ENU mutagenesis reveals a novel phenotype of reduced limb strength in mice lacking fibrillin 2

Genetic models of fibrillinopathies

The fibrillinopathies represent a group of diseases in which the 10-12 nm extracellular microfibrils are disrupted by genetic variants in one of the genes encoding fibrillin molecules, large glycoproteins of the extracellular matrix. The best-known fibrillinopathy is Marfan syndrome, an autosomal dominant condition affecting the cardiovascular, ocular, skeletal, and other systems, with a prevalence of around 1 in 3,000 across all ethnic groups. It is caused by variants of the FBN1 gene, encoding fibrillin-1, which interacts with elastin to provide strength and elasticity to connective tissues. A number of mouse models have been created in an attempt to replicate the human phenotype, although all have limitations. There are also natural bovine models and engineered models in pig and rabbit. Variants in FBN2 encoding fibrillin-2 cause congenital contractural arachnodactyly and mouse models for this condition have also been produced. In most animals, including birds, reptiles, and amphibians, there is a third fibrillin, fibrillin-3 (FBN3 gene) for which the creation of models has been difficult as the gene is degenerate and nonfunctional in mice and rats. Other eukaryotes such as the nematode C. elegans and zebrafish D. rerio have a gene with some homology to fibrillins and models have been used to discover more about the function of this family of proteins. This review looks at the phenotype, inheritance, and relevance of the various animal models for the different fibrillinopathies.

The fibrillinopathies: new insights with focus on the paradigm of opposing phenotypes for both FBN1 and FBN2

Different pathogenic variants in the fibrillin‐1 gene (FBN1) cause Marfan syndrome and acromelic dysplasias. Whereas the musculoskeletal features of Marfan syndrome involve tall stature, arachnodactyly, joint hypermobility, and muscle hypoplasia, acromelic dysplasia patients present with short stature, brachydactyly, stiff joints, and hypermuscularity. Similarly, pathogenic variants in the fibrillin‐2 gene (FBN2) cause either a Marfanoid congenital contractural arachnodactyly or a FBN2‐related acromelic dysplasia that most prominently presents with brachydactyly. The phenotypic and molecular resemblances between both the FBN1 and FBN2‐related disorders suggest that reciprocal pathomechanistic lessons can be learned. In this review, we provide an updated overview and comparison of the phenotypic and mutational spectra of both the “tall” and “short” fibrillinopathies. The future parallel functional study of both FBN1/2‐related disorders will reveal new insights into how pathogenic fibrillin variants differently affect the fibrillin microfibril network and/or growth factor homeostasis in clinically opposite syndromes. This knowledge may eventually be translated into new therapeutic approaches by targeting or modulating the fibrillin microfibril network and/or the signaling pathways under its control.

Extracellular matrix remodelling is associated with muscle force increase in overloaded mouse plantaris muscle

AIMS

Transforming growth factor-β (TGF-β) signalling is thought to contribute to the remodelling of extracellular matrix (ECM) of skeletal muscle and to functional decline in patients with muscular dystrophies. We wanted to determine the role of TGF-β-induced ECM remodelling in dystrophic muscle.

METHODS

We experimentally induced the pathological hallmarks of severe muscular dystrophy by mechanically overloading the plantaris muscle in mice. Furthermore, we determined the role of TGF-β signalling on dystrophic tissue modulation and on muscle function by (i) overloading myostatin knockout (Mstn^-/- ) mice and (ii) by additional pharmacological TGF-β inhibition via halofuginone.

RESULTS

Transcriptome analysis of overloaded muscles revealed upregulation predominantly of genes associated with ECM, inflammation and metalloproteinase activity. Histology revealed in wild-type mice signs of severe muscular dystrophy including myofibres with large variation in size and internalized myonuclei, as well as increased ECM deposition. At the same time, muscle weight had increased by 208% and muscle force by 234%. Myostatin deficiency blunted the effect of overload on muscle mass (59% increase) and force (76% increase), while having no effect on ECM deposition. Concomitant treatment with halofuginone blunted overload-induced muscle hypertrophy and muscle force increase, while reducing ECM deposition and increasing myofibre size.

CONCLUSIONS

ECM remodelling is associated with an increase in muscle mass and force in overload-modelled dystrophic muscle. Lack of myostatin is not advantageous and inhibition of ECM deposition by halofuginone is disadvantageous for muscle plasticity in response to stimuli that induce dystrophic muscle.

