PrimPol: A Breakthrough among DNA Replication Enzymes and a Potential New Target for Cancer Therapy

Abstract

DNA replication can encounter blocking obstacles, leading to replication stress and genome instability. There are several mechanisms for evading this blockade. One mechanism consists of repriming ahead of the obstacles, creating a new starting point; in humans, PrimPol is responsible for carrying out this task. PrimPol is a primase that operates in both the nucleus and mitochondria. In contrast with convectional primases, PrimPol is a DNA primase able to initiate DNA synthesis de novo using deoxynucleotides, discriminating against ribonucleotides. In vitro, PrimPol can act as a translesion synthesis (TLS) DNA primase, elongating primers that PrimPol itself sythesizes, or as TLS DNA polymerase, elongating pre-existing primers across lesions. However, the lack of evidence for PrimPol polymerase activity in vivo suggests that PrimPol only uses its TLS abilities to facilitate priming across lesions in cells, thus acting as a TLS DNA primase. Here, we provide a comprehensive review of human PrimPol covering its biochemical properties and structure, in vivo function and regulation, and the processes that take place to fill the gap-containing lesion that PrimPol leaves behind. Finally, we explore the available data on human PrimPol expression in different tissues in physiological conditions and its role in cancer.

3′dNTP Binding Is Modulated during Primer Synthesis and Translesion by Human PrimPol

“…until its complete extension (see scheme at Figure 4A). As shown in the autoradiogram, WT HsPrimPol could efficiently synthetize a dimer and extend it to the final product of 10-mer but also yielded additional bands (indicated by * in Figure 4A), that were previously described as non-canonical products [33]. These non-canonical products are the result of primer realignments and template distortions that enable HsPrimPol to continue primer elongation when the next nucleotide to be incorporated is absent.…”

“…Therefore, the initial synthesis of dimers appears to be the step most affected in both S167A and K297A mutants, implying that Ser 167 and Lys 297 are especially relevant in stabilizing the preternary complex (enzyme:ssDNA:dNTP), which precedes the binding of the 5′ nucleotide and subsequent dimer synthesis. The altered balance of canonical vs. non-canonical products synthesized by K165A, S167A and K297A mutants compared to the WT PrimPol suggests that Lys 165 , Ser 167 and Lys 297 are required for primer realignment-mediated dNTP incorporation [ 12 , 34 ], enabling PrimPol to carry out TLS primase activities [ 33 ]. Interestingly, neither H169A nor H169Y showed any extension of the trimer ( 3p A GT), leading to the conclusion that His 169 is also crucial for 3′ dNTP binding during primer elongation.…”

“…non-canonical products [33]. These non-canonical products are the result of primer realignments and template distortions that enable HsPrimPol to continue primer elongation when the next nucleotide to be incorporated is absent.…”

Transcription–Replication Conflicts as a Source of Genome Instability

“…Both of these fork recovery processes are mediated by replication stress response and checkpoint activation pathways (23). Replication fork recovery relies on chromatin remodelers for fork reversal, homologous recombination machinery for fork protection against nucleases, and translesion synthesis polymerases for fork restart (31,107,130). Lesions impeding replication forks can be bypassed by translesion synthesis polymerases and Fanconi proteins (108).…”

PrimPol Variant V102A with Altered Primase and Polymerase Activities

“…According to the TCGA PanCancer Atlas Studies (cBioPortal), 72 somatic mutations of PRIMPOL gene carrying 59 missense mutations were identified in cancer patients (Cerami et al 2012). Many mutations have potential to disrupt PrimPol activity since they affect the enzyme active site, binding sites for the PolDIP2 and RPA regulatory proteins, and ZnFn motif (Díaz-Talavera et al 2022). Several mutations were identified more than in one patient (R76C/H, L96I, N187H/I, K311I/P, P314H/S, E370G/Q, E407K, R417W/Q, Y521S) that makes them promising targets for future studies of association with the cancer risks and disease prognosis (Cerami et al 2012).…”