Efficient infection of brain microvascular endothelial cells by an in vivo-selected neuroinvasive SIVmac variant

A neuroinvasive/neuropathogenic SIV variant termed SIVmac182 was previously isolated and characterized (Watry et al, 1994). This neuroinvasive strain was derived from the uncloned strain SIVmac251 through serial animal passage of infected microglia, unlike previously reported neurovirulent strains. Importantly, the virus described here was isolated from a strain which already demonstrates limited neuroinvasiveness in vivo, through a route of inoculation which exerts selective pressure for variants in the periphery that can naturally cross the blood-brain barrier and gain access to the brain. Examination of animal tissues indicated that the neuroinvasive strain was capable of replicating in brain microvascular endothelial cells (BMEC). Therefore, we developed an in vitro model of BMEC infection in which to examine mechanisms of virus neuroinvasiveness and neuropathogenicity as well as to address mechanisms of HIV-induced dementia. Results obtained with this in vitro system indicate that growth in BMEC may predict neuroinvasiveness in vivo, and furthermore, that brain passage of virus results in the generation of neuroinvasive strains which demonstrate an increased efficiency of BMEC infection in vitro.

Longitudinal analysis of behavioral, neurophysiological, viral and immunological effects of SIV infection in rhesus monkeys

A model is proposed in which a neurovirulent, microglial-passaged, simian immunodeficiency virus (SIV) is used to produce central nervous system (CNS) pathology and behavioral deficits in rhesus monkeys reminiscent of those seen in humans infected with human immunodeficiency virus (HIV). The time course of disease progression was characterized by using functional measures of cognition and motor skill, as well as neurophysiologic monitoring. Concomitant assessment of immunological and virological parameters illustrated correspondence between impaired behavioral performance and viral pathogenesis. Convergent results were obtained from neuropathological findings indicative of significant CNS disease. In ongoing studies, this SIV model is being used to explore the behavioral sequelae of immunodeficiency virus infection, the viral and host factors leading to neurologic dysfunction, and to begin testing potential therapeutic agents.

Brain capillary endothelial cells express MBEC1, a protein that is related to the Clostridium perfringens enterotoxin receptors

Brain capillary endothelial cells compose the blood-brain barrier, which has a crucial role in maintaining the normal extracellular environment of the central nervous system. We have developed a method to isolate endothelial cells from mouse brain and maintain them in a relatively pure primary culture. Using a subtractive hybridization technique, a novel cDNA, termed MBEC1 (mouse brain endothelial cell 1), has been isolated from the cultured mouse brain capillary endothelial cells. MBEC1 is a 1435-bp cDNA predicted to encode a protein of 218 amino acids. The predicted protein is similar to those of the newly characterized Clostridium perfringens enterotoxin receptors and is the mouse homolog of a recently described human cDNA clone, which is hemizygously deleted in individuals with velo-cardio-facial syndrome and DiGeorge syndrome. MBEC1 was expressed in our cultured MBEC, in freshly isolated MBEC, in a variety of mouse organs, and in mouse embryos as early as embryonic Day 7. In situ hybridization and immunocytochemical analyses revealed the presence of the MBEC1 mRNA and its protein product in brain capillary endothelial cells, as well as in a subset of other endothelial and epithelial cells. Moreover, developmental regulation of expression of MBEC1 was present in respiratory epithelium. Our research thus provides a new molecule for further study of the function of normal and abnormal blood-brain barrier as well as of other specialized endothelia and epithelia.

Analysis of the S3 and S3' subsite specificities of feline immunodeficiency virus (FIV) protease: development of a broad-based protease inhibitor efficacious against FIV, SIV, and HIV in vitro and ex vivo