LC3-II may mediate ATR-induced mitophagy in dopaminergic neurons through SQSTM1/p62 pathway

Atrazine (2-chloro-4-ethylamino-6-isopropylamine-1,3,5-triazine; ATR) has been demonstrated to regulate autophagy- and apoptosis-related proteins in doparminergic neuronal damage. In our study, we investigated the role of LC3-II in ATR-induced degeneration of dopaminergic neurons. In vivo dopaminergic neuron degeneration model was set up with ATR treatment and confirmed by the behavioral responses and pathological analysis. Dopaminergic neuron cells were transfected with LC3-II siRNA and treated with ATR to observe cell survival and reactive oxygen species release. The process of mitochondrial autophagy and the neurotoxic effects of mitochondrial autophagy were detected by immunofluorescence assay, immunohistochemical analysis, real-time PCR, and western blot analysis. Results showed that after ATR treatment, the grip strength of Wistar rats was significantly decreased, and behavioral signs of anxiety were clearly observed. The mRNA and protein levels of tyrosine hydroxylase, LC3-II, PINK1, and Parkin were significantly decreased in ATR-induced rat dopaminergic neurons and PC-12 cells, while the mRNA expression and protein levels of SQSTM1/p62 and Parl were increased. Exposure to ATR also led to accumulation of autophagic lysosomes and autophagic bodies along with significantly decreased levels of dopaminergic neurons and alterations in mitochondrial homeostasis, which was reversed by LC3-II siRNA. Our results suggest that ATR affects the mitochondria-mediated dopaminergic neuronal death, which may be mediated by LC3-II and other autophagy markers in vivo and in vitro through SQSTM1/p62 signaling pathway.

Optimizing Genomic Methods for Mapping and Identification of Candidate Variants in ENU Mutagenesis Screens Using Inbred Mice

Positional cloning of ENU-induced mutations has traditionally relied on analysis of polymorphic variation between two strains. In contrast, the application of whole-genome sequencing (WGS) has enabled gene discovery in mutant lines maintained on an inbred genetic background. This approach utilizes genetic variation derived from ENU-induced variants for mapping and reduces the likelihood of phenotypic variation, making it an ideal method for genetic modifier screening. Here, we describe the results of such a screen, wherein we determined the minimal number of mutant genomic DNA samples to include in our analyses and improved the sensitivity of our screen by individually barcoding each genomic DNA library. We present several unique cases to illustrate this approach's efficacy, including the discovery of two distinct mutations that generate essentially identical mutant phenotypes, the ascertainment of a non-ENU-induced candidate variant through homozygosity mapping, and an approach for the identification of putative dominant genetic modifiers.

Identification of a Novel Missense FBN2 Mutation in a Chinese Family with Congenital Contractural Arachnodactyly Using Exome Sequencing

Congenital contractural arachnodactyly (CCA, OMIM 121050), also known as Beals-Hecht syndrome, is an autosomal dominant disorder of connective tissue. CCA is characterized by arachnodactyly, dolichostenomelia, pectus deformities, kyphoscoliosis, congenital contractures and a crumpled appearance of the helix of the ear. The aim of this study is to identify the genetic cause of a 4-generation Chinese family of Tujia ethnicity with congenital contractural arachnodactyly by exome sequencing. The clinical features of patients in this family are consistent with CCA. A novel missense mutation, c.3769T>C (p.C1257R), in the fibrillin 2 gene (FBN2) was identified responsible for the genetic cause of our family with CCA. The p.C1257R mutation occurs in the 19th calcium-binding epidermal growth factor-like (cbEGF) domain. The amino acid residue cysteine in this domain is conserved among different species. Our findings suggest that exome sequencing is a powerful tool to discover mutation(s) in CCA. Our results may also provide new insights into the cause and diagnosis of CCA, and may have implications for genetic counseling and clinical management.

