Biology subjects

Ebrahimi, D.

Publications and source records attributed to Ebrahimi, D..

10 recordsLinked to original sources

bioRxiv2026

CODMA-A3: A Competitive Oligo-based DNA Modification Assay Applied to APOBEC Enzymes

Quantifying the functional effects of human variation in proteins is a challenge because bioinformatic pipelines allow variant detection to outpace experimental characterization. This results in a practical problem of resource allocation to identify the most scientifically significant variants to prioritize for mechanistic investigation. There are many approaches appropriate depending on class of enzymatic activity being investigated. However, fundamental to many of these approaches is a trade-off between mechanistic insight and throughput. In seeking to understand how activity changes across human variants of the APOBEC3 family of proteins we sought to create an early screening assay to prioritize variants for mechanistic study. We wanted an assay that was not dependent on protein purification, allowed for multiple-substrate competition and could be executed in a microtiter plate allowing for high throughput and approximate normalized substrate turnover without requiring classic Michaelis-Menten approaches. Here we describe CODMA (Competitive Oligo-based DNA Modification Assay) and applied it to A3 enzymology as a method to rapidly screen A3 constructs for changes in substrate utilization fingerprints. CODMA-A3 detected effects of substrate competition, distinguished fluorophore identity from substrate sequence effects, resolved activity changes caused by a single nucleotide substitution within a competing substrate, and identified shifts in substrate utilization following modification of lysate composition, oligonucleotide architecture, or antibody-mediated protein perturbation. CODMA-A3 occupies a methodological niche between low-throughput mechanistic assays and sequencing-intensive approaches by providing a rapid, inexpensive screening platform for comparative characterization of 20 to 40 enzyme variants prior to detailed mechanistic investigation.

biochemistry↗

bioRxiv2026

Deconvolution of HIV-1 Mutational Signatures Reveals Dominant and Donor-Specific APOBEC3-Associated Mutagenesis Across Anatomical Compartments

HIV-1 persists across multiple anatomical compartments, yet the mutational processes shaping viral diversity within these reservoirs remain incompletely understood. We investigated HIV-1 mutational signatures across blood, spleen, gut, and brain tissues from 21 people with HIV enrolled in the Last Gift rapid autopsy program. Mutations in single-genome HIV-1 env DNA sequences were quantified by trinucleotide context and deconvolved using non-negative matrix factorization. Two APOBEC3-associated signatures were resolved and together accounted for approximately 50% of all mutations; this contribution remained stable after normalization by the number of potential APOBEC3 target sites. Normalization further revealed a minor signature consistent with methylation-induced cytosine deamination. Mutational patterns were driven primarily by donor identity rather than anatomical compartment. Identical APOBEC3-hypermutated sequences were recovered across brain, gut, and blood, consistent with clonal expansion and trafficking of infected cells. These findings establish APOBEC3 activity as a major contributor to HIV-1 mutagenesis across tissue reservoirs and demonstrate that APOBEC3-associated mutagenesis is strongly donor-specific.

genomics↗

bioRxiv2026

Deciphering the Network Architecture of APOBEC3-Driven Mutagenesis in HPV-Positive Head and Neck Cancers

APOBEC3A (A3A) and APOBEC3B (A3B) are cytosine deaminases that restrict viral infection and can also mutate the host genome. In human papillomavirus (HPV)-positive head and neck squamous cell carcinoma (HNSCC), expression of both enzymes is elevated, but bulk sequencing averages their effects across mixed cell populations. Here we profile single cells from HPV16-positive HNSCC tumors and matched normal tissue. We find that the APOBEC3 (A3) single base substitution mutational signature, SBS2, is enriched in cells expressing more A3A than A3B, whereas copy number alteration (CNA) burden is enriched in cells expressing more A3B than A3A. Tumor versus normal co-expression networks identify the A3 interactors RALY and HNRNPA2B1 as candidate A3 activators. We found that SBS2 and CNA mark the maintenance and productive stages of HPV16 lifecycle, and their ratio may offer a molecular estimate of tumor age. The neoantigens from immune-visible SBS2-HIGH and immune-evasive CNA-HIGH cells identify candidates for mRNA vaccines matched to a tumor's viral state.

cancer biology↗

bioRxiv2026

APOBEC3G Splicing Defects in Nonhuman Primate Models Result in Disparate Viral Mutational Profiles Relative to Humans