The S3 and S3' subsite binding specificities of HIV and feline immunodeficiency virus proteases (FIV) proteases (PRs) have been explored by using C2-symmetric competitive inhibitors. The inhibitors evaluated contained (1S, 2R, 3R, 4S)-1,4-diamino-1, 4-dibenzyl-2,3-diol as P1 and P1' units, Val as P2 and P2' residues, and a variety of amino acids at the P3 and P3' positions. All inhibitors showed very high potency against HIV PR in vitro, and their Ki values ranged between 1.1 and 2.6 nM. In contrast to the low restriction of P3 and P3' residues observed in HIV PR, FIV PR exhibited strong preference for small hydrophobic groups at the S3 and S3' subsites. Within this series, the most effective inhibitor against FIV PR contained Ala at P3 and P3'. Its Ki of 41 nM was 415- and 170-fold lower than those of the inhibitors without the P3 and P3' moieties or with the Phe at these positions, respectively. In addition, these compounds were tested against mutant FIV PRs, which contain amino acid substitutions corresponding to those in native HIV PR at homologous sites, and their efficacy of inhibition progressively increased up to 5-fold. The most potent FIV PR inhibitor was selected for examination of its effectiveness in tissue culture, and it was able to block nearly 100% of virus production in an acute infection at 1 microg/ml (1.1 microM) against HIV, FIV, and simian immunodeficiency virus. Furthermore, it was not toxic to cells, and even after 2 months of culture there was no sign of resistance development by virus. The findings suggest that inhibitors with small P3 residue may be efficacious against a broad range of HIV variants as well as interspecies PRs.

Simian immunodeficiency virus: a model for neuroAIDS

In addition to its profound effects on the immune system, HIV also infects the CNS and can cause abnormalities in infected individuals ranging from mild cognitive and motor disorders to frank dementia. We have been actively investigating the molecular and cellular mechanisms underlying the CNS manifestations of lentivirus infection through the comparative evaluation of brain pathophysiology under a number of parallel interrelated strategies. Here we describe our ongoing studies with the SIV/rhesus macaque system. We have applied an interdisciplinary multistep approach, utilizing viral, immunological, pathological, behavioral, and electrophysiological techniques to assess disease and study CNS dysfunction induced by SIV. The profile of the infection and the host response, and the resulting cognitive, motor, and neurophysiological abnormalities in SIV-infected monkeys, recapitulates many aspects of the functional impairments associated with HIV-induced CNS disease in humans. Consequently, the SIV model is ideal for examining the mechanisms underlying these functional abnormalities and for testing potential therapeutic agents.

Cortical neuronal cytoskeletal changes associated with FIV infection

HIV-1 infection is often complicated by central nervous system (CNS) dysfunction. Degenerative neuronal changes as well as neuronal loss have been documented in individuals with AIDS. Feline immunodeficiency virus (FIV) infection of cats provides a model for both the immune and the central nervous system manifestations of HIV infection of humans. In this study we have examined neurons in the frontal cortex of feline immunodeficiency virus-infected cats and controls for immunoreactivity with SMI 32, an antibody recognizing a non-phosphorylated epitope on neurofilaments. We noted a significant increase in the number of immunoreactive pyramidal cells in infected animals compared to controls. The changes seen in the neuronal cytoskeleton as a consequence of the inoculation with FIV were similar to those seen in humans undergoing the normal aging process as well as those suffering from neurological diseases, including Alzheimer's and dementia pugilistica. The changes we noted in the feline brain were also similar to that reported in animals with traumatic injuries or with spontaneously occurring or induced motor neuron diseases, suggesting that the increase in reactivity represents a deleterious effect of FIV on the central nervous system.

Neurologic dysfunctions caused by a molecular clone of feline immunodeficiency virus, FIV-PPR

FIV is a lentivirus of domestic cats that causes a spectrum of diseases that is remarkably similar to the clinical syndrome produced by HIV infection in people. Both HIV and FIV has been shown to cause neurologic dysfunction. Specific Pathogen-Free (SPF) cats were placed into one of three groups: FIV-PPR infected; DU-FIV-PPR (a dUTPase mutant of the FIV-PPR clone) infected; or an age-matched control group. In both infected groups, the general clinical signs of infection included lymphadenopathy, oral ulcerations, rough hair coat, and conjuntivitis. Specific neurological changes in the FIV-PPR infected cats included hind limb paresis; delayed righting and pupillary reflexes; behavioral changes; delayed visual and auditory evoked potentials; decreased spinal and peripheral nerve conduction velocities; and marked alterations in sleep patterns. Most of these changes were also observed in the DU-FIV-PPR infected cats. However, these cats tended to have a slightly less severe disease. In this study, we have demonstrated that an infectious molecular clone of FIV closely parallels the disease course of wild type FIV-infected cats. By using a knockout gene mutant of this clone, we were able to demonstrate that the dUTPase gene is not essential for neuropathogenesis. Further use of the FIV-PPR clone should prove useful in determining the essential viral elements that are important in the neuropathogenesis of lentiviral infections.

