- Email: [email protected]
Mouse
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Chromosome-Specific Factors Within the above broad picture of CPM, each chromosome has its own highly individual pattern of frequency and behavior, with much detail yet to be elucidated. Of the aneuploidies commonly seen in this form, trisomy 16 CPM is almost exclusively a consequence of correction of errors arising in maternal meiosis I, whereas trisomy 7 CPM is primarily due to mitotic errors in normal conceptions; trisomy 15 CPM orignates through both mechanisms. CPM for trisomies 2 and 3 have an additional complication in that they demonstrate a cell-lineage-specific distribution, with abnormal cells being preferentially encountered in inner cell mass-derived and trophoblast-derived components of chorionic tissue, respectively. The mechanisms underlying these patterns of chromosome specific behavior are not understood. In a small subgroup of pregnancies, mosaicism within the placenta may actually result in greater survival of an abnormal fetus. Studies of trisomy 13 and 18 fetuses reaching term (or late terminations of pregnancy) show a much greater incidence of placental mosaicism, with a normal karyotype in trophoblast cells, than in studies of those pregnancies lost in the first trimester. This indicates that the presence of normal trophoblast cells somehow interferes with the processes by which the maternal system recognizes these as abnormal conceptions. Such effects on enhanced survival are not seen among trisomy 21 pregnancies.
Mosaicism in Preimplantation Embryos Most of our understanding of mosaicism in humans has been deduced from the analysis of diagnostic samples derived from continuing and noncontinuing pregnancies and from liveborns. Cytogenetic analysis of small numbers of nonreturned embryos from IVF programs has allowed some direct observation of aspects of mosaicism and related phenomena; caution should be exercised, however, in extrapolating this to `normal' conceptions. Most information comes from limited fluorescence in situ hybridization (FISH) analysis of interphase cells, which detects three overlapping groups of embryos: those with essentially uniform normal or abnormal karotypes; those with two or more cell lines present; and those with large numbers of cells with diverse karyotypes (chaotic embryos). Data from fully karyotyped blastocyst metaphases support this broad classification, but additionally suggests that the abnormal cells seen, particularly those in chaotic embryos, may have gross structural chromosome errors as well as simple gain or loss of whole chromosomes. Mosaicism for ploidy, notably tetraploidy, is common. In general, although
some of the abnormal karyotypes, e.g., trisomy 16, seen in IVF-derived embryos, are detected at frequencies broadly comparable to those seen in later conceptions, many of them are not. Either cells with more complex abnormalities are being outgrown or excluded, or the embryos themselves are failing to produce on going pregnancies; it is not possible to exclude that some may also be artifacts of the IVF process itself.
Conclusion Mosaicism is remarkably common in early human development. Its clinical effects are well recognized, but the mechanisms behind its origins are poorly understood. See also: Amniocentesis; Nondisjunction; Prenatal Diagnosis; Trisomy; Uniparental Inheritance
Mouse T H Roderick Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0852
All mammals are very similar in genetics, embryology, biochemistry, physiology, anatomy, and even behavior. Therefore, we could choose any mammal as an experimental object for study to understand ourselves better biologically and medically, and, of course, the closer the relationship the better. However, the mouse, although a distant relative of humans, has distinct advantages. It is one of the smallest mammals, weighing little more than 20 g as a young adult, which permits large numbers to be raised and bred efficiently. Mice breed prolifically as young as 40 days old, which enables a few generations a year to be studied. Mice age about 30 times as rapidly as humans so the study of embryogenesis, development, and aging can be studied in a relatively short period of time. The mouse genome comprises 20 pairs of chromosomes; the human genome has 23 pairs. Genes on the chromosomes are arranged in a very similar order in both species. In fact, if the mouse chromosomal set were broken into about 150 specific pieces and rearranged in the correct way, one could just about reconstruct the arrangement of the human genome. The difference in these arrangements of only about 150 pieces seems small considering the two species have common ancestry 65 million years ago. We now know that all mammals share in this great genomic similarity.
M o u s e 1243 Mice have been cohabitants of humans for thousands of years. For over a hundred years, and probably much longer, mouse fanciers have been in the business of selling mice with exotic coat colors and patterns. In the process of living and breeding in a humanmanaged environment, mice were inadvertently selected for tameness. Only within the last 100 years have mice been used seriously for biological research and systematically bred for that purpose. Many of the founders of present-day mouse stocks and strains have their origins in the variety of colored and relatively tame mice that were widely available for sale. More recently, to explore greater genetic variation, strains and stocks have been initiated in laboratories from mice caught in the wild. The use of mice as good genetic, embryological, physiological, developmental, and aging models makes it possible to isolate and examine the various paths of genetics to the development of different diseases.
The Strategy The aim is to find genetic disease in mice that mirrors genetic disease in humans. Presuming a very similar etiology of disease in both species, an observation in one can provide information on the other. Thus, knowing the genetic defect in the mouse and its physiological and developmental consequences, biomedical scientists can devise new ways in which to intervene or alter these defective pathways. Successful intervention or amelioration in the mouse portends the success of the same strategy in humans. The strategy, then, is first to find genetic problems in mice. This can be done by screening and then mating phenotypic deviants. These deviants initially are suspected to be the result of a mutation, which must be confirmed by further breeding of the affected animal with his relatives. Although the mutation rate is low, there are a large number of genes that can mutate, so the appearance of a phenotypic deviant owing to a genetic mutation is not unusual in a sizeable colony. The mutation rate can be enhanced by subjecting the mice to mutagens, such as X-rays. A powerful chemical mutagen, ethyl nitrosourea (ENU), injected subcutaneously has been found to cause a relatively high frequency of point mutations in the male germ cells, thus dramatically increasing the frequency of new mutants for study. When teams of scientists collaborate to examine the offspring of these mice for many different biomedical end points, the result is an effective way in which to increase the numbers of important models. The more we know about genes, the proteins they encode, and the physiological effects of those proteins, the better we can devise schemes to intervene in the
debilitating effects of damaged genes. Certainly a variety of gene therapy techniques now contemplated for humans can be attempted and perfected using homologous or similar mouse models.
