N-Ethyl-N-nitrosourea

ENU, also known as N-ethyl-N-nitrosourea (chemical formula C3H7N3O2), is a highly potent mutagen. For a given gene in mice, ENU can induce 1 new mutation in every 700 loci. It is also toxic at high doses. ENU, also known as N-ethyl-N-nitrosourea (chemical formula C3H7N3O2), is a highly potent mutagen. For a given gene in mice, ENU can induce 1 new mutation in every 700 loci. It is also toxic at high doses. The chemical is an alkylating agent, and acts by transferring the ethyl group of ENU to nucleobases (usually thymine) in nucleic acids. Its main targets are the spermatogonial stem cells, from which mature sperm are derived. Bill Russell (1951) created a landmark in the field of mouse genetics by creating a specifically designed mouse strain, the T (test) stock that was used in genetic screens for testing mutagens such as radiations and chemicals. The T-stock mouse harbors 7 recessive, viable mutations affecting easily recognizable traits. At the Oak Ridge National Laboratory, Russell's initial goal was to determine the rate of inheritable gene mutations in the germ line induced by radiations. Thus he decided to use T-stock mice in order to define how often a set of loci could be mutated with radiations. Since the mutations in the T-stock mouse were recessive, the progeny would have a wild type phenotype (as a result of crossing a mutant to a wild type female ). Thus with any progeny carrying a mutation induced by radiation at one of the 7 loci, would exhibit the mutant phenotype in the first generation itself. This approach, the specific locus test (SLT) allowed Russell to study a wide range of specific mutations and to calculate the mutation rates induced by radiations. In addition to studying the effect of radiation for SLT, Russell et al. were also interested in studying the effect of chemical mutagens such as procarbazine and ethylnitrosourea for SLT. At that time, procarbazine was the most potent chemical mutagen known to cause a significant spermatogonial mutagenesis in an SLT, although at a rate one-third of that of X-rays. Russell's earlier mutagenesis work on Drosophila using diethylnitrosoamine (DEN) triggered them to use DEN for the SLT. However, DEN needs to be enzymatically converted into an alkylating agent in order to be mutagenic and probably this enzymatic activation was not sufficient in mammals. This could be illustrated by the extremely low mutation rate in mice given by DEN (3 in 60,179 offspring). To overcome this problem, a new mutagen, N-ethyl N-nitrosourea (ENU), an alkylating agent, which does not need to be metabolised, was suggested to be used by Ekkehart Vegel to Russell et al. The ENU (250 mg/kg) induced mice underwent a period of sterility for 10 weeks. After recovery, 90 males were crossed to the T-stock females and 7584 pups were obtained. Their results showed that a dose of 250 mg/kg of ENU was capable of producing a mutation rate 5 times higher than that obtained with 600R (1R = 2.6 x10^-4 coulombs/kg) of acute X-irradiation. This rate was also 15 times higher to that obtained with procarbazine (600 mg/kg). To overcome the problem of initial period of sterility, the Russell group showed that instead of injecting one large dose of ENU, a fractionated dose (100 mg/kg) on a weekly schedule permitted a total higher dose (300–400 mg/kg) to be tolerated. This further showed that the mutation frequency improved to be 12 times that of X-rays, 36 times that of procarbazine and over 200 times that of spontaneous mutations. When the mutation rate was averaged across all 7 loci, ENU was found to induce mutations at a frequency of one per locus in every 700 gametes. Ever since the discovery of ENU as the most potent mutagen by Russell et al. it has been used in forward (phenotype based) genetic screens with which one can identify and study a phenotype of interest. As illustrated in Figure 1, the screening process begins with mutagenising a male mouse with ENU. This is followed by systematic phenotypic analysis of the progeny. The progeny are assessed for behavioral, physiological or dysmorphological changes. The abnormal phenotype is identified. Identification of the candidate gene is then achieved by positional cloning of the mutant mice with the phenotype of interest. ENU is used as a genetic tool by designing a variety of genetic screens suitable to the researchers' interests.Depending on the region being assessed, forward genetic screens can be classified as illustrated in Figure 2 as: