Alternative titles; symbols
Other entities represented in this entry:
HGNC Approved Gene Symbol: UNG
Cytogenetic location: 12q24.11 Genomic coordinates (GRCh38): 12:109,097,597-109,110,992 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 12q24.11 | Immunodeficiency with hyper IgM, type 5 | 608106 | Autosomal recessive | 3 |
Uracil-DNA glycosylase (UNG; EC 3.2.2.3) removes uracil in DNA resulting from deamination of cytosine or replicative incorporation of dUMP instead of dTMP. Thus, UNG plays a role in suppressing GC-to-AT transition mutations. The UNG gene encodes 2 isoforms that are individually targeted to the mitochondria and the nucleus. The mitochondrial isoform is referred to as UNG1, UDG1, or UDG1M, and the nuclear isoform is referred to as UNG2, UDG1A, or UDG1N (Caradonna and Muller-Weeks, 2001).
Olsen et al. (1989) cloned and sequenced a cDNA encoding human uracil-DNA glycosylase. They found evidence that uracil-DNA glycosylases from phylogenetically distant species are highly conserved. Using an antibody against placental uracil-DNA glycosylase to screen a human placental cDNA library in phage lambda-gt11, Vollberg et al. (1989) isolated the human uracil-DNA glycosylase gene.
Haug et al. (1998) stated that the nuclear (UNG2) and mitochondrial (UNG1) isoforms of UNG result from alternative splicing and the use of alternative promoters. The UNG1 and UNG2 proteins contain different N-terminal sequences, but the downstream 269 amino acids are common and include a short region that binds replication protein A (see RPA1; 179835) and a larger, compact catalytic domain. Using Northern blot and RNA dot blot analyses, Haug et al. (1998) detected UNG1 in all tissues examined, with highest levels in skeletal muscle, heart, testis, adrenal gland, and thyroid. Appreciable expression of UNG2 was limited to proliferating tissues, with highest levels in testis, followed by placenta, colon, small intestine, and thymus.
Using in vitro mitochondrial import experiments, Caradonna et al. (1996) showed that the 36-kD UDG1 precursor is taken up by mitochondria and proteolytically processed to the 30-kD mature form.
Haug et al. (1998) found that expression of both UNG1 and UNG2 increased in late G1/early S phase in synchronized human keratinocytes, and the increased expression was accompanied by a 4- to 5-fold increase in enzyme activity. Mutation analysis and transient transfection showed that the E2F1 (189971)/DP1 (TFDP1; 189902)-RB (RB1; 614041) transcriptional complex was a strong negative regulator of both UNG promoters, whereas the free E2F1/DP1 dimer was a weak positive regulator, even though a consensus element for E2F binding is present only in the second promoter, which is used for UNG2 only. SP1 (189906)- and MYC (190080)-binding elements close to transcription start areas were positively regulators of both promoters. However, in HeLa cells, overexpression of SP1 stimulated both promoters, and overexpression of MYC and MYC/MAS (MAS1; 165180) was suppressive. CCAAT elements were negative regulators of the second promoter, but they were positive regulators of the first promoter.
Dinner et al. (2001) used a hybrid quantum-mechanical/molecular-mechanical approach to determine the mechanism of catalysis by UDG. In contrast to the concerted associative mechanism proposed initially, Dinner et al. (2001) demonstrated that the reaction proceeds in a stepwise dissociative manner. Cleavage of the glycosylic bond yields an intermediate comprising an oxocarbenium cation and a uracilate anion. Subsequent attack by a water molecule and transfer of a proton to D145 result in the products. Surprisingly, the primary contribution to lowering the activation energy comes from the substrate rather than from the enzyme. This 'autocatalysis' derives from the burial and positioning of 4 phosphate groups that stabilize the rate-determining transition state. The importance of these phosphates explains the residual activity observed for mutants that lack key residues.
Kavli et al. (2002) compared the glycosylase activities of the UNG2, the nuclear UNG isoform, and SMUG1 (607753). Both enzymes were stimulated by physiologic concentrations of Mg(2+), and Mg(2+) increased the preference of UNG2 toward uracil in single-stranded DNA nearly 40-fold. SMUG1 showed broader substrate specificity than UNG2. SMUG1 accumulated within nucleoli in cultured epithelial cells, while UNG2 was excluded from nucleoli. In contrast, only UNG2 accumulated in replication foci during S phase. UNG2 initiated base excision repair of plasmids containing either U:A or U:G pairs in vitro. Kavli et al. (2002) proposed that UNG2 is responsible for both prereplicative removal of deaminated cytosine and postreplicative removal of misincorporated uracil at the replication fork, and that UNG2 is the major enzyme for removal of deaminated cytosine outside of replication foci, with SMUG1 acting as a broad specificity backup.
The apurinic/apyrimidinic (AP) sites generated by UNG are repaired by the general AP repair system with the sequential action of AP endonuclease (APEX; 107748), DNA polymerase-beta (POLB; 174760), and DNA ligase (see LIG1; 126391). Elder et al. (2003) found that overexpression of human UNG2 or S. pombe Ung1, which localizes to the nucleus and is therefore most similar to human UNG2, in yeast induced DNA checkpoint-dependent cell cycle delay and caused cell death, which was enhanced when the checkpoints were inactive. The steady-state level of AP sites increased after Ung1 overexpression, indicating that AP sites are likely to be the DNA damage caused by UNG2 overexpression. Analysis of mutant Ung1 indicated that catalytic activity was not required for toxicity, but that binding of Ung1 or UNG2 to AP sites was important.