Abnormal Activation of BMP Signaling Causes Myopathy in Fbn2 Null Mice

Fibrillins are large extracellular macromolecules that polymerize to form the backbone structure of connective tissue microfibrils. Mutations in the gene for fibrillin-1 cause the Marfan syndrome, while mutations in the gene for fibrillin-2 cause Congenital Contractural Arachnodactyly. Both are autosomal dominant disorders, and both disorders affect musculoskeletal tissues. Here we show that Fbn2 null mice (on a 129/Sv background) are born with reduced muscle mass, abnormal muscle histology, and signs of activated BMP signaling in skeletal muscle. A delay in Myosin Heavy Chain 8, a perinatal myosin, was found in Fbn2 null forelimb muscle tissue, consistent with the notion that muscle defects underlie forelimb contractures in these mice. In addition, white fat accumulated in the forelimbs during the early postnatal period. Adult Fbn2 null mice are already known to demonstrate persistent muscle weakness. Here we measured elevated creatine kinase levels in adult Fbn2 null mice, indicating ongoing cycles of muscle injury. On a C57Bl/6 background, Fbn2 null mice showed severe defects in musculature, leading to neonatal death from respiratory failure. These new findings demonstrate that loss of fibrillin-2 results in phenotypes similar to those found in congenital muscular dystrophies and that FBN2 should be considered as a candidate gene for recessive congenital muscular dystrophy. Both in vivo and in vitro evidence associated muscle abnormalities and accumulation of white fat in Fbn2 null mice with abnormally activated BMP signaling. Genetic rescue of reduced muscle mass and accumulation of white fat in Fbn2 null mice was accomplished by deleting a single allele of Bmp7. In contrast to other reports that activated BMP signaling leads to muscle hypertrophy, our findings demonstrate the exquisite sensitivity of BMP signaling to the fibrillin-2 extracellular environment during early postnatal muscle development. New evidence presented here suggests that fibrillin-2 can sequester BMP complexes in a latent state.

New strategies for treating congenital muscular dystrophies are needed. Current treatments are limited and aim to prolong ambulation and survival. Since most of the genes responsible for congenital muscular dystrophies are still unknown, elucidation of these genes may provide new insights that can lead to novel treatments. Fibrillin-2 null mice are born with myopathy and contractures and demonstrate accumulation of white fat during the early postnatal period. Both the histological features of myopathy and the accumulation of fat are rescued by inhibiting BMP signaling. Results indicate that FBN2 is a candidate gene for congenital muscular dystrophy and that strategies aimed at inhibition of abnormal BMP signaling may be applicable to muscular dystrophies. Furthermore, results reveal the importance of extracellular control of BMP signaling in skeletal muscle.

Gestational and lactational exposure to atrazine via the drinking water causes specific behavioral deficits and selectively alters monoaminergic systems in C57BL/6 mouse dams, juvenile and adult offspring

Atrazine (ATR) is one of the most frequently detected pesticides in the U.S. water supply. This study aimed to investigate neurobehavioral and neurochemical effects of ATR in C57BL/6 mouse offspring and dams exposed to a relatively low (3 mg/l, estimated intake 1.4 mg/kg/day) concentration of ATR via the drinking water (DW) from gestational day 6 to postnatal day (PND) 23. Behavioral tests included open field, pole, grip strength, novel object recognition (NOR), forced swim, and marble burying tests. Maternal weight gain and offspring (PND21, 35, and 70) body or brain weights were not affected by ATR. However, ATR-treated dams exhibited decreased NOR performance and a trend toward hyperactivity. Juvenile offspring (PND35) from ATR-exposed dams were hyperactive (both sexes), spent less time swimming (males), and buried more marbles (females). In adult offspring (PND70), the only behavioral change was a sex-specific (females) decreased NOR performance by ATR. Neurochemically, a trend toward increased striatal dopamine (DA) in dams and a significant increase in juvenile offspring (both sexes) was observed. Additionally, ATR exposure decreased perirhinal cortex serotonin in the adult female offspring. These results suggest that perinatal DW exposure to ATR targets the nigrostriatal DA pathway in dams and, especially, juvenile offspring, alters dams' cognitive performance, induces sex-selective changes involving motor and emotional functions in juvenile offspring, and decreases cognitive ability of adult female offspring, with the latter possibly associated with altered perirhinal cortex serotonin homeostasis. Overall, ATR exposure during gestation and lactation may cause adverse nervous system effects to both offspring and dams.

