Molecular analysis of viable spontaneous and radiation-induced albino (c)-locus mutations in the mouse

Interaction between p22phox and Nox4 in the endoplasmic reticulum suggests a unique mechanism of NADPH oxidase complex formation

The p22^phox protein is an essential component of the phagocytic- and inner ear NADPH oxidases but its relationship to other Nox proteins is less clear. We have studied the role of p22^phox in the TGF-β1-stimulated H₂O₂ production of primary human and murine fibroblasts. TGF-β1 induced H₂O₂ release of the examined cells, and the response was dependent on the expression of both Nox4 and p22^phox. Interestingly, the p22^phox protein was present in the absence of any detectable Nox/Duox expression, and the p22^phox level was unaffected by TGF-β1. On the other hand, Nox4 expression was dependent on the presence of p22^phox, establishing an asymmetrical relationship between the two proteins. Nox4 and p22^phox proteins localized to the endoplasmic reticulum and their distribution was unaffected by TGF-β1. We used a chemically induced protein dimerization method to study the orientation of p22^phox and Nox4 in the endoplasmic reticulum membrane. This technique is based on the rapamycin-mediated heterodimerization of the mammalian FRB domain with the FK506 binding protein. The results of these experiments suggest that the enzyme complex produces H₂O₂ into the lumen of the endoplasmic reticulum, indicating that Nox4 contributes to the development of the oxidative milieu within this organelle.

Genome-Wide Deletion Screening with the Array CGH Method in Mouse Offspring Derived from Irradiated Spermatogonia Indicates that Mutagenic Responses are Highly Variable among Genes

Until the end of the 20th century, mouse germ cell data on induced mutation rates, which were collected using classical genetic methods at preselected specific loci, provided the principal basis for estimates of genetic risks from radiation in humans. The work reported on here is an extension of earlier efforts in this area using molecular methods. It focuses on validating the use of array comparative genomic hybridization (array CGH) methods for identifying radiation-induced copy number variants (CNVs) and specifically for DNA deletions. The emphasis on deletions stems from the view that it constitutes the predominant type of radiation-induced genetic damage, which is relevant for estimating genetic risks in humans. In the current study, deletion mutations were screened in the genomes of F1 mice born to unirradiated or 4 Gy irradiated sires at the spermatogonia stage (100 offspring each). The array CGH analysis was performed using a "2M array" with over 2 million probes with a mean interprobe distance of approximately 1 kb. The results provide evidence of five molecularly-confirmed paternally-derived deletions in the irradiated group (5/100) and one in the controls (1/100). These data support a calculation, which estimates that the mutation rate is 1 × 10^-2/Gy per genome for induced deletions; this is much lower than would be expected if one assumes that the specific locus rate of 1 × 10^-5/locus per Gy (at 34 loci) is applicable to other genes in the genome. The low observed rate of induced deletions suggests that the effective number of genes/genomic regions at which recoverable deletions could be induced would be only approximately 1,000. This estimate is far lower than expected from the size of the mouse genome (>20,000 genes). Such a discrepancy between observation and expectation can occur if the genome contains numerous genes that are far less sensitive to radiation-induced deletions, if many deletion-bearing offspring are not viable or if the current method is substandard for detecting small deletions.

Ionizing radiation and genetic risks. XVII. Formation mechanisms underlying naturally occurring DNA deletions in the human genome and their potential relevance for bridging the gap between induced DNA double-strand breaks and deletions in irradiated germ cells

While much is known about radiation-induced DNA double-strand breaks (DSBs) and their repair, the question of how deletions of different sizes arise as a result of the processing of DSBs by the cell's repair systems has not been fully answered. In order to bridge this gap between DSBs and deletions, we critically reviewed published data on mechanisms pertaining to: (a) repair of DNA DSBs (from basic studies in this area); (b) formation of naturally occurring structural variation (SV) - especially of deletions - in the human genome (from genomic studies) and (c) radiation-induced mutations and structural chromosomal aberrations in mammalian somatic cells (from radiation mutagenesis and radiation cytogenetic studies). The specific aim was to assess the relative importance of the postulated mechanisms in generating deletions in the human genome and examine whether empirical data on radiation-induced deletions in mouse germ cells are consistent with predictions of these mechanisms. The mechanisms include (a) NHEJ, a DSB repair process that does not require any homology and which functions in all stages of the cell cycle (and is of particular relevance in G0/G1); (b) MMEJ, also a DSB repair process but which requires microhomology and which presumably functions in all cell cycle stages; (c) NAHR, a recombination-based DSB repair mechanism which operates in prophase I of meiosis in germ cells; (d) MMBIR, a microhomology-mediated, replication-based mechanism which operates in the S phase of the cell cycle, and (e) strand slippage during replication (involved in the origin of small insertions and deletions (INDELs). Our analysis permits the inference that, between them, these five mechanisms can explain nearly all naturally occurring deletions of different sizes identified in the human genome, NAHR and MMBIR being potentially more versatile in this regard. With respect to radiation-induced deletions, the basic studies suggest that those arising as a result of the operation of NHEJ/MMEJ processes, as currently formulated, are expected to be relatively small. However, data on induced mutations in mouse spermatogonial stem cells (irradiation in G0/G1 phase of the cell cycle and DSB repair presumed to be via NHEJ predominantly) show that most are associated with deletions of different sizes, some in the megabase range. There is thus a 'discrepancy' between what the basic studies suggest and the empirical observations in mutagenesis studies. This discrepancy, however, is only an apparent but not a real one. It can be resolved by considering the issue of deletions in the broader context of and in conjunction with the organization of chromatin in chromosomes and nuclear architecture, the conceptual framework for which already exists in studies carried out during the past fifteen years or so. In this paper, we specifically hypothesize that repair of DSBs induced in chromatin loops may offer a basis to explain the induction of deletions of different sizes and suggest an approach to test the hypothesis. We emphasize that the bridging of the gap between induced DSB and resulting deletions of different sizes is critical for current efforts in computational modeling of genetic risks.