Microglia-passaged simian immunodeficiency virus induces neurophysiological abnormalities in monkeys

Four rhesus macaques were inoculated intravenously with a cryopreserved stock of microglia obtained from a simian immunodeficiency virus (SIV)-infected rhesus macaque. Before infection, three of the four monkeys were trained and tested daily on a computerized neuropsychological test battery. After SIV infection, behavioral testing continued to monitor deficits associated with disease progression. Five additional age-matched, behaviorally trained monkeys served as controls. Neurophysiological testing for visual and auditory evoked responses was accomplished 37-52 weeks after infection in all monkeys. Subsequently, all four SIV-infected monkeys and one control subject were sacrificed, and samples of brain tissue were taken for pathological analysis. SIV-infected monkeys demonstrated abnormal responses in both auditory and visual evoked responses. In addition, around the time of electrophysiological recording, all three SIV-infected, behaviorally trained monkeys exhibited significant decreases in progressive-ratio performance, reflecting a reduction in reinforcer efficacy. One subject also demonstrated impairments in shifting of attentional set and motor ability at that time. Neuropathological evaluation revealed that all four SIV-infected monkeys exhibited numerous perivascular and parenchymal infiltrating T cells. These findings document that SIV causes electrophysiological, behavioral, and neuropathological sequelae similar to what has been observed in the human neuroAIDS syndrome. Our observations further validate the simian model for the investigation of the pathogenesis of AIDS dementia and for the investigation of drugs with potential therapeutic benefits.

Expression of inflammatory cytokines and inducible nitric oxide synthase in brains of SIV-infected rhesus monkeys: applications to HIV-induced central nervous system disease

BACKGROUND

Human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system (CNS) can lead to severe impairments in cognition, behavior, and motor skills. The mechanism(s) by which HIV-1 induces CNS disease are not well understood. Recent evidence suggests that expression of inducible nitric oxide synthase (iNOS) and nitric oxide (NO) may contribute to HIV-1-induced neurologic disease. We sought to determine if these factors were present in the CNS of rhesus monkeys with simian immunodeficiency virus (SIV)-induced CNS disease.

MATERIALS AND METHODS

Total NO production in cerebral spinal fluid (CSF) from infected monkeys was determined by measuring nitrite (NO2-) and nitrate (NO3-) (stable NO degradation products) utilizing Greiss reagents. In situ hybridization revealed iNOS, interferon-gamma (IFNgamma), and interleukin 1 beta (IL-1 beta) mRNA in the brains of SIV-infected monkeys. Microglia were isolated from animals infected with SIV. Following stimulation with LPS, induction of iNOS mRNA in isolated microglia was analyzed by reverse transcriptase-polymerase chain reaction.

RESULTS

Serial CSF samples from an SIV-infected monkey reveal increased levels of NO2-/NO3-. In situ hybridization demonstrated iNOS, IFN gamma, and IL-1 beta mRNAs in post-mortem brain tissue of SIV-infected monkeys. Furthermore, stimulated microglia from an SIV-infected monkey could produce iNOS mRNA.

CONCLUSIONS

The presence of iNOS in the brain and NO2-/NO3- in the CSF indicates that NO is produced in the CNS of SIV-infected monkeys. The data suggest that iNOS and NO may be contributing to SIV-induced CNS disease.