Using B-cell lines expressing or lacking CD19 (107265), Imai et al. (2003) found that CD40LG (300386) plus IL4 (147780) treatment induced nuclear UNG transcripts only in CD19-positive B cells. They proposed a model of class-switch recombination (CSR) and somatic hypermutation in which AICDA (605257) deaminates cytosine into uracil in targeted DNA (e.g., in immunoglobulin switch or variable regions) followed by uracil removal by UNG.
Using Ugi, an inhibitor of UNG, Begum et al. (2004) confirmed the importance of UNG in CSR. Ugi, however, did not inhibit DNA cleavage in the immunoglobulin heavy chain locus during CSR, even though Ugi blocked UNG binding to DNA and inhibited CSR. Furthermore, mutants of UNG unable to remove uracil were capable of rescuing CSR in UNG -/- cells, suggesting that UNG has unknown functions other than uracil removal that are critical in the repair step of CSR. In addition, the authors noted that Ung -/- mice have residual CSR activity, indicating that UNG and other mismatch repair proteins may serve as scaffolds to recruit different error-prone polymerases to repair cleaved ends during CSR and somatic hypermutation.
Kavli et al. (2005) found that UNG2 in nuclear extracts of B cells from UNG-proficient individuals efficiently removed uracil from single-stranded DNA. SMUG1 could not compensate for UNG2 deficiency in B cells from UNG-deficient patients with hyper-IgM syndrome-5 (HIGM5; 608106). In HIGM5 patients, UNG mutations led to an accumulation of genomic uracil. Immunoblot and confocal microscopy analyses showed that UNG2 with the phe251-to-ser substitution (F251S; 191525.0003) in its catalytic domain was fully active and stable when expressed in E. coli, but it was mistargeted to mitochondria rather than the nucleus, likely due to interaction with UNG1, and was degraded in mammalian cells. Kavli et al. (2005) proposed a model for the initiation of CSR in which UNG2 is essential for CSR in the MutS-alpha (see MSH6; 600678)-independent pathway and is also important for CSR in the MutS-alpha-dependent pathway.
Studebaker et al. (2005) found that inhibition of UNG in human cell lines decreased cell proliferation, but had no effect on cell viability.
Haug et al. (1994) demonstrated that the UNG gene contains 4 exons and has an approximate size of 13 kb. The promoter is GC rich and lacks a TATA box.
Haug et al. (1996) demonstrated that the UNG gene spans approximately 13.5 kb, including the promoter region. UNG comprises 6 exons and 5 introns. It was found to exhibit typical features of housekeeping genes, including a 5-prime CpG island of 1.2 kb and a very GC-rich TATA-less promoter containing a number of elements involved in constitutive expression and cell cycle regulation.
Haug et al. (1998) stated that the UNG gene contains 7 exons. It has 2 alternate first exons located within a partially methylated CpG island that provide functional alternate promoters.
Aasland et al. (1990) assigned the DGU gene to chromosome 12 by Southern blot analysis of DNA from a panel of rodent-human somatic cell hybrids.
By radiation hybrid mapping, Haug et al. (1996) assigned the UNG gene to chromosome 12q23-q24.1.
In 3 patients with hyper-IgM syndrome-5 (HIGM5; 608106), Imai et al. (2003) identified mutations in the UNG gene. All 4 mutations occurred within the catalytic domain of the UNG protein. Patient B cells were incapable of CSR after activation with anti-CD40 (109535) or with soluble CD40LG plus IL4. The phenotype was similar to that observed in Ung -/- mice, although the CSR defect was more severe.
Nilsen et al. (2000) generated knockout mice lacking Ung. In contrast to Ung-deficient mutants of bacteria and yeast, these mice did not exhibit a greatly increased spontaneous mutation frequency. There was, however, only slow removal of uracil from misincorporated dUMP in isolated Ung -/- nuclei and an elevated steady-state level of uracil in DNA in dividing Ung -/- cells. A backup uracil-excising activity in tissue extracts from Ung null mice, with properties indistinguishable from the mammalian SMUG1 DNA glycosylase, may account for the repair of premutagenic U:G mispairs resulting from cytosine deamination in vivo. The authors suggested that the nuclear UNG protein has evolved a specialized role in mammalian cells counteracting U:A base pairs formed by use of dUTP during DNA synthesis.
The gene symbol UNG corresponds to the nomenclature used for E. coli and Saccharomyces cerevisiae.
In a patient with hyper-IgM syndrome-5 (HIGM5; 608106), Imai et al. (2003) identified compound heterozygosity for a 1-bp deletion (C) at nucleotide 462 in exon 2 and a 2-bp deletion (TA) at nucleotide 639 in exon 4 (191525.0002) of the UNG gene. Both mutations resulted in the generation of premature stop codons (at positions 141 and 224 of nuclear UNG, respectively). The parents of the patient were nonconsanguineous.
For discussion of the 2-bp deletion in the UNG gene (639_640delTA) that was found in compound heterozygous state in a patient with HIGM5 (608106) by Imai et al. (2003), see 191525.0001.
In a patient with immunodeficiency with hyper-IgM (HIGM5; 608106), Imai et al. (2003) identified a homozygous 822T-C transition in exon 5 of the UNG gene, resulting in a phe251-to-ser (F251S) substitution in nuclear UNG. The parents of the patient were nonconsanguineous.
Using immunoblot and confocal microscopy analyses, Kavli et al. (2005) showed that UNG2 with the F251S substitution in its catalytic domain was fully active and stable when expressed in E. coli, but it was mistargeted to mitochondria rather than the nucleus, likely due to interaction with UNG1, and was degraded in mammalian cells.
In a patient with hyper-IgM syndrome-5 (HIGM5; 608106), who was born to first-cousin parents, Imai et al. (2003) identified a homozygous 2-bp deletion (AT) at nucleotide 497 in exon 2 of the UNG gene. The mutation resulted in the generation of a premature stop codon at position 159 of nuclear UNG.
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