The effect of dose rate on the frequency of specific-locus mutations induced in mouse spermatogonia is restricted to larger lesions; a retrospective analysis of historical data

A series of 19 large-scale germ-cell mutagenesis experiments conducted several decades ago led to the conclusion that low-LET radiation delivered to mouse spermatogonia at dose rates of 0.8 R/min and below induced only about one-third as many specific-locus mutations as did single, acute exposures at 24 R/min and above. A two-hit origin of the mutations was deemed unlikely in view of the then prevailing evidence for the small size of genetic lesions in spermatogonia. Instead, the dose-rate effect was hypothesized to be the result of a repair system that exists in spermatogonia, but not in more mature male reproductive cells. More recent genetic and molecular studies on the marker genes have identified the phenotypes associated with specific states of the mutant chromosomes, and it is now possible retrospectively to classify individual past mutations as "large lesions" or "other lesions". The mutation-frequency difference between high and low dose rates is restricted to the large lesion mutations, for which the dose-curve slopes differ by a factor exceeding 3.4. For other lesion mutations, there is essentially no difference between the slopes for protracted and acute irradiations; induced other lesions frequencies per unit dose remain similar for dose rates ranging over more than 7 orders of magnitude. For large lesions, these values rise sharply at dose rates >0.8 R/min, though they remain similar within the whole range of protracted doses, failing to provide evidence for a threshold dose rate. The downward bend at high doses that had been noted for X-ray-induced specific-locus mutations as a whole and ascribed to a positive correlation between spermatogonial death and mutation load is now found to be restricted to large lesion mutations. There is a marked difference between the mutation spectra (distributions among the seven loci) for large lesions and other lesions. Within each class, however, the spectra are similar for acute and protracted irradiation.

Chromosome aberrations after high-dose 131I and 99mTc-MIBI administration using a micronucleus assay

OBJECTIVE

We evaluated the potential detrimental cytogenetic effects of Tc-methoxyisobutyl isonitrile (MIBI) and I on patients who were exposed to the radiopharmaceutics for cardiac imaging or thyroid cancer therapy, respectively.

METHODS

Mononuclear leukocytes were isolated both before and after radiopharmaceutical administration and subsequently cultured. Micronuclei frequency was then assessed and microscopic evaluation of apoptosis was conducted.

RESULTS

Small statistically insignificant augmentation in the percentage of micronuclei from 10.9+/-3.8 to 11.3+/-2.4% was observed in the Tc-MIBI group. In contrast, I elicited a notable augmentation of micronuclei from 6.3+/-2.2 to 9.6+/-3.1 at 3.7 GBq, and 6+/-1.5 to 9.2+/-2.7 at 5.55 GBq (P<0.05).

CONCLUSION

Our results showed that there were no remarkable alterations either in the micronuclei incidence or in the percentage of apoptotic lymphocytes after in-vivo exposure to radiopharmaceutical imaging, which provides evidence to reduce the growing concern about the safety issue of cardiac imaging with Tc-MIBI, whereas the deleterious effects of I must be considered when it is applied to thyroid cancer treatment.

Mechanisms of spatial and temporal development of autoimmune vitiligo in tyrosinase-specific TCR transgenic mice

Generalized vitiligo is thought to have an autoimmune etiology and has been correlated with the presence of CD8 T cells specific for melanocyte differentiation Ag. However, limited animal models for the disease have hampered its understanding. Thus, we generated TCR transgenic mice that recognize an epitope of the melanocyte protein, tyrosinase. These animals develop vitiligo with strikingly similar characteristics to the human disease. Vitiligo develops temporally and spatially, with juvenile lesions forming bilaterally in head and facial areas, and only arising later in the body of adult animals. Vitiligo is entirely dependent on CD8 T cells, whereas CD4 T cells exert a negative regulatory effect. Importantly, CD8 T cells can be pervasively present in the skin in the steady state without inducing vitiligo in most areas. This points to developmental differences in melanocyte susceptibility and/or immunological effector mechanisms over time, or in different body locations. Disease is strongly dependent on both IFN-gamma and CXCR3, whereas dependence on CCR5 is more limited, and both CCR4 and perforin are dispensable. Genetic ablation of CXCR3 or IFN-gamma also resulted in scarce CD8 T cell infiltration into the skin. Our results identify unexpected complexity in vitiligo development and point toward possible therapeutic interventions.