Serial passage of microglial SIV results in selection of homogeneous env quasispecies in the brain

The pathogenic effects of HIV include infection of the central nervous system (CNS) which can result in cognitive and motor dysfunction. Simian immunodeficiency virus (SIV) infection of rhesus macaques provides an excellent model of HIV-induced disease. We have achieved a reproducible infection of the CNS using a stock of virus obtained by serial passage of microglia-associated SIV. Since the envelope genes of both HIV and SIV encode determinants important in viral pathogenesis, and the variability inherent in these viruses provides a molecular footprint of viral quasispecies, we analyzed the viral env sequences resulting from this serial passage. SIV env sequences were analyzed by direct PCR amplification of DNA isolated from microglia from infected animals. Nucleotide sequence comparison reveals that serial passage of microglia-associated SIV resulted in divergence from the donor stock of virus. Furthermore, an enrichment of unique env quasispecies which is maintained through the serial passage was found in the diseased brains.

Increased mutation frequency of feline immunodeficiency virus lacking functional deoxyuridine-triphosphatase

Feline immunodeficiency virus (FIV) encodes the enzyme deoxyuridine-triphosphatase (DU; EC 3.6.1.23) between the coding regions for reverse transcriptase and integrase in the pol gene. Here, we report the in vivo infection of cats with a DU- variant of the PPR strain of FIV and compare its growth properties and tissue distribution with those of wild-type FIV-PPR. The results reveal several important points: (i) DU- FIV is able to infect the cat, with kinetics similar to that observed with wild-type FIV; (ii) both wild-type and DU- FIV-infected specific-pathogen free cats mount a strong humoral antibody response which is able to limit the virus burden in both groups of animals; (iii) the virus burden is reduced in the DU- FIV-infected cats, particularly in tissues such as spleen and salivary gland; and (iv) the mutation frequency in DU- FIVs integrated in the DNA of primary macrophages after 9 months of infection is approximately 5-fold greater than the frequency observed in DU- FIV DNA integrated in T lymphocytes. Mutation rate with wild-type FIV remains the same in both cell types in vivo. The dominant mutations seen in macrophages with DU- FIV are G-->A base changes, consistent with an increased misincorporation of deoxyuridine into viral DNA of DU- FIVs during reverse transcription. Because this enzyme is absent from human immunodeficiency virus type 1 and other primate lentiviruses, virus replication in cell environments with low DU activity may lead to increased mutation and contribute to the rapid expansion of the viral repertoire.

Simian immunodeficiency virus (SIV)-specific CTL in cerebrospinal fluid and brains of SIV-infected rhesus macaques

Simian immunodeficiency virus (SIV)-specific CD8+ CTL were isolated from blood, cerebrospinal fluid, and brains of rhesus macaques infected i.v. with SIV. CTL were found as early as 1 wk postinfection and their appearance correlated with a decrease of viral Ag (p27) found in the blood. CTL isolated from cerebrospinal fluid and/or brain often recognized different viral proteins than CTL isolated from blood, suggesting either a unique migratory pattern to the central nervous system or a difference in activation/retention of lymphocytes in these compartments.

Simian immunodeficiency virus (SIV)-specific CTL in cerebrospinal fluid and brains of SIV-infected rhesus macaques

Simian immunodeficiency virus (SIV)-specific CD8+ CTL were isolated from blood, cerebrospinal fluid, and brains of rhesus macaques infected i.v. with SIV. CTL were found as early as 1 wk postinfection and their appearance correlated with a decrease of viral Ag (p27) found in the blood. CTL isolated from cerebrospinal fluid and/or brain often recognized different viral proteins than CTL isolated from blood, suggesting either a unique migratory pattern to the central nervous system or a difference in activation/retention of lymphocytes in these compartments.

Transfer of neuropathogenic simian immunodeficiency virus with naturally infected microglia

The central nervous system (CNS) is a target for human immunodeficiency virus infection, and, in individuals with acquired immune deficiency syndrome, this can lead to a devastating dementia. Only certain viral variants appear capable of invading the CNS and infecting microglia and brain macrophages. To determine whether the virus entering the brain may be particularly pathogenic to the CNS, we isolated microglia from the brains of simian immunodeficiency virus-infected rhesus monkeys. Serial transfer of these cells into naive animals indicated that productive simian immunodeficiency virus infection could indeed be transferred. Furthermore, CNS infection occurred within a relatively short time span and was associated with viral gene expression in the brain and pathology characteristic of human immunodeficiency virus encephalitis. While demonstrating that neuropathogenic variants partition into the CNS, our approach will allow the dissection of functional neuropathogenic elements present in these viruses.

Myasthenia gravis-like syndrome induced by expression of interferon gamma in the neuromuscular junction

Abnormal humoral responses toward motor end plate constituents in muscle induce myasthenia gravis (MG). To study the etiology of this disease, and whether it could be induced by host defense molecules, we examined the consequences of interferon (IFN) gamma production within the neuromuscular junction of transgenic mice. The transgenic mice exhibited gradually increasing muscular weakness, flaccid paralysis, and functional disruption of the neuromuscular junction that was reversed after administration of an inhibitor of acetylcholinesterase, features which are strikingly similar to human MG. Furthermore, histological examination revealed infiltration of mononuclear cells and autoantibody deposition at motor end plates. Immunoprecipitation analysis indicated that a previously unidentified 87-kD target antigen was recognized by sera from transgenic mice and also by sera from the majority of human MG patients studied. These results suggest that expression of IFN-gamma at motor end plates provokes an autoimmune humoral response, similar to human MG, thus linking the expression of this factor with development of this disease.

Feline immunodeficiency virus as a model for study of lentivirus infection of the central nervous system

Feline immunodeficiency virus infects the CNS and results in predictable pathophysiology strikingly similar to that seen with HIV-1 infection of humans. The observed pathophysiology is mimicked in several physiologically assessed modalities, further supporting the validity of the feline model. Peripheral and control evoked potential findings and the occurrence of the sleep architecture changes in both cat and human disease provide an intriguing focus for further investigation. Although structurally diverse in an absolute sense, FIV and HIV-1 share basic structural features and commonalities of their life cycle. It is likely that by understanding the common mechanisms by which these lentiviruses influence CNS function, a more complete understanding of the neurological deficits seen in HIV-1 infected patients will be obtained. The cat model is particularly valuable for study of CNS disease, since it allows detailed analyses of events during the acute phase of infection, under circumstances in which the nature and timing of the infection are carefully controlled. The availability of molecular clones for mutational analysis will facilitate mapping of genomic regions critical to the perturbation of CNS function. It is suggested that development of intervention strategies in the cat model will yield treatment modalities directly applicable to HIV-1 infection of humans.

Neurological abnormalities associated with feline immunodeficiency virus infection

Specific pathogen-free cats were infected with the Maryland strain of FIV (FIV-MD) for the purpose of assessing the effects of FIV infection on the central nervous system (CNS). Two separate studies were performed, involving a total of 13 infected cats and six age-matched, sham-inoculated controls. All animals infected with FIV-MD seroconverted by 8 weeks post-infection and virus was recovered from peripheral blood mononuclear cells of all infected cats. All of the infected animals had lower absolute CD4+ cells counts and decreased CD4+/CD8+ ratios. Virus was recovered from the cerebrospinal fluid (CSF) of certain infected individuals, and antiviral antibody and pleocytosis were evident in the CSF of the majority of infected cats. Additionally, virus was recovered from tissue explants from the cerebellum, midbrain and brainstem of one sacrificed FIV+ cat. Specific neurological changes included anisocoria, delayed righting reflex and delayed pupillary reflex, as well as delayed visual and auditory evoked potentials, and marked alterations in sleep patterns similar to those reported for human immunodeficiency virus (HIV)-positive individuals. Histological evaluation revealed the presence of perivascular cuffing and glial nodules in FIV-infected cats. These results indicate that FIV causes an acute neurological disease that closely resembles the early neurological effects of HIV infection in humans and should serve well as an animal model for lentivirus-induced CNS disease.