A trans-acting protein effect causes severe eye malformation in the Mp mouse

Mp is an irradiation-induced mouse mutation associated with microphthalmia, micropinna and hind limb syndactyly. We show that Mp is caused by a 660 kb balanced inversion on chromosome 18 producing reciprocal 3-prime gene fusion events involving Fbn2 and Isoc1. The Isoc1-Fbn2 fusion gene (Isoc1^Mp) mRNA has a frameshift and early stop codon resulting in nonsense mediated decay. Homozygous deletions of Isoc1 do not support a significant developmental role for this gene. The Fbn2-Isoc1 fusion gene (Fbn2^Mp) predicted protein consists of the N-terminal Fibrillin-2 (amino acids 1–2646, exons 1–62) lacking the C-terminal furin-cleavage site with a short out-of-frame extension encoded by the final exon of Isoc1. The Mp limb phenotype is consistent with that reported in Fbn2 null embryos. However, severe eye malformations, a defining feature of Mp, are not seen in Fbn2 null animals. Fibrillin-2^Mp forms large fibrillar structures within the rough endoplasmic reticulum (rER) associated with an unfolded protein response and quantitative mass spectrometry shows a generalised defect in protein secretion in conditioned media from mutant cells. In the embryonic eye Fbn2 is expressed within the peripheral ciliary margin (CM). Mp embryos show reduced canonical Wnt-signalling in the CM – known to be essential for ciliary body development - and show subsequent aplasia of CM-derived structures. We propose that the Mp “worse-than-null” eye phenotype plausibly results from a failure in normal trafficking of proteins that are co-expressed with Fbn2 within the CM. The prediction of similar trans-acting protein effects will be an important challenge in the medical interpretation of human mutations from whole exome sequencing.

With the current increase in large-scale sequencing efforts, correct interpretation of mutation consequences has never been more important. Here, we present evidence for a trans-acting protein effect in a novel mutation of Fbn2, associated with severe developmental eye defects not found in loss of function Fibrillin-2 alleles. The mutant protein is expressed in the developing eye but is unable to exit the cells, instead forming large protein aggregates within the endoplasmic reticulum. We observed ER-stress in mutant eyes, and detected a general reduction to secretion of co-expressed proteins in cell cultures. We propose that similar effects could be caused by mutations to other proteins that are trafficked through the ER, highlighting a disease mechanism that results in different clinical outcomes than observed, or predicted, from loss-off-function alleles.

Ocular phenotype of Fbn2-null mice

PURPOSE

Fibrillin-2 (Fbn2) is the dominant fibrillin isoform expressed during development of the mouse eye. To test its role in morphogenesis, we examined the ocular phenotype of Fbn2(-/-) mice.

METHODS

Ocular morphology was assessed by confocal microscopy using antibodies against microfibril components.

RESULTS

Fbn2(-/-) mice had a high incidence of anterior segment dysgenesis. The iris was the most commonly affected tissue. Complete iridal coloboma was present in 37% of eyes. Dyscoria, corectopia and pseudopolycoria were also common (43% combined incidence). In wild-type (WT) mice, fibrillin-2-rich microfibrils are prominent in the pupillary membrane (PM) during development. In Fbn2-null mice, the absence of Fbn2 was partially compensated for by increased expression of fibrillin-1, although the resulting PM microfibrils were disorganized, compared with WTs. In colobomatous adult Fbn2(-/-) eyes, the PM failed to regress normally, especially beneath the notched region of the iris. Segments of the ciliary body were hypoplastic, and zonular fibers, although relatively plentiful, were unevenly distributed around the lens equator. In regions where the zonular fibers were particularly disturbed, the synchronous differentiation of the underlying lens fiber cells was affected.

CONCLUSIONS

Fbn2 has an indispensable role in ocular morphogenesis in mice. The high incidence of iris coloboma in Fbn2-null animals implies a previously unsuspected role in optic fissure closure. The observation that fiber cell differentiation was disturbed in Fbn2(-/-) mice raises the possibility that the attachment of zonular fibers to the lens surface may help specify the equatorial margin of the lens epithelium.