Strategies and challenges in eliciting immunity to melanoma

The ability of CD8+ T cells to recognize melanoma tumors has led to the development of immunotherapeutic approaches that use the antigens CD8+ T cells recognize. However, clinical response rates have been disappointing. Here we summarize our work to understand the mechanisms of self-tolerance that limit responses to currently utilized antigens and our approach to identify new antigens directly tied to malignancy. We also explore several aspects of the anti-tumor immune response induced by peptide-pulsed dendritic cells (DCs). DCs differentially augment the avidity of recall T cells specific for self-antigens and overcome a process of aberrant CD8+ T-cell differentiation that occurs in tumor-draining lymph nodes. DC migration is constrained by injection route, resulting in immune responses in localized lymphoid tissue, and differential control of tumors depending on their location in the body. We demonstrate that CD8+ T-cell differentiation in different lymphoid compartments alters the expression of homing receptor molecules and leads to the presence of systemic central memory cells. Our studies highlight several issues that must be addressed to improve the efficacy of tumor immunotherapy.

Deletional Self-Tolerance to a Melanocyte/Melanoma Antigen Derived from Tyrosinase Is Mediated by a Radio-Resistant Cell in Peripheral and Mesenteric Lymph Nodes

Self-tolerance to melanocyte differentiation Ags limits the ability to generate therapeutic antimelanoma responses. However, the mechanisms responsible for CD8 T cell tolerance to these Ags are unknown. We have used a newly generated TCR-transgenic mouse to establish the basis of tolerance to one such Ag from tyrosinase. Despite expression of tyrosinase transcripts in the thymus, central deletion does not shape the tyrosinase-specific CD8 T cell repertoire. We demonstrate that this endogenously expressed melanocyte Ag is constitutively presented in both peripheral and mesenteric lymph nodes, leading to abortive activation and deletion of tyrosinase-specific CD8 T cells. Importantly, this Ag is not presented by either radio-sensitive dendritic cells, or by radio-resistant Langerhans cells. Thus, for this endogenous Ag, cross-tolerization does not appear to be an operative mechanism. Instead, we find radioresistant tyrosinase mRNA expression in lymphoid compartments where CD8 T cell deletion occurs. This suggests that direct presentation of tyrosinase by radio-resistant lymph node resident cells is entirely responsible for tolerance to this endogenous melanocyte differentiation Ag.

Comparison of the genetic effects of equimolar doses of ENU and MNU: while the chemicals differ dramatically in their mutagenicity in stem-cell spermatogonia, both elicit very high mutation rates in differentiating spermatogonia

Mutagenic, reproductive, and toxicity effects of two closely related chemicals, ethylnitrosourea (ENU) and methylnitrosourea (MNU), were compared at equimolar and near-equimolar doses in the mouse specific-locus test in a screen of all stages of spermatogenesis and spermiogenesis. In stem-cell spermatogonia (SG), ENU is more than an order of magnitude more mutagenic than MNU. During post-SG stages, both chemicals exhibit high peaks in mutation yield when differentiating spermatogonia (DG) and preleptotene spermatocytes are exposed. The mutation frequency induced by 75mgMNU/kg during this peak interval is, to date, the highest induced by any single-exposure mutagenic treatment - chemical or radiation - that allows survival of the exposed animal and its germ cells, producing an estimated 10 new mutations per genome. There is thus a vast difference between stem cell and differentiating spermatogonia in their sensitivity to MNU, but little difference between these stages in their sensitivity to ENU. During stages following meiotic metaphase, the highest mutation yield is obtained from exposed spermatids, but for both chemicals, that yield is less than one-quarter that obtained from the peak interval. Large-lesion (LL) mutations were induced only in spermatids. Although only a few of the remaining mutations were analyzed molecularly, there is considerable evidence from recent molecular characterizations of the marker genes and their flanking chromosomal regions that most, if not all, mutations induced during the peak-sensitive period did not involve lesions outside the marked loci. Both ENU and MNU treatments of post-SG stages yielded significant numbers of mutants that were recovered as mosaics, with the proportion being higher for ENU than for MNU. Comparing the chemicals for the endpoints studied and additional ones (e.g., chromosome aberrations, toxicity to germ cells and to animals, teratogenicity) revealed that while MNU is generally more effective, the opposite is true when the target cells are SG.

Immunity to melanoma antigens: from self-tolerance to immunotherapy

The development of effective immune therapy for cancer is a central goal of immunologists in the 21st century. Our laboratories have been deeply involved in characterization of the immune response to melanoma and translation of laboratory discoveries into clinical trials. We have identified a cohort of peptide antigens presented by Major Histocompatibility Complex (MHC) molecules on melanoma cells and widely recognized by T cells from melanoma patients. These have been incorporated into peptide-based vaccines that induce CD8(+) and CD4(+) T-cell responses in 80-100% of patients. Major objective clinical tumor regressions have been observed in some patients, and overall survival in vaccinated patients exceeds expected stage-specific survival. New clinical trials will determine the value of combination of melanoma helper peptides (MHP) into multipeptide vaccines targeting CD8 cells. New trials will also evaluate new approaches to modulating the host-tumor relationship and will develop new combination therapies. Parallel investigations in murine models are elucidating the immunobiology of the melanoma-host relationship and addressing issues that are not feasible to approach in human trials. Based on the fact that the largest cohort of melanoma antigens are derived from normal proteins concerned with pigment production, we have evaluated the mechanisms of self-tolerance to tyrosinase (Tyr) and have determined how T cells in an environment of self-tolerance are impacted by immunization. Using peptide-pulsed dendritic cells as immunogens, we have also used the mouse model to establish strategies for quantitative and qualitative enhancement of antitumor immunity. This information creates opportunities for a new generation of therapeutic interventions using cancer vaccines.

The Tyr (albino) locus of the laboratory mouse

The albino mouse was already known in ancient times and was apparently selectively bred in Egypt, China, and Japan. Thus, it is not surprising that the c or albino locus (now the Tyr locus) was among the first used to demonstrate Mendelian inheritance in mammals at the dawn of the past century. This locus is now known to encode tyrosinase, the rate-limiting enzyme in the production of melanin pigment, and the molecular basis of the albino ( Tyr(c)) mutation is known. Here we describe the congenic series of Tyr-locus alleles, from wild type to null ( albino). We compare eye and skin pigmentation phenotypes and the genetic lesions that cause each. We suggest that this panel of congenic mutants contains rich, untapped resources for the study of many questions of basic cell biological interest.

A strategy to study tyrosinase transgenes in mouse melanocytes

BACKGROUND

A number of transgenic mice carrying different deletions in the Locus Control Region (LCR) of the mouse tyrosinase (Tyr) gene have been developed and analysed in our laboratory. We require melanocytes from these mice, to further study, at the cellular level, the effect of these deletions on the expression of the Tyr transgene, without potential interference with or from the endogenous Tyr alleles. It has been previously reported that it is possible to obtain and immortalize melanocyte cell cultures from postnatal mouse skin.

RESULTS

Here, we describe the efforts towards obtaining melanocyte cultures from our Tyr transgenic mice. We have bred our Tyr transgenic mice into Tyr c-32DSD mutant background, lacking the endogenous Tyr locus. In these conditions, we failed to obtain immortalized melanocytes. We decided to include the inactivation of the Ink4a-Arf locus to promote melanocyte immortalisation. For this purpose, we report the segregation of the Ink4a-Arf null allele from the brown (Tyrp1b) mutation in mice. Finally, we found that Ink4a-Arf +/- and Ink4a-Arf -/- melanocytes had undistinguishable tyrosine hydroxylase activities, although the latter showed reduced cellular pigmentation content.

CONCLUSION

The simultaneous presence of precise genomic deletions that include the tyrosinase locus, such as the Tyr c-32DSD allele, the Tyr transgene itself and the inactivated Ink4a-Arf locus in Tyrp1B genetic background appear as the crucial combination to perform forthcoming experiments. We cannot exclude that Ink4a-Arf mutations could affect the melanin biosynthetic pathway. Therefore, subsequent experiments with melanocytes will have to be performed in a normalized genetic background regarding the Ink4a-Arf locus.

Effects of Male Germ-Cell Stage on the Frequency, Nature, and Spectrum of Induced Specific-Locus Mutations in the Mouse

By means of the mouse specific-locus test (SLT) with visible markers, which is capable of detecting intragenic mutations as well as larger lesions, about 20 mutagens have been studied comparatively across arrays of male germ-cell stages. In addition, a very large historical control, accumulated over decades, provides data on spontaneous mutations in males. Each mutagen has a characteristic germ-cell-stage sensitivity pattern. Although most chemicals yield their maximum numbers of mutations following exposure of spermatozoa and late spermatids, mutagens have now been identified that peak in each of the major stages of spermatogenesis and spermiogenesis, including those in which effects on recombination can also be induced. Stem-cell spermatogonia have yielded positive results with only five of 15 mutagenic chemicals. In postspermatogonial stages, all chemicals, as well as radiations, induce primarily large lesions (LL). By contrast, in spermatogonia (either stem-cell or differentiating) all chemicals except one (bleomycin) produce very few such lesions. The spectrum of relative mutation frequencies at the seven loci of the SLT is characteristic for treated germ-cell stage and mutagen. Treatments that induce primarily LL are characterized by a great preponderance of s (Ednrb)-locus mutations (possibly due to a paucity of haplo-insufficient genes in the surrounding region); and those that induce very few, if any, LL by a great preponderance of p-locus mutations. Spontaneous locus-spectra differ from both types of treatment-induced spectra; moreover, there are two distinct types of spontaneous spectra, depending on whether mutations occurred in mitotic cells or during the perigametic interval.

Route of immunization with peptide-pulsed dendritic cells controls the distribution of memory and effector T cells in lymphoid tissues and determines the pattern of regional tumor control

We have established that the route of immunization with peptide-pulsed, activated DC leads to memory CD8⁺ T cells with distinct distributions in lymphoid tissues, which determines the ability to control tumors growing in different body sites. Both intravenous (i.v.) and subcutaneous (s.c.) immunization induced memory T cells in spleen and control of metastatic-like lung tumors. s.c. immunization also induced memory T cells in lymph nodes (LNs), imparting protection against subcutaneously growing tumors. In contrast, i.v. immunization-induced memory was restricted to spleen and failed to impart protective immunity against subcutaneously growing tumors. Memory cell distribution and tumor control were both linked to injection route–dependent localization of DCs in lymphoid compartments. Using peripheral LN–ablated mice, these LNs were shown to be essential for control of subcutaneously growing tumors but not lung metastases; in contrast, using immunized asplenic mice, we found that the spleen is necessary and sufficient for control of lung tumors, but unnecessary for control of subcutaneously growing tumors. These data demonstrate the existence of a previously undescribed population of splenic-resident memory CD8 T cells that are essential for the control of lung metastases. Thus, regional immunity based on memory T cell residence patterns is an important factor in DC-based tumor immunotherapy.

Identification of four highly conserved genes between breakpoint hotspots BP1 and BP2 of the Prader-Willi/Angelman syndromes deletion region that have undergone evolutionary transposition mediated by flanking duplicons

Prader-Willi and Angelman syndromes (PWS and AS) typically result from an approximately 4-Mb deletion of human chromosome 15q11-q13, with clustered breakpoints (BP) at either of two proximal sites (BP1 and BP2) and one distal site (BP3). HERC2 and other duplicons map to these BP regions, with the 2-Mb PWS/AS imprinted domain just distal of BP2. Previously, the presence of genes and their imprinted status have not been examined between BP1 and BP2. Here, we identify two known (CYFIP1 and GCP5) and two novel (NIPA1 and NIPA2) genes in this region in human and their orthologs in mouse chromosome 7C. These genes are expressed from a broad range of tissues and are nonimprinted, as they are expressed in cells derived from normal individuals, patients with PWS or AS, and the corresponding mouse models. However, replication-timing studies in the mouse reveal that they are located in a genomic domain showing asynchronous replication, a feature typically ascribed to monoallelically expressed loci. The novel genes NIPA1 and NIPA2 each encode putative polypeptides with nine transmembrane domains, suggesting function as receptors or as transporters. Phylogenetic analyses show that NIPA1 and NIPA2 are highly conserved in vertebrate species, with ancestral members in invertebrates and plants. Intriguingly, evolutionary studies show conservation of the four-gene cassette between BP1 and BP2 in human, including NIPA1/2, CYFIP1, and GCP5, and proximity to the Herc2 gene in both mouse and Fugu. These observations support a model in which duplications of the HERC2 gene at BP3 in primates first flanked the four-gene cassette, with subsequent transposition of these four unique genes by a HERC2 duplicon-mediated process to form the BP1-BP2 region. Duplicons therefore appear to mediate genomic fluidity in both disease and evolutionary processes.

Antigens derived from melanocyte differentiation proteins: self-tolerance, autoimmunity, and use for cancer immunotherapy

A large set of peptide antigens presented by class I major histocompatibility complex (MHC) molecules on human and murine melanomas and recognized by CD8+ T cells have been defined. These peptides represent attractive candidates for the development of therapeutic and/or prophylactic approaches to treat this cancer. However, the majority of the peptides that are presented by multiple tumors and recognized by T cells from multiple patients arise from proteins that are also expressed in normal melanocytes. It is expected that immune responses to such peptides will be compromised by self-tolerance or, alternatively, that stimulation of effective immune responses will be accompanied by autoimmune vitiligo. In this review, we describe a preclinical model to evaluate these issues and recent data to suggest that tolerance can be overcome to generate effective antitumor responses. This model also allows the rapid and systematic examination of parameters for the effective use of synthetic peptide vaccines.

Molecular and phenotypic analysis of 25 recessive, homozygous-viable alleles at the mouse agouti locus

Agouti is a paracrine-acting, transient antagonist of melanocortin 1 receptors that specifies the subapical band of yellow on otherwise black hairs of the wild-type coat. To better understand both agouti structure/function and the germline damage caused by chemicals and radiation, an allelic series of 25 recessive, homozygous-viable agouti mutations generated in specific-locus tests were characterized. Visual inspection of fur, augmented by quantifiable chemical analysis of hair melanins, suggested four phenotypic categories (mild, moderate, umbrous-like, severe) for the 18 hypomorphs and a single category for the 7 amorphs (null). Molecular analysis indicated protein-coding alterations in 8 hypomorphs and 6 amorphs, with mild-moderate phenotypes correlating with signal peptide or basic domain mutations, and more devastating phenotypes resulting from C-terminal lesions. Ten hypomorphs and one null demonstrated wild-type coding potential, suggesting that they contain mutations elsewhere in the > or = 125-kb agouti locus that either reduce the level or alter the temporal/spatial distribution of agouti transcripts. Beyond the notable contributions to the field of mouse germ cell mutagenesis, analysis of this allelic series illustrates that complete abrogation of agouti function in vivo occurs most often through protein-coding lesions, whereas partial loss of function occurs slightly more frequently at the level of gene expression control.

Untargeted mutation of the maternally derived mouse hypervariable minisatellite allele in F1 mice born to irradiated spermatozoa

Length change mutation at the Ms6hm hypervariable mouse minisatellite locus was analyzed in C57BL/6N x C3H/HeN F(1) mice and the F(1) of the reciprocal cross born to irradiated male parents. Spontaneous mutant frequencies were 8.4% and 9.8% for the paternally derived and maternally derived C3H/HeN alleles, respectively. The mutant frequencies for the paternally derived allele increased to 22% and 19% when the male parents were irradiated with 6 Gy at the postmeiotic spermatozoa stage and the spermatogonia stage, respectively. These increases in the mutant frequency were at least 10 to 100 times higher than those expected from the frequency of hits to the 3- to 4-kb allele, suggesting that the length change mutation at this minisatellite locus was not a targeted event due directly to DNA damage in the region. Further analysis demonstrated that the mutant frequency increased also at the maternally derived C3H/HeN allele to 20% when the male parents were irradiated at the spermatozoa stage. This increase in the maternal allele mutation was not observed in F(1) born to irradiated spermatogonia. The present study suggests that introduction of DNA damage by irradiated sperm triggers genomic instability in zygotes and in embryos of subsequent developmental stages, and this genomic instability induces untargeted mutation in cis at the paternally derived minisatellite allele and in trans at the maternally derived unirradiated allele. Untargeted mutation revealed in the present study defines a previously unnoticed genetic hazard to the maternally derived genome by the paternally introduced DNA damage.

Untargeted mutation of the maternally derived mouse hypervariable minisatellite allele in F1 mice born to irradiated spermatozoa

Length change mutation at the Ms6hm hypervariable mouse minisatellite locus was analyzed in C57BL/6N x C3H/HeN F(1) mice and the F(1) of the reciprocal cross born to irradiated male parents. Spontaneous mutant frequencies were 8.4% and 9.8% for the paternally derived and maternally derived C3H/HeN alleles, respectively. The mutant frequencies for the paternally derived allele increased to 22% and 19% when the male parents were irradiated with 6 Gy at the postmeiotic spermatozoa stage and the spermatogonia stage, respectively. These increases in the mutant frequency were at least 10 to 100 times higher than those expected from the frequency of hits to the 3- to 4-kb allele, suggesting that the length change mutation at this minisatellite locus was not a targeted event due directly to DNA damage in the region. Further analysis demonstrated that the mutant frequency increased also at the maternally derived C3H/HeN allele to 20% when the male parents were irradiated at the spermatozoa stage. This increase in the maternal allele mutation was not observed in F(1) born to irradiated spermatogonia. The present study suggests that introduction of DNA damage by irradiated sperm triggers genomic instability in zygotes and in embryos of subsequent developmental stages, and this genomic instability induces untargeted mutation in cis at the paternally derived minisatellite allele and in trans at the maternally derived unirradiated allele. Untargeted mutation revealed in the present study defines a previously unnoticed genetic hazard to the maternally derived genome by the paternally introduced DNA damage.

Mouse fzd4 maps within a region of chromosome 7 important for thymus and cardiac development

The cardiac neural crest (CNC) plays a central role in development of the thymus gland and cardiovascular system. Through morphological and histological characterization of embryos homozygous for the Del(7)Tyr(c-112K) and Del(7)Tyr(c-3H) albino deletions, we identified abnormalities that are consistent with aberrant development of tissues requiring CNC contributions. The defects include incompletely penetrant heart and great vessel patterning defects and hypoplastic thymus glands. The CNC phenotype is complemented by the partially overlapping deletion Del(7)Tyr(c-23DVT). Combined, these results suggest that a functional region necessary for development of CNC derived tissues is located between the Del(7)Tyr(c-23DVT) and Del(7)Tyr(c-112K) distal deletion breakpoints. This interval encompasses a functional region previously identified as important for juvenile survival (juvenile development and fertility, jdf). Using deletion mapping, we localized the Frizzled4 (Fzd4) gene to the jdf/thymus and cardiac development intervals.

Self-tolerance to the murine homologue of a tyrosinase-derived melanoma antigen: implications for tumor immunotherapy

The human tyrosinase-derived peptide YMDGTMSQV is presented on the surface of human histocompatibility leukocyte antigen (HLA)-A*0201(+) melanomas and has been suggested to be a tumor antigen despite the fact that tyrosinase is also expressed in melanocytes. To gain information about immunoreactivity and self-tolerance to this antigen, we established a model using the murine tyrosinase-derived homologue of this peptide FMDGTMSQV, together with transgenic mice expressing the HLA-A*0201 recombinant molecule AAD. The murine peptide was processed and presented by AAD similarly to its human counterpart. After immunization with recombinant vaccinia virus encoding murine tyrosinase, we detected a robust AAD-restricted cytotoxic T lymphocyte (CTL) response to FMDGTMSQV in AAD transgenic mice in which the entire tyrosinase gene had been deleted by a radiation-induced mutation. A residual response was observed in the AAD(+)tyrosinase(+) mice after activation under certain conditions. At least some of these residual CTLs in AAD(+)tyrosinase(+) mice were of high avidity and induced vitiligo upon adoptive transfer into AAD(+)tyrosinase(+) hosts. Collectively, these data suggest that FMDGTMSQV is naturally processed and presented in vivo, and that this presentation leads to substantial but incomplete self-tolerance. The relevance of this model to an understanding of the human immune response to tyrosinase is discussed.