Androgen treatment prevents diabetes in nonobese diabetic mice

The nonobese diabetic (NOD) mouse strain provides a model system for human autoimmune diabetes. This disease model is extensively used not only to examine the etiology and pathogenesis of diabetes, but also as a means to evaluate therapies. In NOD mice, the disease progresses from insulitis to islet destruction and clinical diabetes in a high percentage of female mice. In this study, androgen therapy, begun after the onset of insulitis, was found to prevent islet destruction and diabetes without eliminating the islet inflammation in female NOD mice. However, diabetes can be adoptively transferred into such hormone-treated recipients. The prevention of disease onset by androgen is likely due to the hormonal alteration of the development or function of the immune cells necessary for islet destruction.

Estrogen regulates the IFN-gamma promoter

The greater immune reactivity of females has been attributed in part to the influence of sex steroid hormones, but the underlying mechanisms are unknown. Here we report evidence that expression of the IFN-gamma gene may be subject to direct hormonal control. In a transient expression assay, the sex steroid 17 beta-estradiol markedly increases activity of the IFN-gamma promoter in lymphoid cells that express the appropriate hormone receptor. This effect is mediated by sequences in the 5'-flanking region of the gene, and can augment the effect of T cell-activating agents. Short term exposure to estradiol also increases IFN-gamma mRNA expression in Con A-treated murine spleen cells. Hormonal regulation of this pleiotropic cytokine may account in part for the ability of estrogen to potentiate many types of immune responses, and for the disproportionate susceptibility of females to autoimmune disease.

Estrogen regulates the IFN-gamma promoter

The greater immune reactivity of females has been attributed in part to the influence of sex steroid hormones, but the underlying mechanisms are unknown. Here we report evidence that expression of the IFN-gamma gene may be subject to direct hormonal control. In a transient expression assay, the sex steroid 17 beta-estradiol markedly increases activity of the IFN-gamma promoter in lymphoid cells that express the appropriate hormone receptor. This effect is mediated by sequences in the 5'-flanking region of the gene, and can augment the effect of T cell-activating agents. Short term exposure to estradiol also increases IFN-gamma mRNA expression in Con A-treated murine spleen cells. Hormonal regulation of this pleiotropic cytokine may account in part for the ability of estrogen to potentiate many types of immune responses, and for the disproportionate susceptibility of females to autoimmune disease.

Cladosporium trichoides cerebral phaeohyphomycosis in a liver transplant recipient. Report of a case

Cerebral phaeohyphomycosis (also referred to as cerebral chromomycosis), one of the diseases caused by the dematiaceous (black) fungi, is most commonly caused by Cladosporium trichoides (referred to by some as Xylohypha bantiana) and is a rare disease, with 31 culture-proven cases reported to date. Although most cases have occurred in immunocompetent hosts, recent experimental evidence suggests that host immunosuppression may predispose patients to the disease. The authors report a case of fatal cerebral phaeohyphomycosis in a liver transplant patient, the first to occur in a transplant patient of any type, to the best of the authors' knowledge. This case provides support for the hypothesis that immunosuppressed patients may be at increased risk for development of this disease.

Detection and typing of human papillomavirus in archival cervical cancer specimens by DNA amplification with consensus primers

We developed a polymerase chain reaction DNA amplification system using two distinct consensus oligonucleotide primer sets for the improved detection and typing of a broad spectrum of human genital papillomavirus (HPV) sequences, including those of novel viruses. The system incorporates one primer set designed to amplify a highly conserved L1 domain and a second primer set designed to amplify a domain within the E6 gene. We used this system to analyze 48 fixed, paraffin-embedded tissue sections (41 specimens from 33 cervical carcinomas, four normal cervical tissues, and several control tissues) for the presence of HPV DNA. HPV sequences were detected in all carcinoma samples and none of the control samples. Hybridization analyses showed that the results obtained with the two amplification schemes concurred completely. This approach allowed rapid confirmation of typing results and may improve the likelihood of detecting a wide variety of HPV sequences, including those of novel HPVs.

Interferon-gamma and the sexual dimorphism of autoimmunity

The sexual difference in the incidence of autoimmune diseases has remained an enigma for many years. In the examination of the induction of autoimmunity in transgenic mice, evidence has been obtained further implicating the lymphokine interferon-gamma in the etiology of autoimmunity. Sex steroid regulation of the production of this molecule, as well as other cytokines, may help explain the gender-specific differences in the immune system, including autoimmunity.

Establishment of embryonic stem cell lines from preimplantation mouse embryos homozygous for lethal mutations in the t-complex

We have determined the frequency at which embryonic stem cell (ESC) lines can be established from inner cell masses (ICMs) isolated from blastocysts homozygous for lethal mutations in the mouse t-complex. Approximately one-third of the expected number, 3/29, of the ESC lines established from embryos obtained by inter-se mating of +/tw18 mice are homozygous for the tw18 haplotype. These tw18/tw18 ESC lines form a variety of cell types in vitro and in vivo, including mesodermal derivatives such as cartilage and muscle. On the basis of these and data from other studies, we suggest that the normal function of the gene represented by the tw18 lethal allele is required for multiplication/survival of mesodermal precursors in the embryo rather than the specification of the mesodermal lineage, and that the lethal effects of this mutation are expressed in only the highly structured environment of the early postimplantation embryo. In studies of the lethal tw5 haplotype, we found that 2/2 ESC lines obtained are mutant homozygotes. Analysis of these data, in conjunction with the results of our earlier study (Magnuson, T., Epstein, C. J., Silver, L. M., and Martin, G. R. (1982), Nature (London) 298, 750-753), suggests that homozygosity for the genes found in the tw5 haplotype does not reduce cell viability. By contrast, 0/16 ESC lines isolated from embryos obtained from matings of +/t0 mice are mutant homozygotes. Analysis of the genotypes of ICM-derived primary stem cell colonies suggests that t0 homozygous ICM cells are unable to undergo sufficient proliferation in vitro to give rise to ESC lines.

Nonhomologous pairing in mice heterozygous for a t haplotype can produce recombinant chromosomes with duplications and deletions

We have investigated the structure and properties of a chromosomal product recovered from a rare recombination event between a t haplotype and a wild-type form of mouse chromosome 17. Our embryological and molecular studies indicate that this chromosome (twLub2) is characterized by both a deletion and duplication of adjacent genetic material. The deletion appears to be responsible for a dominant lethal maternal effect and a recessive embryonic lethality. The duplication provides an explanation for the twLub2 suppression of the dominant T locus phenotype. A reanalysis of previously described results with another chromosome 17 variant called TtOrl indicates a structure for this chromosome that is reciprocal to that observed for twLub2. We have postulated the existence of an inversion over the proximal portion of all complete t haplotypes in order to explain the generation of the partial t haplotypes twLub2 and TtOrl. This proximal inversion and the previously described distal inversion are sufficient to account for all of the recombination properties that are characteristic of complete t haplotypes. The structures determined for twLub2 and TtOrl indicate that rare recombination can occur between nonequivalent genomic sequences within the inverted proximal t region when wild-type and t chromosomes are paired in a linear, nonhomologous configuration.

Molecular probes define different regions of the mouse t complex

Four genomic clones obtained from microdissected fragments of the proximal portion of mouse chromosome 17 have been used to identify a series of t-haplotype-specific restriction fragments. Their specificity is defined by presence in eight complete t haplotypes and absence from 18 inbred strains of wild-type mice. Partial t haplotypes contain subsets of the t-specific fragments, and each can be classified according to the t-specific fragments it contains. This is the first molecular evidence that independent partial t haplotypes contain different lengths of t haplotype DNA. Recombination studies indicate that partial t haplotypes suppress recombination in proportion to the extent of t haplotype DNA they contain. Molecular analysis of partial t haplotypes shows that the t-specific fragments map to and thus define different regions of the t complex. Certain regions of t haplotype DNA defined by t-specific restriction fragments can be correlated with loci involved in the control of transmission ratio distortion.

An alpha globin pseudogene is located within the mouse t complex

Restriction fragment length polymorphisms have been identified in the alpha-globin pseudogene carried on chromosome 17. Different inbred strains of mice carry one of two forms of the sequence, whereas all complete t haplotypes carry a third form. With the use of partial t haplotypes, the alpha-psi 4 sequence has been mapped to the distal end of the t complex. This sequence appears to be closely linked to the tufted locus in the t complex.