Congenie and Coisogenic Strains

Definition and Development

A pair of strains is said to be coisogenic if they differ at only a single genetic locus as a result of a mutation within one branch of the strain. Such strains are useful, because the effects of the mutation can be studied without the complication of genetic segregation in the genetic background. Unfortunately, coisogenic strains cannot be produced to order, as they depend on the mutation occurring within the inbred strain, which is uncontrollable. However, targeted mutations produced by homologous recombination using embryonic stem (ES) cells, which usually have the 129 inbred strain genotype, are basically coisogenic with strain 129 unless they are outcrossed to another strain. Thus, someone who has produced such mice should give careful consideration to maintaining it on the 129 genetic background in spite of its disadvantages. Outcrossing is usually done because strain 129 breeds poorly and is not well characterized. If or when ES cells of other strains become readily available, it will be possible to produce coisogenic strains for any locus, which can be manipulated in this way.

A pair of strains is said to be congenic if it approximates the coisogenic state as a result of backcrossing a gene (or, more strictly, an allele at a particular locus), known as the differential allele, to an inbred strain. Several methods of backcrossing can be used, depending on the mode of inheritance and method of identification of the gene. Basically, a donor strain is mated with the chosen background strain (often C57BL/6, but any strain may also be used depending on the aim of the study) to produce an F1 hybrid, designated N1. These animals are then mated to the inbred strain to produce the first backcross generation, designated N2. This will be segregating for the gene of interest, and carriers of the gene need to be identified. Many transgenes can be identified from a sample of DNA, and in this case, animals that carry the transgene would be selected for further backcrossing. If the gene can only be identified by its phenotypic effect, and if it is recessive, then it will be necessary to breed some of the N2 generation together in order to produce some homozygous mutant animals (the N is not incremented in this case as it does not count as a backcross generation) to continue with the backcrossing program. These procedures are repeated, ideally, until at least the N10 generation has been reached. At this stage, the strain carrying the mutation is said to be congenic with the background strain. If the differential allele can be made homozygous, it can be maintained just like an ordinary inbred strain (as above) with no further backcrossing.

One problem with this procedure is that it takes a lot of time. It is difficult to get even four generations of mice per year, so producing a congenic strain will often take as long as three years. Modern research requires instant results, so people are tempted not to go through the whole process before using the animals. One alternative is to produce "speed congenics."40 From the offspring of the N2 generation onwards, about 20 males tested and known to carry the gene of interest are tested for their genotype at about 80 or more microsatellite genetic markers. The male who has the most alleles of the background strain summed across these loci is then chosen for further backcrossing. He will be mated to several females, as he needs to produce about 80 offspring. Half of these will be females, which will not be used, and half of the males will not carry the desired gene, leaving about 20 offspring to be tested for the next generation, from which the best male is again selected. Such a breeding program will approximately halve the time that it takes to produce a congenic strain, but it is expensive and needs careful organization.

Nomenclature of Coisogenic and Congenic Strains

Coisogenic strains usually have the designation of the background strains followed by a hyphen, then the symbol of the differential allele, shown in italics (the nomenclature of genes is discussed below), e.g., C57BL/6J-Lepob. Where the gene is maintained in a heterozygous state, this is indicated by a + sign, a slash, and the gene symbol: C3H/N- +/W.v

Congenic strains usually have the background strain designation (which is often abbreviated), a period, the donor strain designation, a dash, and the allele designation, e.g., B10.129-_fflz*.. B10 is an abbreviation for C57BL/10, and a full list of such abbreviations is given in the relevant nomenclature Web site. If the donor strain is not inbred or the genetic difference is complex, involving more than one allele, a less complete symbol may be used, such as the background strain and the gene symbols of the differential locus. Note that at present, gene symbols are changing quite frequently as the genes responsible for mutant phenotypes are identified, so the designation of the coisogenic and congenic strains also needs to change.

Research Uses of Congenic Strains

Congenic strains have been used extensively by immunogeneticists to separate the genes responsible for the rejection of allografts, and subsequently, to study the genetics of the major histocompatibility complex without the additional complexity and "noise" created by segregation of the background.41 Many sets of congenic strains, which differ at the MHC, are available. These have been used, for example, to study the effects of this complex locus on response to microorganisms such as the Leishmania parasite42 and carcinogens such as urethane.43 Such strains can be used without the necessity of genotyping individual animals. Many strains congenic for a mutation are also available, such as C57BL/6-Lep^b, the diabetes mutation maintained on the C57BL/6 genetic background.

More recently, geneticists have emphasized the great importance of transferring transgenes to an inbred background.32 The expression of many mutants, knockouts, and transgenes depends on the rest of the genome. On a segregating background, the expression may be variable. Even more seriously, unless the strain is maintained in large numbers like a properly maintained outbred stock (see above), directional selection and genetic drift can drastically alter the expression of the gene over a period of a few generations. The directional selection in this case may be natural selection for reduced expression of the mutant. If the mutant is deleterious, then animals, which express it to a lesser extent, will have a reproductive advantage so that over a period of a few generations the expression may become much less severe. On the other hand, if the mutant or transgene can be backcrossed to an inbred background, then its characteristics will become largely fixed. If resources allow the transgene to be backcrossed to more than one strain, then the extent to which the background affects expression can also be studied. In some cases, genetic modifiers of transgene expression have been mapped.44 Controlling such modifiers may provide a method of controlling the expression of some mutants in animals and in human medicine.45

Maintenance of Congenic Strains

Once a strain is fully congenic, it can be maintained in the same way as any other inbred strain.

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