Ionizing radiation and genetic risks. X. The potential "disease phenotypes" of radiation-induced genetic damage in humans: perspectives from human molecular biology and radiation genetics

Estimates of genetic risks of radiation exposure of humans are traditionally expressed as expected increases in the frequencies of genetic diseases (single-gene, chromosomal and multifactorial) over and above those of naturally-occurring ones in the population. An important assumption in expressing risks in this manner is that gonadal radiation exposures can cause an increase in the frequency of mutations and that this would result in an increase in the frequency of genetic diseases under study. However, despite compelling evidence for radiation-induced mutations in experimental systems, no increases in the frequencies of genetic diseases of concern or other adverse effects (i.e., those which are not formally classified as genetic diseases), have been found in human studies involving parents who have sustained radiation exposures. The known differences between spontaneous mutations that underlie naturally-occurring single-gene diseases and radiation-induced mutations studied in experimental systems now permit us to address and resolve these issues to some extent. The fact that spontaneous mutations (among which are point mutations and DNA deletions generally restricted to the gene) originate through a number of different mechanisms and that the latter are intimately related to the DNA organization of the genes, are now well-documented. Further, spontaneous mutations include those that cause diseases through loss of function as well as gain of function of genes. In contrast, most radiation-induced mutations studied in experimental systems (although identified through the phenotypes of the marker genes) are predominantly multigene deletions which cause loss of function; the recoverability of an induced deletion in a livebirth seems dependent on whether the gene and the genomic region in which it is located can tolerate heterozygosity for the deletion and yet be compatible with viability. In retrospect, the successful mutation test systems (such as the mouse specific locus test) used in radiation studies have involved genes which are non-essential for survival and are also located in genomic regions, likewise non-essential for survival. In contrast, most of the human genes at which induced mutations have been looked for, do not seem to have these attributes. The inference therefore is that the failure to find induced germline mutations in humans is not due to the resistance of human genes to induced mutations but due to the structural and functional constraints associated with their recoverability in livebirths. Since the risk of inducible genetic diseases in humans is estimated using rates of "recovered" mutations in mice, there is a need to introduce appropriate correction factors to bridge the gap between these rates and the rates at which mutations causing diseases are potentially recoverable in humans. Since the whole genome is the "target" for radiation-induced genetic damage, the failure to find increases in the frequencies of specific single-gene diseases of societal concern does not imply that there are no genetic risks of radiation exposures: the problem lies in delineating the phenotypes of recoverable genetic damage that are recognizable in livebirths. Data from studies of naturally-occurring microdeletion syndromes in humans and those from mouse radiation studies are instructive in this regard. They (i) support the view that growth retardation, mental retardation and multisystem developmental abnormalities are likely to be among the quantitatively more important adverse effects of radiation-induced genetic damage than mutations in a few selected genes and (ii) underscore the need to expand the focus in risk estimation from known genetic diseases (as has been the case thus far) to include these induced adverse developmental effects although most of these are not formally classified as "genetic diseases". (ABSTRACT TRUNCATED)

Differential response of mouse male germ-cell stages to radiation-induced specific-locus and dominant mutations

In an attempt to provide a systematic assessment of the frequency and nature of mutations induced in successive stages of spermato- and spermiogenesis, X-irradiated male mice were re-mated at weekly intervals, and large samples of progeny, observed from birth onward, were scored and genetically tested for recessive mutations at seven specific loci and for externally recognizable dominant mutations. Productivity findings provided a rough measure of induced dominant-lethal frequencies. A qualitative assessment of specific-locus mutations (which include deletions and other rearrangements) was made on the basis of homozygosity test results, as well as from information derived from more recent complementation studies and molecular analyses. Both recessive and dominant visibles revealed clear distinctions between spermatogonia and postspermatogonial stages. In addition, differences for both of these endpoints, as well as for presumed dominant lethals, were found among various postspermatogonial stages. It may be concluded that radiation produces its maximum rates of genetic damage in germ-cell stages ranging from midpachytene spermatocytes through early spermatids, a pattern unlike any of those that have been defined for chemicals; further, the frequency peaks for radiation are lower and broader. The difference between post-stem-cell stages overall and stem-cell spermatogonia was smaller than is generally found with chemicals, not only with respect to the frequency but also the nature of mutations.

Generation, identification, and recovery of mouse mutations

Mouse mutations can be generated by a variety of techniques including those that rely on inducing agents such as X rays or chemicals and those that involve genetic manipulations such as in transgene insertions and gene knockouts. Each technique has its advantages and disadvantages. Inducing agents are often more efficient when random mutations in as yet unknown genes are desired. In contrast, genetic manipulations are advantageous when the mutagenesis needs to be targeted to certain genes or regions. Once these mutations are produced, they must be systematically identified and characterized to confirm their distinction from other known mutations and environmental influences. Allelism and linkage tests should be performed. Finally, methods for maintaining these mutations should be applied so that studies of them can be pursued in the most efficient manner.

Type I oculocutaneous albinism associated with a full-length deletion of the tyrosinase gene

Type I oculocutaneous albinism is an autosomal recessive disorder in which the biosynthesis of melanin is reduced or absent in skin, hair, and eyes because of deficient activity of tyrosinase (EC 1.14.18.1). Type I oculocutaneous albinism is caused by mutations in the tyrosinase structural gene, TYR; however, no large TYR gene deletions have been identified previously in humans. Here we report a patient with type IB oculocutaneous albinism who is a compound heterozygote for TYR allele containing a mutation that is likely to affect pre-RNA splicing and a paternally inherited allele in which the TYR gene is completely deleted, the first such allele described to date. Aside from the albinism in the proband, his phenotype and that of his normally pigmented father is otherwise normal, suggesting that this TYR deletion does not involve other functionally important contiguous genes.

Insertional mutagenesis in transgenic mice

Increasing numbers of transgenic mouse lines have resulted in several dozens of mutants created by insertional mutagenesis. The advantages of different vector systems and the problems associated with the analysis of mutations and the cloning of the affected genes are discussed in this review.

Molecular basis of dark-eyed albinism in the mouse

Dark-eyed albino (C44H) is a recessive allele at the mouse albino (c) locus, which encodes tyrosinase (monophenol,L-dopa:oxygen oxidoreductase, EC 1.14.18.1), the key enzyme in melanin synthesis. Similar to type IB oculocutaneous albinism in humans, overall production of pigment is greatly reduced in dark-eyed albino mice and obvious only in the eyes. We have studied the molecular basis of the c44H mutation and show that expression of the tyrosinase gene is not affected. After sequencing tyrosinase cDNA isolated from c44H/c44H homozygotes, we uncovered a single base alteration from wild type leading to a serine-to-isoleucine exchange. The importance of this mutation was demonstrated by generating transgenic mice containing a mutated tyrosinase minigene. This showed that the single base change was sufficient to severely depress pigment production in transgenic mice. We therefore conclude that the point mutation is responsible and sufficient to generate the dark-eyed albino phenotype.

Role of mouse germ-cell mutagenesis in understanding genetic risk and in generating mutations that are prime tools for studies in modern biology

Highlights are presented on (1) the role mouse germ-cell mutagenesis has played in assessing the genetic harm from radiations and chemicals, and (2) the contributions to the field of modern biology that are being made by the products of this research--the propagated mutations. Among the numerous findings in radiation mutagenesis were the humped dose-effect curve for spermatogonial stem cells, the major differences between the sexes and between germ-cell stages of each sex in both yield and nature of mutations, the dose-rate effect, which provided the first evidence for repair of mutational (or premutational) damage, the augmenting effect of certain regimes of dose fractionation, and many others. Chemical mutagenesis studies that followed revealed at least three patterns of mutation yield and demonstrated that germ-cell stage--much more than the nature of the chemical--governs the nature of the DNA lesions induced. Two "supermutagens," one for intragenic mutations and one for deletions and other rearrangements, have become very useful in the manufacture of mutations for specific purposes. The mutations propagated from radiation- and chemical-mutagenesis experiments are providing prime resources for basic studies in genome organization, gene structure, and function. DNA lesions that involve specific loci have made possible increasingly detailed characterization of extensive deletion complexes that facilitate high-intensity physical and functional mapping within them. Numerous loci associated with interesting developmental anomalies have been identified and have become accessible to positional cloning. Several of the genes accessed with the aid of induced mutations (deletions, other rearrangements, and point mutations) are furnishing prime reagents for elucidating human disease conditions.

High-frequency induction of chromosomal rearrangements in mouse germ cells by the chemotherapeutic agent chlorambucil

Recent mutagenesis studies have demonstrated that the chemotherapeutic agent, chlorambucil (CHL), is highly mutagenic in male germ cells of the mouse. Post-meiotic germ cells, and especially early spermatids, are the most sensitive to the cytotoxic and mutagenic effects of this agent. Genetic, cytogenetic and molecular analyses of many induced mutations have shown that, in these germ-cell stages, CHL induces predominantly chromosomal rearrangements (deletions and translocations), and mutation-rate studies show that, in terms of tolerated doses, CHL is perhaps five to ten times more efficient in inducing rearrangements than is radiation exposure. Appropriate breeding protocols, along with knowledge of the advantages and limitations associated with the use of CHL, can be used to expand the current resource of chromosomal rearrangements in the mouse and to provide new phenotype-associated mutations amenable to positional-cloning techniques. The analysis of CHL-induced mutations has also contributed to understanding the factors that affect the yield and nature of chemically induced germline mutations in mammals.

Structural differences between specific-locus mutations induced by different exposure regimes in mouse spermatogonial stem cells

It was first shown by W.L. Russell (1962), and confirmed by him and others, that a 24-h interval between dose fractions (but not shorter or longer ones) elevates the rate of radiation-induced spermatogonial specific-locus mutations to levels considerably above the linear extrapolation made from lower-dose results. We have now analyzed the nature of mutations induced either in previously undisturbed or in "sensitized" spermatogonial stem cells, i.e., those that received a challenging dose of X-rays 24 h following a priming dose. Results are based on molecular studies of a large set of viable albino mutations [using probes derived from the tyrosinase (c) gene and from the regions surrounding c], and on retrospective classifications of mutations at c and two additional loci into LL (large lesions), IG (intragenic mutations), and OL (other lesions), utilizing criteria developed earlier. A significant difference (P = 0.016) was found between previously undisturbed and sensitized stem-cell spermatogonia; the latter have a higher LL/IG ratio, similar to the ratio observed for mutations induced in poststem-cell stages. This finding of a qualitative difference indicates that the additional mutations produced by a 24-h fractionated treatment are the result of the second (challenging) dose. The qualitative difference, further, indicates that the mutation-rate-augmenting effects of 24-h fractionation are not due, merely, to an increase (caused by the priming dose) of a normally responsive component of the spermatogonial population. The finding that the additional mutations that are produced by the challenging dose are primarily large DNA lesions suggests that the nuclear state of sensitized stem-cell spermatogonia may be different from the state of previously undisturbed spermatogonia. This state, which appears to be similar to that of postspermatonial stages, may be conducive to the formation of LLs, even by agents that are not LL inducers in other systems. The results further indicate that the relative paucity of LLs characteristic of treated (previously undisturbed) spermatogonial stem cells is probably not the result of selection against such mutations during subsequent germ-cell development.