DNA polymerase POLQ and cellular defense against DNA damage

Abstract

In mammalian cells, POLQ (pol θ) is an unusual specialized DNA polymerase whose in vivo function is under active investigation. POLQ has been implicated by different experiments to play a role in resistance to ionizing radiation and defense against genomic instability, in base excision repair, and in immunological diversification. The protein is formed by an N-terminal helicase-like domain, a C-terminal DNA polymerase domain, and a large central domain that spans between the two. This arrangement is also found in the Drosophila Mus308 protein, which functions in resistance to DNA interstrand crosslinking agents. Homologs of POLQ and Mus308 are found in multicellular eukaryotes, including plants, but a comparison of phenotypes suggests that not all of these genes are functional orthologs. Flies defective in Mus308 are sensitive to DNA interstrand crosslinking agents, while mammalian cells defective in POLQ are primarily sensitive to DNA double-strand breaking agents. Cells from Polq(-/-) mice are hypersensitive to radiation and peripheral blood cells display increased spontaneous and ionizing radiation-induced levels of micronuclei (a hallmark of gross chromosomal aberrations), though mice apparently develop normally. Loss of POLQ in human and mouse cells causes sensitivity to ionizing radiation and other double strand breaking agents and increased DNA damage signaling. Retrospective studies of clinical samples show that higher levels of POLQ gene expression in breast and colorectal cancer are correlated with poorer outcomes for patients. A clear understanding of the mechanism of action and physiologic function of POLQ in the cell is likely to bear clinical relevance.

Figure 1. Structural comparison and related grouping of POLQ gene family members

The orange regions and blue shaded regions are affiliated with the respective helicase and polymerase domains of the proteins. Numbers above the proteins indicate amino acid lengths. Percentages indicating the individual % similarity between the helicase-like and polymerase domains of POLQ were generated in MacVector.

Figure 2. Map and alignment of POLQ polymerase domain

(A) The C-terminal portion of POLQ necessary for polymerase activity is displayed. The polymerase domain of POLQ is displayed in blue. Conserved A-family DNA polymerase motifs are noted in black. The unique sequence inserts found in POLQ are shown in yellow. An inactive DnaQ-like 3'→5' exonuclease domain and a fragment similar to the E. coli 5'→3' exonuclease domain and the 5'-dRP lyase activity of POLQ mapped to the polymerase domain are noted. An asterisk notes the tyrosine residue in motif 4 of the polymerase domain that is responsible for ddNTP incorporation. The location of the serine to proline mutation found in the Chaos1 mutant mouse is noted in the central domain. (B) Sequence alignment of POLQ family members constructed in MacVector. Starting and ending amino acid numbers for each species in the alignment are notated in brackets. (C). The dendogram was created SeaView. The scale bar indicates a distance of 0.1 amino acid substitution per site. MycoPolI: Mycobacterium tuberculosis pol I; EcPolI: Escherichia coli pol I; TaqPolI: Thermus aquaticus pol I; TEBICHI: Arabidopsis thaliana TEBICHI; Rice: Oryza sativa pol I; Soybean: Glycine max POLQ-like; CePolq-1: Caenorhabditis elegans Polq-1; Mosquito: Aedes aegypti POLQ; Mus308: Drosophlia melanogaster Mus308; Anole: Anolis carolinensis POLQ-like; Tilapia: Oreochromis niloticus POLQ; Zebrafish: Danio rerio POLQ; Xenopus: Xenopus tropicalis POLQ; Chicken: Gallus gallus POLQ; Finch: Taeniopygia guttata POLQ; Dog: Canis lupus POLQ; Rabbit: Oryctolagus cuniculus; Horse: Equus caballus POLQ; Cow: Bos taurus POLQ; Rat: Rattus norvegicus POLQ; Mouse: Mus musculus POLQ; Rhesus: Macaca mulatta POLQ; Chimp: Pan troglodytes POLQ; Human: Homo sapiens POLQ; HsPOLN Homo sapiens POLN; PtPOLN: Pan troglodytes POLN; MmPOLN: Mus musculus POLN; DrPOLN: Danio rerio POLN.

Figure 2. Map and alignment of POLQ polymerase domain

(A) The C-terminal portion of POLQ necessary for polymerase activity is displayed. The polymerase domain of POLQ is displayed in blue. Conserved A-family DNA polymerase motifs are noted in black. The unique sequence inserts found in POLQ are shown in yellow. An inactive DnaQ-like 3'→5' exonuclease domain and a fragment similar to the E. coli 5'→3' exonuclease domain and the 5'-dRP lyase activity of POLQ mapped to the polymerase domain are noted. An asterisk notes the tyrosine residue in motif 4 of the polymerase domain that is responsible for ddNTP incorporation. The location of the serine to proline mutation found in the Chaos1 mutant mouse is noted in the central domain. (B) Sequence alignment of POLQ family members constructed in MacVector. Starting and ending amino acid numbers for each species in the alignment are notated in brackets. (C). The dendogram was created SeaView. The scale bar indicates a distance of 0.1 amino acid substitution per site. MycoPolI: Mycobacterium tuberculosis pol I; EcPolI: Escherichia coli pol I; TaqPolI: Thermus aquaticus pol I; TEBICHI: Arabidopsis thaliana TEBICHI; Rice: Oryza sativa pol I; Soybean: Glycine max POLQ-like; CePolq-1: Caenorhabditis elegans Polq-1; Mosquito: Aedes aegypti POLQ; Mus308: Drosophlia melanogaster Mus308; Anole: Anolis carolinensis POLQ-like; Tilapia: Oreochromis niloticus POLQ; Zebrafish: Danio rerio POLQ; Xenopus: Xenopus tropicalis POLQ; Chicken: Gallus gallus POLQ; Finch: Taeniopygia guttata POLQ; Dog: Canis lupus POLQ; Rabbit: Oryctolagus cuniculus; Horse: Equus caballus POLQ; Cow: Bos taurus POLQ; Rat: Rattus norvegicus POLQ; Mouse: Mus musculus POLQ; Rhesus: Macaca mulatta POLQ; Chimp: Pan troglodytes POLQ; Human: Homo sapiens POLQ; HsPOLN Homo sapiens POLN; PtPOLN: Pan troglodytes POLN; MmPOLN: Mus musculus POLN; DrPOLN: Danio rerio POLN.

Figure 2. Map and alignment of POLQ polymerase domain

(A) The C-terminal portion of POLQ necessary for polymerase activity is displayed. The polymerase domain of POLQ is displayed in blue. Conserved A-family DNA polymerase motifs are noted in black. The unique sequence inserts found in POLQ are shown in yellow. An inactive DnaQ-like 3'→5' exonuclease domain and a fragment similar to the E. coli 5'→3' exonuclease domain and the 5'-dRP lyase activity of POLQ mapped to the polymerase domain are noted. An asterisk notes the tyrosine residue in motif 4 of the polymerase domain that is responsible for ddNTP incorporation. The location of the serine to proline mutation found in the Chaos1 mutant mouse is noted in the central domain. (B) Sequence alignment of POLQ family members constructed in MacVector. Starting and ending amino acid numbers for each species in the alignment are notated in brackets. (C). The dendogram was created SeaView. The scale bar indicates a distance of 0.1 amino acid substitution per site. MycoPolI: Mycobacterium tuberculosis pol I; EcPolI: Escherichia coli pol I; TaqPolI: Thermus aquaticus pol I; TEBICHI: Arabidopsis thaliana TEBICHI; Rice: Oryza sativa pol I; Soybean: Glycine max POLQ-like; CePolq-1: Caenorhabditis elegans Polq-1; Mosquito: Aedes aegypti POLQ; Mus308: Drosophlia melanogaster Mus308; Anole: Anolis carolinensis POLQ-like; Tilapia: Oreochromis niloticus POLQ; Zebrafish: Danio rerio POLQ; Xenopus: Xenopus tropicalis POLQ; Chicken: Gallus gallus POLQ; Finch: Taeniopygia guttata POLQ; Dog: Canis lupus POLQ; Rabbit: Oryctolagus cuniculus; Horse: Equus caballus POLQ; Cow: Bos taurus POLQ; Rat: Rattus norvegicus POLQ; Mouse: Mus musculus POLQ; Rhesus: Macaca mulatta POLQ; Chimp: Pan troglodytes POLQ; Human: Homo sapiens POLQ; HsPOLN Homo sapiens POLN; PtPOLN: Pan troglodytes POLN; MmPOLN: Mus musculus POLN; DrPOLN: Danio rerio POLN.

Figure 2. Map and alignment of POLQ polymerase domain

(A) The C-terminal portion of POLQ necessary for polymerase activity is displayed. The polymerase domain of POLQ is displayed in blue. Conserved A-family DNA polymerase motifs are noted in black. The unique sequence inserts found in POLQ are shown in yellow. An inactive DnaQ-like 3'→5' exonuclease domain and a fragment similar to the E. coli 5'→3' exonuclease domain and the 5'-dRP lyase activity of POLQ mapped to the polymerase domain are noted. An asterisk notes the tyrosine residue in motif 4 of the polymerase domain that is responsible for ddNTP incorporation. The location of the serine to proline mutation found in the Chaos1 mutant mouse is noted in the central domain. (B) Sequence alignment of POLQ family members constructed in MacVector. Starting and ending amino acid numbers for each species in the alignment are notated in brackets. (C). The dendogram was created SeaView. The scale bar indicates a distance of 0.1 amino acid substitution per site. MycoPolI: Mycobacterium tuberculosis pol I; EcPolI: Escherichia coli pol I; TaqPolI: Thermus aquaticus pol I; TEBICHI: Arabidopsis thaliana TEBICHI; Rice: Oryza sativa pol I; Soybean: Glycine max POLQ-like; CePolq-1: Caenorhabditis elegans Polq-1; Mosquito: Aedes aegypti POLQ; Mus308: Drosophlia melanogaster Mus308; Anole: Anolis carolinensis POLQ-like; Tilapia: Oreochromis niloticus POLQ; Zebrafish: Danio rerio POLQ; Xenopus: Xenopus tropicalis POLQ; Chicken: Gallus gallus POLQ; Finch: Taeniopygia guttata POLQ; Dog: Canis lupus POLQ; Rabbit: Oryctolagus cuniculus; Horse: Equus caballus POLQ; Cow: Bos taurus POLQ; Rat: Rattus norvegicus POLQ; Mouse: Mus musculus POLQ; Rhesus: Macaca mulatta POLQ; Chimp: Pan troglodytes POLQ; Human: Homo sapiens POLQ; HsPOLN Homo sapiens POLN; PtPOLN: Pan troglodytes POLN; MmPOLN: Mus musculus POLN; DrPOLN: Danio rerio POLN.