Nonhuman primates (NHPs), particularly macaques, are indispensable models for studying human infectious diseases due to their close immunological and physiological similarities. Understanding species-specific molecular differences is essential for maximizing the translational value of these models. Here we report that APOBEC3G (A3G), a potent antiviral restriction factor and the major source of genetic variations in HIV, exhibits a widespread mRNA splicing defect in the Cercopithecinae subfamily, which includes the commonly used NHP models. Driven by intronic polymorphisms, this splicing defect substantially reduces A3G protein levels and consequently results in a markedly reduced A3G-mediated mutation signatures, fewer defective viral genomes, and greater viral diversification in SIV compared to HIV. This species-specific effect is not restricted to lentiviruses: reduced A3G signatures have also been reported in simian foamy virus and simian T-cell leukemia virus, suggesting broader effects across primate retroviruses. These findings reveal a lineage-specific alteration in a major antiviral restriction factor, with important implications for viral restriction, evolution, drug resistance, and immune evasion. They also highlight the importance of incorporating naturally occurring genetic variation into NHP model selection to improve the reproducibility, translational fidelity, and biological relevance of preclinical research.

genomics↗

bioRxiv2025

HIV-1 Vpr drives epigenetic remodeling to enhance virus transcription and latency reactivation

Despite decades of research, the primary proviral function of the HIV-1 Vpr accessory protein remains enigmatic. Vpr is essential for pathogenesis in vivo and for virus replication in myeloid cells, but the underlying cause-and-effect mechanism(s) driving these phenomena are poorly understood. Canonically, Vpr hijacks a cellular ubiquitin ligase complex to target several dozen host proteins for proteasomal degradation. Many of these substrates were recently revealed to be involved in DNA damage repair (DDR), which rationalizes the longstanding observation that Vpr induces constitutive activation of DDR signaling. Here, we use a combination of functional, biochemical, and genetic approaches establish a clear mechanistic link between Vpr-induced DDR signaling and remodeling of the epigenetic landscape to enhance HIV-1 promoter activity during acute infection and virus reactivation from latency. Functional, genetic, and bimolecular fluorescence complementation experiments reveal that Vpr utilizes degradation-dependent and -independent mechanisms to induce epigenetic remodeling and that Vpr segregates into two discrete pools with dedicated activities--A multimeric pool in the nucleus that is associated with chromatin and a monomeric pool associated with DCAF1 in the cytoplasm. Vpr function in remodeling the nuclear environment is present in common HIV-1 subtypes worldwide and provides a mechanistic rationale for its essentiality in virus replication. Author summaryWhile HIV-1 Vpr plays an essential role in virus replication, the molecular mechanisms underlying its essentiality remain enigmatic. Vprs best characterized function is the ability to induce the depletion of several dozen host proteins by hijacking a cellular E3-ubiquitin ligase complex. Here, we establish that Vpr promotes global epigenetic remodeling to enhance HIV-1 promoter activity during acute infection and virus reactivation from latency. We demonstrate that epigenetic remodeling activity is linked to Vprs ability to induce constitutive DNA damage repair signaling, and that it occurs through both degradation-dependent and -independent mechanisms. Moreover, this Vpr function is present in common HIV-1 subtypes circulating globally. This study provides novel mechanistic insights into how HIV-1 exploits host DNA repair pathways and sheds light on Vprs proviral function.

microbiology↗

bioRxiv2024

Molecular mechanism for regulating APOBEC3G DNA editing function by the non-catalytic domain

APOBEC3G (A3G) belongs to the AID/APOBEC cytidine deaminase family and is essential for antiviral immunity. It contains two zinc-coordinated cytidine-deaminase (CD) domains. The N-terminal CD1 domain is non-catalytic but has a strong affinity for nucleic acids, whereas the C-terminal CD2 domain catalyzes C-to-U editing in single-stranded DNA. The interplay between the two domains in DNA binding and editing is not fully understood. Here, our studies on rhesus macaque A3G (rA3G) show that the DNA editing function in linear and hairpin loop DNA is greatly enhanced by AA or GA dinucleotide motifs present downstream (in the 3-direction) but not upstream (in the 5-direction) of the target-C editing sites. The effective distance between AA/GA and the target-C sites depends on the local DNA secondary structure. We present two co-crystal structures of rA3G bound to ssDNA containing AA and GA, revealing the contribution of the non-catalytic CD1 domain in capturing AA/GA DNA and explaining our biochemical observations. Our structural and biochemical findings elucidate the molecular mechanism underlying the cooperative function between the non-catalytic and the catalytic domains of A3G, which is critical for its antiviral role and its contribution to genome mutations in cancer.

biochemistry↗

bioRxiv2021

Signatures of Mutational Processes in Human DNA Evolution

The human genome contains over 100 million SNPs, most of which are C/T (G/A) variations. The type and sequence context of these SNPs are not random, suggesting that they are caused by distinct mutational processes. Deciphering the mutational signatures is a crucial step to discovering the molecular processes responsible for DNA variations across human populations, and potentially for causing genetic diseases. Our analyses of the 1000 Genomes Project SNPs and germline de novo mutations suggest that at least four mutational processes are responsible for human genetic variations. One process is European-specific and no longer active. The remaining three processes are currently active in all human populations. Two of the active processes co-occur and leave a single joint mutational signature in human nuclear DNA. The third active process is specific to mitochondrial DNA, and inflicts C-to-T mutations at mostly non-CG sites. We found neither evidence of APOBEC-induced cytosine deamination in the human germline, nor de novo mutation enrichment within certain regions of the human genome.

evolutionary biology↗

bioRxiv2020

Evidence for ZAP-independent CpG reduction in SARS-CoV-2 genome, and pangolin coronavirus origin of 5'UTR

SARS-CoV-2, the causative agent of COVID-19, has an RNA genome, which is, overall, closely related to the bat coronavirus sequence RaTG13. However, the ACE2-binding domain of this virus is more similar to a coronavirus isolated from a Guangdong pangolin. In addition to this unique feature, the genome of SARS-CoV-2 (and its closely related coronaviruses) has a low CpG content. This has been postulated to be the signature of an evolutionary pressure exerted by the host antiviral protein ZAP. Here, we analyzed the sequences of a wide range of viruses using both alignment-based and alignment free approaches to investigate the origin of SARS-CoV-2 genome. Our analyses revealed a high level of similarity between the 5UTR of SARS-CoV-2 and that of the Guangdong pangolin coronavirus. This suggests bat and pangolin coronaviruses might have recombined at least twice (in the 5UTR and ACE2 binding regions) to seed the formation of SARS-CoV-2. An alternative hypothesis is that the lineage preceding SARS-CoV-2 is a yet to be sampled bat coronavirus whose ACE2 binding domain and 5UTR are distinct from other known bat coronaviruses. Additionally, we performed a detailed analysis of viral genome compositions as well as expression and RNA binding data of ZAP to show that the low CpG abundance in SARS-CoV-2 is not related to an evolutionary pressure from ZAP.

genomics↗

bioRxiv2020

Integrative analysis of mutated genes and mutational processes reveals seven colorectal cancer subtypes

Colorectal cancer (CRC) is one of the leading causes of cancer-related deaths in the world. It has been reported that [~]10%-15% of individuals with colorectal cancer experience a causative mutation in the known susceptibility genes, highlighting the importance of identifying mutations for early detection in high risk individuals. Through extensive sequencing projects such as the International Cancer Genome Consortium (ICGC), a large number of somatic point mutations have been identified that can be used to identify cancer-associated genes, as well as the signature of mutational processes defined by the tri-nucleotide sequence context (motif) of mutated sites. Mutation is the hallmark of cancer genome, and many studies have reported cancer subtyping based on the type of frequently mutated genes, or the proportion of mutational processes, however, none of these cancer subtyping methods consider these features simultaneously. This highlights the need for a better and more inclusive subtype classification approach to enable biomarker discovery and thus inform drug development for CRC. In this study, we developed a statistical pipeline based on a novel concept gene-motif, which merges mutated gene information with tri-nucleotide motif of mutated sites, to identify cancer subtypes, in this case CRCs. Our analysis identified for the first time, 3,131 gene-motif combinations that were significantly mutated in 536 ICGC colorectal cancer samples compared to other cancer types, identifying seven CRC subtypes with distinguishable phenotypes and biomarkers. Interestingly, we identified several genes that were mutated in multiple subtypes but with unique sequence contexts. Taken together, our results highlight the importance of considering both the mutation type and mutated genes in identification of cancer subtypes and cancer biomarkers.

cancer biology↗

bioRxiv2018

CANCERSIGN: a user-friendly and robust tool for identification and classification of mutational signatures and patterns in cancer genomes

Analyses of large somatic mutation datasets, using advanced computational algorithms, have revealed at least 30 independent mutational signatures in tumor samples. These studies have been instrumental in identification and quantification of responsible endogenous and exogenous molecular processes against cancer. The quantitative approach used to deconvolute mutational signatures is becoming an integral part of cancer research. Therefore, development of a stand-alone tool with a user-friendly graphical interface for analysis of cancer mutational signatures is necessary. In this manuscript, we introduce CANCERSIGN as an open access1 bioinformatics tool that uses raw mutation data (BED files) as input, and generates 3-mer and 5-mer mutational signatures. Additionally, this tool enables users to perform clustering on tumor samples based on the raw mutation counts as well as using the proportion of mutational signatures in each sample. Using this tool, we analysed all the whole genome somatic mutation datasets of International Cancer Genome Consortium (ICGC) samples and identified a number of novel signatures.

bioinformatics↗