UPDATE ONRGeEvNieEwCOMPLETIONS AND ANNOTATIONS Vasiliou, Ross and Nebert Discovery of DT diaphorase Ernster and Navazio are credited with discovering (in the rat) the ﬁrst member of the NQO family in the late 1950s.7 – 9 This enzyme is now ofﬁcially named NQO1 (EC 1.6.99.2; pre- vious names included DT diaphorase and aminoazo dye reductase, NQOR1, NMO1, NMOR1 and QR1). It was later concluded9,10 that this enzyme is probably identical to a vitamin K reductase that had been isolated by Martius and colleagues. The mammalian NQO1 enzyme is expressed constitutively in most tissues and is strikingly upregulated during oxidative or electrophilic stress; NQO1 has been shown to be a member of the [AHR ] gene battery.11 The crystal structure,12 enzymic functions13 and polymorphisms14 in the NQO1 gene have all recently been summarised. The human NQO1 gene on chromosome 16q22.1 comprises six exons, spanning approximately 18 kilobases (kb), encodes 274 amino acids and contains, to date, 93 single nucleotide polymorphisms (SNPs) within 10 kb 50-ward of the ﬁrst exon and 2 kb 30-ward of the last exon (http://SNPper. chip.org/). Two additional variants, besides the consensus gene, were found in the Entrez Gene database (Figure 1). Compared with the human consensus (reference, ‘wild-type’) NQO1*1 allele coding for normal NQO1 enzyme and activity, the NQO1*2 allele encodes a non-synonymous mutation (Pro187Ser) that has negligible NQO1 enzyme activity. The NQO1*2 allelic frequency ranges between 0.22 (Caucasian) and 0.45 (Asian) in various ethnic populations. A large epidemiological investigation of a ben- zene-exposed population15 has shown that NQO1*2 homo- zygotes exhibit as much as a seven-fold greater risk of bone marrow toxicity, leading to diseases such as aplastic anaemia and leukaemia. The NQO1*3 allele (Arg139Try) represents an additional NQO1 polymorphism16 that has been character- ised; allelic frequencies have been reported as high as 0.05 in some ethnic groups,17 and the NQO1*3 allele has been linked to a higher rate of exon 4 deletion — an alternatively spliced form of NQO1 mRNA.14 The other member18,19 of this mammalian gene family is NQO2, which has recently been demonstrated to be identical to a protein isolated from human kidney in 1962. NQO2 is 43 residues shorter than NQO1 at its C-terminus and shows 49 per cent sequence identity with NQO1. The human NQO2 gene on chromosome 6p25.2 comprises seven exons (ﬁrst exon non-coding) spanning 19.8 kb, encodes 231 amino acids and contains, to date, 254 SNPs within 10 kb 50-ward of the ﬁrst exon and 2 kb 30-ward of the last exon (http://SNPper. chip.org/). Whereas earlier reports20,21 had suggested an association between both NQO1 and NQO2 polymorphisms and Parkinson disease, a more recent study22 showed no association between alleles of either NQO1 or NQO2 and Parkinson’s disease. NQO1 efﬁciently utilises NAD(P)H as the source of redu- cing equivalents, but — because the 43-amino acid deletion in NQO2 contains a portion of the pyridine nucleotide binding site — NQO2 has been shown to prefer dihydronicotinamide ribosyl, and other similar derivatives, as a cofactor.23 Similar to NQO1, NQO2 can also bioactivate certain anti-tumour agents, such as the CB1954 nitro compound.13 A crystal structure of human NQO2 has been published,23 but not many biological functions of this enzyme have so far been well characterised. Curiously, NQO2 contains a melatonin-binding site,24 which might be a reasonable explanation for the anti-oxidant role ascribed to melatonin. Both NQO1 and NQO2 activities are observed at high levels in many human solid tumours, including lung, colon, liver and breast; this ﬁnding has encouraged oncologists to explore the possibility of NQO1 and NQO2 as drug targets for the development of novel anti-tumour agents that can be activated by these enzymes.13 Deﬁnition of ‘Pfam’ and ‘clan’ Pfam is a means of identifying similar domain structures of all proteins that are summarised in UniProt (http://www.ebi. uniprot.org/). Both NQO1 and NQO2 were found to belong 1 1 kb 2 3 4 5 6 NQO1 NQO1v2 NQO1v3 Figure 1. Gene structure of the human NQO1 gene and two splice variants listed in the Entrez Gene database. Variant 2 has exon 5 missing and variant 3 has exon 4 missing. The complete gene and two variants are also included in the phylogenetic analysis of Figure 2. 330 q HENRY STEWART PUBLICATIONS 1473-9542. HUMAN GENOMICS. VOL 2. NO 5. 329–335 MARCH 2006