Inbred Strains Inbred strains are defined as the product of 20 consecutive generations of brother±sister matings. Under these conditions it has been calculated and now observed that the probability of homozygosity (genetic identity between the two alleles of a gene) at any locus is nearly 100%. Having achieved status as an inbred strain, it receives a name by strictly agreed upon nomenclature rules, and the strain, through the scientific literature, becomes known world-wide for its genetic and phenotypic traits. The strain usually then becomes available to any researcher in the world. Genetically independent strains, i.e., strains independently initiated from different founder populations, are the most likely to be useful in finding phenotypic differences between inbred strains. This is because the strains themselves, by virtue of their distinct origins, have the greatest chance of being genetically different.
Crosses The study of inbred strains is also a powerful method for finding genes that cause important biomedical phenotypes. If animals are raised in the same environment, the phenotypic differences between inbred strains are mainly owing to genetic differences between the strains. Often there are many genes contributing to these differences. To determine the nature and number of the genetic differences, crosses between mice of two strains are made followed by breeding the offspring (called the F1) back to either parental strain (backcross) or by breeding the F1 with another F1 of the same parental origin. These crosses produce offspring (called the F2) in which the intensities of the trait can be quantified and an estimate made of the number of genes causing the trait of interest. If the mice in the backcross or F2 fall into a very few categories, then there are probably very few gene differences causing the variation in the trait. If the mice fall broadly across a continuum, then there are probably several genes involved and possibly a greater relative influence of environmental factors as well. Crossing mice of inbred strains that differ in specific characteristics is also a way of uncovering important genetic effects. Recombinant inbred strains are derived by continual brother±sister matings from independent mated pairs from the F2 of a cross
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between two inbred strains. The resulting set of strains provides a powerful method for genetic analysis of any traits by which the parental strains differ. Congenic and consomic inbred strains are produced by repeatedly backcrossing a gene or a chromosome from one strain onto the background of another. The single donated gene can usually be recognized in the phenotype of the backcrossed mice in making a congenic strain, but the animals must be typed for microsatellite markers or other genetic polymorphisms in making consomics to be sure that the chromosome of interest has not recombined with the new host strain chromosome.
Selection Selection experiments can also be carried out to greatly exaggerate the population mean of any trait with a reasonable heritability, that is, where the phenotypic difference is owing in large part to a genetic etiology. Many such experiments have been attempted. A far greater mean difference for study can probably be achieved by selection than can be found among inbred strains. But one can argue that other techniques, such as choosing specific strain combinations to make recombinant inbred lines, are better. Usually they provide sufficient strain differences, and they offer powerful opportunities for genetic analysis, which includes determining the number and influence of genes affecting the trait and initial mapping of the genes. There are hundreds of good mouse models for human genetic disease. Our experience so far indicates that mouse and human will share most of their symptoms in genetic disease. Some important examples are given below.
Chromosomal Aberrations Humans have various kinds of chromosomal defects and rearrangements. For example, there are trisomies, duplications, aneuploids, translocations, and inversions, all with potential major effects on viability, reproduction, and developmental abnormalities, including physical deformities and mental retardation. Mice have numerous examples of the same conditions, many having been studied to provide insight into the human condition. There is a mouse segmental trisomy, T(16;17)65Dn, that emulates Down syndrome and is currently being widely studied. Of particular importantance to families with these chromosomal problems is information on the probability of recurrence in subsequent pregnancies. Mouse models provide the best material to understand causation and recurrence.
A Specific Disease Analogy Osteoporosis
The mouse has a number of genes that control bone density, a fact originally discovered because differences in this trait were found among inbred strains. The many possible paths through which these genes act to regulate bone density are open to study, understanding, and probable therapy.
Specific Gene Homologies or Analogies Obesity
Obesity is found in mice just as it is in humans. Since about 60% of cases of diabetes, 30% of gall bladder disease, 20% of cardiovascular disease, 10% of musculoskeletal disease, and 2% of cancer is attributed to obesity in humans, exploring its genetic causes in mice is very useful. More than half a dozen mutations have been found in mice that cause obesity by differing physiological actions. One example is the obese gene (Lepob) which has been cloned and is now known to produce a hormone called leptin. An understanding of this gene may permit a medical regimen utilizing leptin to control this aspect of human obesity.
Muscular Dystrophy
The Dmdmdx gene mutation in the mouse causes a muscular dystrophy similar to Duchenne muscular dystrophy in humans. This gene is located on the X chromosome of both the mouse and human; therefore, it usually affects males.
Spinal Cord Injury
Spinal cord injury in humans leading to lower leg and sometimes arm immobility is a great problem in terms of health, emotional burden, and economic cost. A huge research effort has been made in attempting to make spinal cords rejoin and mend, thus restoring function. There are mice that are knockouts for neurotrophins which keep the nervous system intact. These animals are ideal subjects for new treatments or regimens to revive neuron growth and restore function.
Parkinson's Disease
In this mouse model, the dopamine receptor gene, Drd1a, has been subjected to targeted mutation providing a model for Parkinson's disease, schizophrenia, and diseases of addiction to amphetamine, cocaine, and alcohol.
Severe Combined Immunodeficiency
The mouse with severe combined immunodeficiency (Prkdcscid) has a severely weakened immune system, making it difficult for it to fight infections and reject
M o u s e 1245 foreign tissue. These mice can be engrafted with human lymphocytes with the full human immunological power to attack foreign tissue. Thus, the human immunological response to many foreign bodies, such as the HIV virus, can be studied in the laboratory mouse. Furthermore, antibiotics and other drugs can be evaluated relatively easily under these defined experimental conditions.
Heart Disease
The build-up of plaque in the arteries is a major cause of human heart disease. The Apoe deficient mouse shows arterial plaque accumulation as early as 3 months of age even when raised on low fat diets.
Cancer
Mice can develop all the same cancers that humans do. There is a naturally occurring variant the Apc gene in the mouse that causes colon cancer similar to that in humans. An understanding of the malfunction of this gene could lead to ways in which to cure the human genetic condition and also the vast majority of colon cancers caused by environmental factors. A major characteristic of cancer is the uncontrolled growth of cells. The Trp53 gene makes a protein that controls cell division and therefore controls wild tumor growth. The Trp53 targeted mutant mouse has a damaged form of this cancer suppressor gene, making the mutant animal highly susceptible to many different cancers. The model is important in studies of breast and ovarian cancers. Exciting opportunities for a cure lie in immunotherapy, which employs the immune system's ability to recognize `foreign,' including cancerous, tissues and to attack and destroy the tumors. The immunology of the mouse is strikingly similar to that of humans and can easily be manipulated in the mouse.
Juvenile Diabetes
Juvenile diabetes or insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease. Mice of inbred strain NOD develop this disease and are currently being widely studied to identify the diabetes susceptibility genes and the mechanism of the disease. Mice of the TH stock are a good model for adult onset diabetes or noninsulin dependent diabetes (NIDDM).
Cystic Fibrosis
Cystic fibrosis is a very widespread, fatal human genetic disease. A targeted mutation of the mouse Cftr gene causes many of the symptoms of human cystic fibrosis.
Epilepsy
A mouse model exists that shows both major forms of human epilepsy, i.e., petit mal and grand mal. It will be
particularly useful in the study of the petit mal form, which usually occurs in children.
Eye Genetics
There are many genes that cause cataracts in humans, and this high frequency is also found in the mouse, from which it may be concluded that we are dealing with the same spectrum of genes and mutational events in both species. Furthermore, mice can have corneal disorders, glaucoma, and retinal degeneration, which together cause most cases of blindness in human populations. The DBA/2J mouse strain has been found to have several symptoms common in human glaucoma, a leading cause of human blindness. The strain is now widely used to investigate the nature of the development of glaucoma.
Aging
Inbred strains are well known for their different life spans. Some strains die young from specific diseases such as leukemia. But it is more difficult to determine genes that have an effect in extending life span beyond the normal range. Recently, in a selection experiment for life span, a significant association was found for two unlinked genes and longevity. With further elucidation of the effects of these genes, an understanding of the mechanisms for prolonging life will probably be revealed. In addition to genetic models there are hundreds of good tissue and developmental models for study.
Other Models Germ Cells
All germ cells of both male and female mice can be studied histologically or manipulated in vivo in situations impossible or difficult to simulate in humans.
Embryogenesis
The early embryology of the mouse is nearly identical to that of humans. In fact, except for size, it is difficult to distinguish the mouse embryo from the human embryo throughout the first trimester. This means that the potentially thousands of genes that bring the embryo to this stage from a fertilized ovum are doing virtually the same thing in mouse and humans. Therefore, studying any developmental anomaly in the mouse caused by a defective gene can lead us to knowledge of the homologous human condition. Later in embryogenesis one can see the human head enlarge extensively, the mouse head elongate with a snout, the mouse tail elongate, and the digits for toes and fingers differentiating from paws.
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Nevertheless at birth, similarities are still apparent in anatomy and continuing developmental patterns. After birth there are genetic problems associated with the onset of maturity and reproductive functioning. Again the mouse emulates humans in postnatal development and aging, although in the mouse the process is 30 times more rapid. Finally as humans are living longer, we are more commonly finding human genetic disorders that appear in middle age or later in life, such as hemochromatosis, Huntington disease, type II diabetes, glaucoma, and many forms of cancer. Again the study of mice in their rapid transition through these developmental periods into old age is of enormous value.
Advanced Protocols With the large amount of information acquired on mice, several distinct subdisciplines have grown dedicated to the raising, care, and use of mice. The advanced state of these protocols makes the mouse an even more important laboratory asset.
Nomenclature Without a specific nomenclature, it would be impossible for scientists to communicate about specific genes,chromosomalvariants,andstrains.A16-member International Committee on Standardized Genetic Nomenclature for Mice is responsible for ensuring cohesive guidelines for nomenclature. Collaboration with committees overseeing nomenclature of other species is important, especially in the naming of genes shown to be orthologous between species.
Breeding Systems The purpose of a breeding system is to preserve and control the genetic causes of variability in the biological traits of interest. Important considerations are the avoidance of inbreeding, which is more complex than random breeding alone. Other crosses have been designed to manipulate gene and chromosomal transfer from one strain to another permitting several kinds of biological analysis.
Record Keeping and Colony Management One must be able to identify each mouse in a genetically heterogeneous colony. Thus, it is necessary to maintain a perfect association between the mouse and its location, its ancestry, descendants, and relatives and all the biological information acquired on it. Furthermore, animals need an optimally comfortable environment in which to live. Thus, there is considerable
knowledge based on tested protocols of proper care, feeding, sterilizing of feed, providing clean water, and cleaning of equipment. Good physical and comfortable surroundings, along with continual concern for the health and well being of each animal, is essential. A most important concern is the humane treatment of animals in research. Protocols for proper humane handling of mice are in common practice and are continually reviewed and improved.
Cryobiology Now that the freezing of early-stage embryos is virtually routine, it is possible to keep colonies of mice that have great potential for research but which are not at the present time being used. With no adverse effects, the embryos can be thawed, transplanted into pseudopregnant females, and brought to an otherwise normal birth. Frozen embryos can also be used as insurance against loss of strains or stocks that are kept in very small living colonies. The freezing of sperm, which is more advanced in human reproductive science, can now be done with mice, making possible an effective and inexpensive way to preserve specific haplotypes for future use.
Informatics The acquisition and assembly of information in computer-accessible form is greatly advanced for the mouse. The large recent growth in information on the genetics and biology of the laboratory mouse fortunately has advanced with the similar exponential development of the computer and its programing applications. This happy coincidence has made it possible for the information to be immediately made available to researchers in laboratories world-wide. Curators systematically read the scientific literature and put data selectively into a database from which it can be systematically accessed through the internet. Now major databases, all accessible on-line, include genomic sequencing, gene descriptions, genetic and strain nomenclature, experimental mapping data, linkage maps, cytogenetic maps, physical maps, gene homologies among mammals, phenotypes, allelic variants, strain data, and committee reports. Also, there is an index of various types of gene expression during mousedevelopment,whichwillbeincreasinglyimportant as biology proceeds beyond genomic sequencing.
Further Reading
Altman PL and Katz DD (eds) (2000) Inbred and Genetically Defined Strains of Laboratory Animals, Part 1, Mouse and Rat. Bethesda, MD: Federation of American Societies for Experimental Biology.
Mouse , Classical Genetics 1247 Festing MFW (1979) Inbred Strains in Biomedical Research. New York: Oxford University Press. Foster HL, Small JD and Fox JG (eds) (1982) The Mouse in Biomedical Research. New York: Academic Press. Green EL (ed.) (1966) Biology of the Laboratory Mouse, 2nd edn. New York: McGraw-Hill. Green EL (1981) Genetics and Probability in Animal Breeding Experiments. London: Macmillan. Lyon MF, Sohaila R and Brown SDM (eds) (1996) Genetic Variants and Strains of the Laboratory Mouse, 3rd edn. Oxford: Oxford University Press. Morse HC III (ed.) (1978) Origins of Inbred Mice. New York: Academic Press. Mouse Genome Informatics: http://www.informatics.jax.org/ Rugh R (1968) The Mouse, its Reproduction and Development. Minneapolis, MN: Burgess Publishing. Silver LM (1995) Mouse Genetics, Concepts and Applications. New York: Oxford University Press.
See also: Inbred Strain; Little, Clarence; Mammalian Genetics (Mouse Genetics); Mouse, Classical Genetics; Mus musculus
Mouse, Classical Genetics L Silver Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0225
The Early Years of Genetic Analysis Although its significance was not immediately recognized, the first demonstration of linkage in the mouse was published in 1915 by the great twentieth-century geneticist J.B.S. Haldane. What Haldane found was evidence for coupling between mutations at the albino (c) and pink-eyed dilution ( p) loci, which we now know to lie 15 cM apart on chromosome 7. Since that time, the linkage map of the mouse has expanded steadily at a near-exponential pace. During the first 65 years of work on the mouse map, this expansion took place one locus at a time. First, each new mutation had to be bred into a strain with other phenotypic markers. Then further breeding was pursued to determine whether the new mutation showed linkage to any of these other markers. This process had to be repeated with different groups of phenotypic markers until linkage to one other previously mapped marker was established. At this point, further breeding studies could be conducted with additional phenotypic markers from the same linkage group to establish a more refined map position. In the first compendium of mouse genetic data published in The Biology of the Laboratory Mouse in
1941, a total of 24 independent loci were listed, of which 15 could be placed into seven linkage groups containing either two or three loci each; the remaining nine loci were found not to be linked to each other or to any of the seven confirmed linkage groups. By the time the second edition of The Biology of the Laboratory Mouse was published in 1966, the number of mapped loci had grown to 250, and the number of linkage groups had climbed to 19, although in four cases, these included only two or three loci. With the 1989 publication of the second edition of Genetic Variants and Strains of the Laboratory Mouse, 965 loci had been mapped on all 20 recombining chromosomes. However, even at the time that this map was actually prepared for publication (circa late 1987), it was still the case that the vast majority of mapped loci were defined by mutations that had been painstakingly incorporated into the whole genome map through extensive breeding studies.
The Middle Ages: Recombinant Inbred Strains The first important conceptual breakthrough aimed at reducing the time, effort, and animals required to map single loci came with the conceptualization and establishment of recombinant inbred (RI) strains by Donald Bailey and Benjamin Taylor at the Jackson Laboratory. A set of RI strains provides a collection of samples in which recombination events between homologs from two different inbred strains are preserved within the context of new inbred strains. The power of the RI approach is that loci can be mapped relative to each other within the same `cross' even though the analyses themselves may be performed many years apart. Since the RI strains are essentially preformed and immortal, typing a newly defined locus requires only as much time as the typing assay itself. Although the RI mapping approach was extremely powerful in theory, during the first two decades after its appearance, its use was rather limited because of two major problems. First, analysis was only possible with loci present as alternative alleles in the two inbred parental strains used to form each RI set. This ruled out nearly all of the many loci that were defined by gross phenotypic effects. Only a handful of such loci ± primarily those that affect coat color ± were polymorphic among different inbred strains. In fact, in the prerecombinant DNA era, the only other loci that were amenable to RI analysis were those that encoded: (1) polymorphic enzymes (called allozymes or isozymes) that were observed as differentially migrating bands on starch gels processed for the specific enzyme activity under analysis; (2) immunological
Mouse
Chromosome-Specific Factors Within the above broad picture of CPM, each chromosome has its own highly individual pattern of frequency and behavior, with much detail yet to be elucidated. Of the aneuploidies commonly seen in this form, trisomy 16 CPM is almost exclusively a consequence of correction of errors arising in maternal meiosis I, whereas trisomy 7 CPM is primarily due to mitotic errors in normal conceptions; trisomy 15 CPM orignates through both mechanisms. CPM for trisomies 2 and 3 have an additional complication in that they demonstrate a cell-lineage-specific distribution, with abnormal cells being preferentially encountered in inner cell mass-derived and trophoblast-derived components of chorionic tissue, respectively. The mechanisms underlying these patterns of chromosome specific behavior are not understood. In a small subgroup of pregnancies, mosaicism within the placenta may actually result in greater survival of an abnormal fetus. Studies of trisomy 13 and 18 fetuses reaching term (or late terminations of pregnancy) show a much greater incidence of placental mosaicism, with a normal karyotype in trophoblast cells, than in studies of those pregnancies lost in the first trimester. This indicates that the presence of normal trophoblast cells somehow interferes with the processes by which the maternal system recognizes these as abnormal conceptions. Such effects on enhanced survival are not seen among trisomy 21 pregnancies.
Mosaicism in Preimplantation Embryos Most of our understanding of mosaicism in humans has been deduced from the analysis of diagnostic samples derived from continuing and noncontinuing pregnancies and from liveborns. Cytogenetic analysis of small numbers of nonreturned embryos from IVF programs has allowed some direct observation of aspects of mosaicism and related phenomena; caution should be exercised, however, in extrapolating this to `normal' conceptions. Most information comes from limited fluorescence in situ hybridization (FISH) analysis of interphase cells, which detects three overlapping groups of embryos: those with essentially uniform normal or abnormal karotypes; those with two or more cell lines present; and those with large numbers of cells with diverse karyotypes (chaotic embryos). Data from fully karyotyped blastocyst metaphases support this broad classification, but additionally suggests that the abnormal cells seen, particularly those in chaotic embryos, may have gross structural chromosome errors as well as simple gain or loss of whole chromosomes. Mosaicism for ploidy, notably tetraploidy, is common. In general, although
some of the abnormal karyotypes, e.g., trisomy 16, seen in IVF-derived embryos, are detected at frequencies broadly comparable to those seen in later conceptions, many of them are not. Either cells with more complex abnormalities are being outgrown or excluded, or the embryos themselves are failing to produce on going pregnancies; it is not possible to exclude that some may also be artifacts of the IVF process itself.
Conclusion Mosaicism is remarkably common in early human development. Its clinical effects are well recognized, but the mechanisms behind its origins are poorly understood. See also: Amniocentesis; Nondisjunction; Prenatal Diagnosis; Trisomy; Uniparental Inheritance
Mouse T H Roderick Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0852
All mammals are very similar in genetics, embryology, biochemistry, physiology, anatomy, and even behavior. Therefore, we could choose any mammal as an experimental object for study to understand ourselves better biologically and medically, and, of course, the closer the relationship the better. However, the mouse, although a distant relative of humans, has distinct advantages. It is one of the smallest mammals, weighing little more than 20 g as a young adult, which permits large numbers to be raised and bred efficiently. Mice breed prolifically as young as 40 days old, which enables a few generations a year to be studied. Mice age about 30 times as rapidly as humans so the study of embryogenesis, development, and aging can be studied in a relatively short period of time. The mouse genome comprises 20 pairs of chromosomes; the human genome has 23 pairs. Genes on the chromosomes are arranged in a very similar order in both species. In fact, if the mouse chromosomal set were broken into about 150 specific pieces and rearranged in the correct way, one could just about reconstruct the arrangement of the human genome. The difference in these arrangements of only about 150 pieces seems small considering the two species have common ancestry 65 million years ago. We now know that all mammals share in this great genomic similarity.
M o u s e 1243 Mice have been cohabitants of humans for thousands of years. For over a hundred years, and probably much longer, mouse fanciers have been in the business of selling mice with exotic coat colors and patterns. In the process of living and breeding in a humanmanaged environment, mice were inadvertently selected for tameness. Only within the last 100 years have mice been used seriously for biological research and systematically bred for that purpose. Many of the founders of present-day mouse stocks and strains have their origins in the variety of colored and relatively tame mice that were widely available for sale. More recently, to explore greater genetic variation, strains and stocks have been initiated in laboratories from mice caught in the wild. The use of mice as good genetic, embryological, physiological, developmental, and aging models makes it possible to isolate and examine the various paths of genetics to the development of different diseases.
The Strategy The aim is to find genetic disease in mice that mirrors genetic disease in humans. Presuming a very similar etiology of disease in both species, an observation in one can provide information on the other. Thus, knowing the genetic defect in the mouse and its physiological and developmental consequences, biomedical scientists can devise new ways in which to intervene or alter these defective pathways. Successful intervention or amelioration in the mouse portends the success of the same strategy in humans. The strategy, then, is first to find genetic problems in mice. This can be done by screening and then mating phenotypic deviants. These deviants initially are suspected to be the result of a mutation, which must be confirmed by further breeding of the affected animal with his relatives. Although the mutation rate is low, there are a large number of genes that can mutate, so the appearance of a phenotypic deviant owing to a genetic mutation is not unusual in a sizeable colony. The mutation rate can be enhanced by subjecting the mice to mutagens, such as X-rays. A powerful chemical mutagen, ethyl nitrosourea (ENU), injected subcutaneously has been found to cause a relatively high frequency of point mutations in the male germ cells, thus dramatically increasing the frequency of new mutants for study. When teams of scientists collaborate to examine the offspring of these mice for many different biomedical end points, the result is an effective way in which to increase the numbers of important models. The more we know about genes, the proteins they encode, and the physiological effects of those proteins, the better we can devise schemes to intervene in the
debilitating effects of damaged genes. Certainly a variety of gene therapy techniques now contemplated for humans can be attempted and perfected using homologous or similar mouse models.
Inbred Strains Inbred strains are defined as the product of 20 consecutive generations of brother±sister matings. Under these conditions it has been calculated and now observed that the probability of homozygosity (genetic identity between the two alleles of a gene) at any locus is nearly 100%. Having achieved status as an inbred strain, it receives a name by strictly agreed upon nomenclature rules, and the strain, through the scientific literature, becomes known world-wide for its genetic and phenotypic traits. The strain usually then becomes available to any researcher in the world. Genetically independent strains, i.e., strains independently initiated from different founder populations, are the most likely to be useful in finding phenotypic differences between inbred strains. This is because the strains themselves, by virtue of their distinct origins, have the greatest chance of being genetically different.
Crosses The study of inbred strains is also a powerful method for finding genes that cause important biomedical phenotypes. If animals are raised in the same environment, the phenotypic differences between inbred strains are mainly owing to genetic differences between the strains. Often there are many genes contributing to these differences. To determine the nature and number of the genetic differences, crosses between mice of two strains are made followed by breeding the offspring (called the F1) back to either parental strain (backcross) or by breeding the F1 with another F1 of the same parental origin. These crosses produce offspring (called the F2) in which the intensities of the trait can be quantified and an estimate made of the number of genes causing the trait of interest. If the mice in the backcross or F2 fall into a very few categories, then there are probably very few gene differences causing the variation in the trait. If the mice fall broadly across a continuum, then there are probably several genes involved and possibly a greater relative influence of environmental factors as well. Crossing mice of inbred strains that differ in specific characteristics is also a way of uncovering important genetic effects. Recombinant inbred strains are derived by continual brother±sister matings from independent mated pairs from the F2 of a cross
1244
Mouse
between two inbred strains. The resulting set of strains provides a powerful method for genetic analysis of any traits by which the parental strains differ. Congenic and consomic inbred strains are produced by repeatedly backcrossing a gene or a chromosome from one strain onto the background of another. The single donated gene can usually be recognized in the phenotype of the backcrossed mice in making a congenic strain, but the animals must be typed for microsatellite markers or other genetic polymorphisms in making consomics to be sure that the chromosome of interest has not recombined with the new host strain chromosome.
Selection Selection experiments can also be carried out to greatly exaggerate the population mean of any trait with a reasonable heritability, that is, where the phenotypic difference is owing in large part to a genetic etiology. Many such experiments have been attempted. A far greater mean difference for study can probably be achieved by selection than can be found among inbred strains. But one can argue that other techniques, such as choosing specific strain combinations to make recombinant inbred lines, are better. Usually they provide sufficient strain differences, and they offer powerful opportunities for genetic analysis, which includes determining the number and influence of genes affecting the trait and initial mapping of the genes. There are hundreds of good mouse models for human genetic disease. Our experience so far indicates that mouse and human will share most of their symptoms in genetic disease. Some important examples are given below.
Chromosomal Aberrations Humans have various kinds of chromosomal defects and rearrangements. For example, there are trisomies, duplications, aneuploids, translocations, and inversions, all with potential major effects on viability, reproduction, and developmental abnormalities, including physical deformities and mental retardation. Mice have numerous examples of the same conditions, many having been studied to provide insight into the human condition. There is a mouse segmental trisomy, T(16;17)65Dn, that emulates Down syndrome and is currently being widely studied. Of particular importantance to families with these chromosomal problems is information on the probability of recurrence in subsequent pregnancies. Mouse models provide the best material to understand causation and recurrence.
A Specific Disease Analogy Osteoporosis
The mouse has a number of genes that control bone density, a fact originally discovered because differences in this trait were found among inbred strains. The many possible paths through which these genes act to regulate bone density are open to study, understanding, and probable therapy.
Specific Gene Homologies or Analogies Obesity
Obesity is found in mice just as it is in humans. Since about 60% of cases of diabetes, 30% of gall bladder disease, 20% of cardiovascular disease, 10% of musculoskeletal disease, and 2% of cancer is attributed to obesity in humans, exploring its genetic causes in mice is very useful. More than half a dozen mutations have been found in mice that cause obesity by differing physiological actions. One example is the obese gene (Lepob) which has been cloned and is now known to produce a hormone called leptin. An understanding of this gene may permit a medical regimen utilizing leptin to control this aspect of human obesity.
Muscular Dystrophy
The Dmdmdx gene mutation in the mouse causes a muscular dystrophy similar to Duchenne muscular dystrophy in humans. This gene is located on the X chromosome of both the mouse and human; therefore, it usually affects males.
Spinal Cord Injury
Spinal cord injury in humans leading to lower leg and sometimes arm immobility is a great problem in terms of health, emotional burden, and economic cost. A huge research effort has been made in attempting to make spinal cords rejoin and mend, thus restoring function. There are mice that are knockouts for neurotrophins which keep the nervous system intact. These animals are ideal subjects for new treatments or regimens to revive neuron growth and restore function.
Parkinson's Disease
In this mouse model, the dopamine receptor gene, Drd1a, has been subjected to targeted mutation providing a model for Parkinson's disease, schizophrenia, and diseases of addiction to amphetamine, cocaine, and alcohol.
Severe Combined Immunodeficiency
The mouse with severe combined immunodeficiency (Prkdcscid) has a severely weakened immune system, making it difficult for it to fight infections and reject
M o u s e 1245 foreign tissue. These mice can be engrafted with human lymphocytes with the full human immunological power to attack foreign tissue. Thus, the human immunological response to many foreign bodies, such as the HIV virus, can be studied in the laboratory mouse. Furthermore, antibiotics and other drugs can be evaluated relatively easily under these defined experimental conditions.
Heart Disease
The build-up of plaque in the arteries is a major cause of human heart disease. The Apoe deficient mouse shows arterial plaque accumulation as early as 3 months of age even when raised on low fat diets.
Cancer
Mice can develop all the same cancers that humans do. There is a naturally occurring variant the Apc gene in the mouse that causes colon cancer similar to that in humans. An understanding of the malfunction of this gene could lead to ways in which to cure the human genetic condition and also the vast majority of colon cancers caused by environmental factors. A major characteristic of cancer is the uncontrolled growth of cells. The Trp53 gene makes a protein that controls cell division and therefore controls wild tumor growth. The Trp53 targeted mutant mouse has a damaged form of this cancer suppressor gene, making the mutant animal highly susceptible to many different cancers. The model is important in studies of breast and ovarian cancers. Exciting opportunities for a cure lie in immunotherapy, which employs the immune system's ability to recognize `foreign,' including cancerous, tissues and to attack and destroy the tumors. The immunology of the mouse is strikingly similar to that of humans and can easily be manipulated in the mouse.
Juvenile Diabetes
Juvenile diabetes or insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease. Mice of inbred strain NOD develop this disease and are currently being widely studied to identify the diabetes susceptibility genes and the mechanism of the disease. Mice of the TH stock are a good model for adult onset diabetes or noninsulin dependent diabetes (NIDDM).
Cystic Fibrosis
Cystic fibrosis is a very widespread, fatal human genetic disease. A targeted mutation of the mouse Cftr gene causes many of the symptoms of human cystic fibrosis.
Epilepsy
A mouse model exists that shows both major forms of human epilepsy, i.e., petit mal and grand mal. It will be
particularly useful in the study of the petit mal form, which usually occurs in children.
Eye Genetics
There are many genes that cause cataracts in humans, and this high frequency is also found in the mouse, from which it may be concluded that we are dealing with the same spectrum of genes and mutational events in both species. Furthermore, mice can have corneal disorders, glaucoma, and retinal degeneration, which together cause most cases of blindness in human populations. The DBA/2J mouse strain has been found to have several symptoms common in human glaucoma, a leading cause of human blindness. The strain is now widely used to investigate the nature of the development of glaucoma.
Aging
Inbred strains are well known for their different life spans. Some strains die young from specific diseases such as leukemia. But it is more difficult to determine genes that have an effect in extending life span beyond the normal range. Recently, in a selection experiment for life span, a significant association was found for two unlinked genes and longevity. With further elucidation of the effects of these genes, an understanding of the mechanisms for prolonging life will probably be revealed. In addition to genetic models there are hundreds of good tissue and developmental models for study.
Other Models Germ Cells
All germ cells of both male and female mice can be studied histologically or manipulated in vivo in situations impossible or difficult to simulate in humans.
Embryogenesis
The early embryology of the mouse is nearly identical to that of humans. In fact, except for size, it is difficult to distinguish the mouse embryo from the human embryo throughout the first trimester. This means that the potentially thousands of genes that bring the embryo to this stage from a fertilized ovum are doing virtually the same thing in mouse and humans. Therefore, studying any developmental anomaly in the mouse caused by a defective gene can lead us to knowledge of the homologous human condition. Later in embryogenesis one can see the human head enlarge extensively, the mouse head elongate with a snout, the mouse tail elongate, and the digits for toes and fingers differentiating from paws.
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Nevertheless at birth, similarities are still apparent in anatomy and continuing developmental patterns. After birth there are genetic problems associated with the onset of maturity and reproductive functioning. Again the mouse emulates humans in postnatal development and aging, although in the mouse the process is 30 times more rapid. Finally as humans are living longer, we are more commonly finding human genetic disorders that appear in middle age or later in life, such as hemochromatosis, Huntington disease, type II diabetes, glaucoma, and many forms of cancer. Again the study of mice in their rapid transition through these developmental periods into old age is of enormous value.
Advanced Protocols With the large amount of information acquired on mice, several distinct subdisciplines have grown dedicated to the raising, care, and use of mice. The advanced state of these protocols makes the mouse an even more important laboratory asset.
Nomenclature Without a specific nomenclature, it would be impossible for scientists to communicate about specific genes,chromosomalvariants,andstrains.A16-member International Committee on Standardized Genetic Nomenclature for Mice is responsible for ensuring cohesive guidelines for nomenclature. Collaboration with committees overseeing nomenclature of other species is important, especially in the naming of genes shown to be orthologous between species.
Breeding Systems The purpose of a breeding system is to preserve and control the genetic causes of variability in the biological traits of interest. Important considerations are the avoidance of inbreeding, which is more complex than random breeding alone. Other crosses have been designed to manipulate gene and chromosomal transfer from one strain to another permitting several kinds of biological analysis.
Record Keeping and Colony Management One must be able to identify each mouse in a genetically heterogeneous colony. Thus, it is necessary to maintain a perfect association between the mouse and its location, its ancestry, descendants, and relatives and all the biological information acquired on it. Furthermore, animals need an optimally comfortable environment in which to live. Thus, there is considerable
knowledge based on tested protocols of proper care, feeding, sterilizing of feed, providing clean water, and cleaning of equipment. Good physical and comfortable surroundings, along with continual concern for the health and well being of each animal, is essential. A most important concern is the humane treatment of animals in research. Protocols for proper humane handling of mice are in common practice and are continually reviewed and improved.
Cryobiology Now that the freezing of early-stage embryos is virtually routine, it is possible to keep colonies of mice that have great potential for research but which are not at the present time being used. With no adverse effects, the embryos can be thawed, transplanted into pseudopregnant females, and brought to an otherwise normal birth. Frozen embryos can also be used as insurance against loss of strains or stocks that are kept in very small living colonies. The freezing of sperm, which is more advanced in human reproductive science, can now be done with mice, making possible an effective and inexpensive way to preserve specific haplotypes for future use.
Informatics The acquisition and assembly of information in computer-accessible form is greatly advanced for the mouse. The large recent growth in information on the genetics and biology of the laboratory mouse fortunately has advanced with the similar exponential development of the computer and its programing applications. This happy coincidence has made it possible for the information to be immediately made available to researchers in laboratories world-wide. Curators systematically read the scientific literature and put data selectively into a database from which it can be systematically accessed through the internet. Now major databases, all accessible on-line, include genomic sequencing, gene descriptions, genetic and strain nomenclature, experimental mapping data, linkage maps, cytogenetic maps, physical maps, gene homologies among mammals, phenotypes, allelic variants, strain data, and committee reports. Also, there is an index of various types of gene expression during mousedevelopment,whichwillbeincreasinglyimportant as biology proceeds beyond genomic sequencing.
Further Reading
Altman PL and Katz DD (eds) (2000) Inbred and Genetically Defined Strains of Laboratory Animals, Part 1, Mouse and Rat. Bethesda, MD: Federation of American Societies for Experimental Biology.
Mouse , Classical Genetics 1247 Festing MFW (1979) Inbred Strains in Biomedical Research. New York: Oxford University Press. Foster HL, Small JD and Fox JG (eds) (1982) The Mouse in Biomedical Research. New York: Academic Press. Green EL (ed.) (1966) Biology of the Laboratory Mouse, 2nd edn. New York: McGraw-Hill. Green EL (1981) Genetics and Probability in Animal Breeding Experiments. London: Macmillan. Lyon MF, Sohaila R and Brown SDM (eds) (1996) Genetic Variants and Strains of the Laboratory Mouse, 3rd edn. Oxford: Oxford University Press. Morse HC III (ed.) (1978) Origins of Inbred Mice. New York: Academic Press. Mouse Genome Informatics: http://www.informatics.jax.org/ Rugh R (1968) The Mouse, its Reproduction and Development. Minneapolis, MN: Burgess Publishing. Silver LM (1995) Mouse Genetics, Concepts and Applications. New York: Oxford University Press.
See also: Inbred Strain; Little, Clarence; Mammalian Genetics (Mouse Genetics); Mouse, Classical Genetics; Mus musculus
Mouse, Classical Genetics L Silver Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0225
The Early Years of Genetic Analysis Although its significance was not immediately recognized, the first demonstration of linkage in the mouse was published in 1915 by the great twentieth-century geneticist J.B.S. Haldane. What Haldane found was evidence for coupling between mutations at the albino (c) and pink-eyed dilution ( p) loci, which we now know to lie 15 cM apart on chromosome 7. Since that time, the linkage map of the mouse has expanded steadily at a near-exponential pace. During the first 65 years of work on the mouse map, this expansion took place one locus at a time. First, each new mutation had to be bred into a strain with other phenotypic markers. Then further breeding was pursued to determine whether the new mutation showed linkage to any of these other markers. This process had to be repeated with different groups of phenotypic markers until linkage to one other previously mapped marker was established. At this point, further breeding studies could be conducted with additional phenotypic markers from the same linkage group to establish a more refined map position. In the first compendium of mouse genetic data published in The Biology of the Laboratory Mouse in
1941, a total of 24 independent loci were listed, of which 15 could be placed into seven linkage groups containing either two or three loci each; the remaining nine loci were found not to be linked to each other or to any of the seven confirmed linkage groups. By the time the second edition of The Biology of the Laboratory Mouse was published in 1966, the number of mapped loci had grown to 250, and the number of linkage groups had climbed to 19, although in four cases, these included only two or three loci. With the 1989 publication of the second edition of Genetic Variants and Strains of the Laboratory Mouse, 965 loci had been mapped on all 20 recombining chromosomes. However, even at the time that this map was actually prepared for publication (circa late 1987), it was still the case that the vast majority of mapped loci were defined by mutations that had been painstakingly incorporated into the whole genome map through extensive breeding studies.
The Middle Ages: Recombinant Inbred Strains The first important conceptual breakthrough aimed at reducing the time, effort, and animals required to map single loci came with the conceptualization and establishment of recombinant inbred (RI) strains by Donald Bailey and Benjamin Taylor at the Jackson Laboratory. A set of RI strains provides a collection of samples in which recombination events between homologs from two different inbred strains are preserved within the context of new inbred strains. The power of the RI approach is that loci can be mapped relative to each other within the same `cross' even though the analyses themselves may be performed many years apart. Since the RI strains are essentially preformed and immortal, typing a newly defined locus requires only as much time as the typing assay itself. Although the RI mapping approach was extremely powerful in theory, during the first two decades after its appearance, its use was rather limited because of two major problems. First, analysis was only possible with loci present as alternative alleles in the two inbred parental strains used to form each RI set. This ruled out nearly all of the many loci that were defined by gross phenotypic effects. Only a handful of such loci ± primarily those that affect coat color ± were polymorphic among different inbred strains. In fact, in the prerecombinant DNA era, the only other loci that were amenable to RI analysis were those that encoded: (1) polymorphic enzymes (called allozymes or isozymes) that were observed as differentially migrating bands on starch gels processed for the specific enzyme activity under analysis; (2) immunological