain of U1A through its C-terminal tyrosine rich region (YY domain). Deletion of the U1A-SAM68 interaction domain or mutation in SAM68-binding sites in intron 5 of mTor abrogates U1A recruitment and 5' splice site recognition by the U1 snRNP, leading to premature intron 5 termination and polyadenylation. Taken together, our results provide the first mechanistic study by which SAM68 modulates alternative splicing decision, by affecting U1 snRNP recruitment at 5' splice sites.

Multifocal demyelinating motor neuropathy and hamartoma syndrome associated with a de novo PTEN mutation.

OBJECTIVE: To describe a patient with a multifocal demyelinating motor neuropathy with onset in childhood and a mutation in phosphatase and tensin homolog (PTEN), a tumor suppressor gene associated with inherited tumor susceptibility conditions, macrocephaly, autism, ataxia, tremor, and epilepsy. Functional implications of this protein have been investigated in Parkinson and Alzheimer diseases. METHODS: We performed whole-exome sequencing in the patient's genomic DNA validated by Sanger sequencing. Immunoblotting, in vitro enzymatic assay, and label-free shotgun proteomic profiling were performed in the patient's fibroblasts. RESULTS: The predominant clinical presentation of the patient was a childhood onset, asymmetric progressive multifocal motor neuropathy. In addition, he presented with macrocephaly, autism spectrum disorder, and skin hamartomas, considered as clinical criteria for PTEN-related hamartoma tumor syndrome. Extensive tumor screening did not detect any malignancies. We detected a novel de novo heterozygous c.269T>C, p.(Phe90Ser) PTEN variant, which was absent in both parents. The pathogenicity of the variant is supported by altered expression of several PTEN-associated proteins involved in tumorigenesis. Moreover, fibroblasts showed a defect in catalytic activity of PTEN against the secondary substrate, phosphatidylinositol 3,4-trisphosphate. In support of our findings, focal hypermyelination leading to peripheral neuropathy has been reported in PTEN-deficient mice. CONCLUSION: We describe a novel phenotype, PTEN-associated multifocal demyelinating motor neuropathy with a skin hamartoma syndrome. A similar mechanism may potentially underlie other forms of Charcot-Marie-Tooth disease with involvement of the phosphatidylinositol pathway.

Quantitative phenotypic and network analysis of 1q44 microdeletion for microcephaly.

As genome wide techniques become more common, an increasing proportion of patients with intellectual disability (ID) are found to have genetic defects allowing genotype-phenotype correlations. Previously, AKT3 deletion was suggested to be responsible for microcephaly in patients with 1q43-q44 deletion syndrome, but this does not correspond to all cases. We report a case of a de novo 1q44 deletion in an 8-year-old boy with microcephaly in whom AKT3 is not deleted. We used a systematic review of the literature, our patient, and network analysis to gain a better understanding of the genetic basis of microcephaly in 1q deletion patients. Our analysis showed that while AKT3 deletion is associated with more severe (≤3 SD) microcephaly in 1q43-q44 deletion patients, other genes may contribute to microcephaly in AKT3 intact patients with microcephaly and 1q43-44 deletion syndrome. We identified a potential role for HNRNPU, SMYD3, NLRP3, and KIF26B in microcephaly. Overall, our study highlights the need for network analysis and quantitative measures reporting in the phenotypic analysis of a complex genetic syndrome related to copy number variation.

Conserved pharmacological rescue of hereditary spastic paraplegia-related phenotypes across model organisms.

Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative diseases causing progressive gait dysfunction. Over 50 genes have now been associated with HSP. Despite the recent explosion in genetic knowledge, HSP remains without pharmacological treatment. Loss-of-function mutation of the SPAST gene, also known as SPG4, is the most common cause of HSP in patients. SPAST is conserved across animal species and regulates microtubule dynamics. Recent studies have shown that it also modulates endoplasmic reticulum (ER) stress. Here, utilizing null SPAST homologues in C. elegans, Drosophila and zebrafish, we tested FDA-approved compounds known to modulate ER stress in order to ameliorate locomotor phenotypes associated with HSP. We found that locomotor defects found in all of our spastin models could be partially rescued by phenazine, methylene blue, N-acetyl-cysteine, guanabenz and salubrinal. In addition, we show that established biomarkers of ER stress levels correlated with improved locomotor activity upon treatment across model organisms. Our results provide insights into biomarkers and novel therapeutic avenues for HSP.

Genomic characterization of chromosome 8 pericentric trisomy.

We present a patient with trisomy 8p11.21q11.21 associated with language, gross motor, fine motor, and cognitive delay. Furthermore, using array-based comparative genomic hybridization, we identify the specific genes duplicated in our patient.

Quantitative Analysis of Climbing Defects in a Drosophila Model of Neurodegenerative Disorders.

Locomotive defects resulting from neurodegenerative disorders can be a late onset symptom of disease, following years of subclinical degeneration, and thus current therapeutic treatment strategies are not curative. Through the use of whole exome sequencing, an increasing number of genes have been identified to play a role in human locomotion. Despite identifying these genes, it is not known how these genes are crucial to normal locomotive functioning. Therefore, a reliable assay, which utilizes model organisms to elucidate the role of these genes in order to identify novel targets of therapeutic interest, is needed more than ever. We have designed a sensitized version of the negative geotaxis assay that allows for the detection of milder defects earlier and has the ability to evaluate these defects over time. The assay is performed in a glass graduated cylinder, which is sealed with a wax barrier film. By increasing the threshold distance to be climbed to 17.5 cm and increasing the experiment duration to 2 min we have observed a greater sensitivity in detecting mild mobility dysfunctions. The assay is cost effective and does not require extensive training to obtain highly reproducible results. This makes it an excellent technique for screening candidate drugs in Drosophila mutants with locomotion defects.