Genome-wide mouse mutagenesis reveals CD45-mediated T cell function as critical in protective immunity to HSV-1

Herpes simplex encephalitis (HSE) is a lethal neurological disease resulting from infection with Herpes Simplex Virus 1 (HSV-1). Loss-of-function mutations in the UNC93B1, TLR3, TRIF, TRAF3, and TBK1 genes have been associated with a human genetic predisposition to HSE, demonstrating the UNC93B-TLR3-type I IFN pathway as critical in protective immunity to HSV-1. However, the TLR3, UNC93B1, and TRIF mutations exhibit incomplete penetrance and represent only a minority of HSE cases, perhaps reflecting the effects of additional host genetic factors. In order to identify new host genes, proteins and signaling pathways involved in HSV-1 and HSE susceptibility, we have implemented the first genome-wide mutagenesis screen in an in vivo HSV-1 infectious model. One pedigree (named P43) segregated a susceptible trait with a fully penetrant phenotype. Genetic mapping and whole exome sequencing led to the identification of the causative nonsense mutation L3X in the Receptor-type tyrosine-protein phosphatase C gene (Ptprc(L3X)), which encodes for the tyrosine phosphatase CD45. Expression of MCP1, IL-6, MMP3, MMP8, and the ICP4 viral gene were significantly increased in the brain stems of infected Ptprc(L3X) mice accounting for hyper-inflammation and pathological damages caused by viral replication. Ptprc(L3X) mutation drastically affects the early stages of thymocytes development but also the final stage of B cell maturation. Transfer of total splenocytes from heterozygous littermates into Ptprc(L3X) mice resulted in a complete HSV-1 protective effect. Furthermore, T cells were the only cell population to fully restore resistance to HSV-1 in the mutants, an effect that required both the CD4⁺ and CD8⁺ T cells and could be attributed to function of CD4⁺ T helper 1 (Th1) cells in CD8⁺ T cell recruitment to the site of infection. Altogether, these results revealed the CD45-mediated T cell function as potentially critical for infection and viral spread to the brain, and also for subsequent HSE development.

Brain deposition and neurotoxicity of manganese in adult mice exposed via the drinking water

Natural leaching processes and/or anthropogenic contamination can result in ground water concentrations of the essential metal manganese (Mn) that far exceed the current regulatory standards. Neurological consequences of Mn drinking water (DW) overexposure to experimental animals, i.e., mice, including its brain deposition/distribution and behavioral effects are understudied. Adult male C57BL/6 mice were exposed to Mn via the DW for 8 weeks. After 5 weeks of Mn exposure, magnetic resonance imaging revealed significant Mn deposition in all examined brain regions; the degree of Mn deposition did not increase further a week later. Behaviorally, early hyperactivity and more time spent in the center of the arenas in an open field test, decreased forelimb grip strength and less time swimming in a forced swim test were observed after 6 weeks of Mn DW exposure. Eight-week Mn DW exposure did not alter striatal dopamine, its metabolites, or the expression of key dopamine homeostatic proteins, but it significantly increased striatal 5-hydroxyindoleacetic acid (a serotonin metabolite) levels, without affecting the levels of serotonin itself. Increased expression (mRNA) of glial fibrillary acidic protein (GFAP, an astrocyte activation marker), heme oxygenase-1 and inducible nitric oxide synthase (oxidative and nitrosative stress markers, respectively) were observed 8 weeks post-Mn DW exposure in the substantia nigra. Besides mRNA increases, GFAP protein expression was increased in the substantia nigra pars reticulata. In summary, the neurobehavioral deficits, characterized by locomotor and emotional perturbations, and nigral glial activation associated with significant brain Mn deposition are among the early signs of Mn neurotoxicity caused by DW overexposure.

Structure and function of the mammalian fibrillin gene family: implications for human connective tissue diseases

Fibrillins and latent transforming growth factor β binding proteins (LTBPs) are components of the extracellular matrix of connective tissue. While fibrillins are integral to the 10nm microfibrils, and often associated with elastin, all family members are likely to have an additional role in regulating the bioavailability of transforming growth factor β (TGBβ). Both fibrillins and LTBPs are large glycoproteins, containing a series of calcium binding epidermal growth factor domains as well as a number of copies of a unique 8 cysteine domain found only in this protein superfamily. There are three mammalian fibrillins and four LTBPs. Fibrillin monomers link head to tail in microfibrils which can then form two and three dimensional structures. In some tissues elastin is recruited to the fibrillin microfibrils to provide elasticity to the tissue. LTBPs are part of the TGBβ large latent complex which sequesters TGBβ in the extracellular matrix. Fibrillin-1 appears to bind to LTBPs to assist in this process and is thus involved in regulating the bioavailability of TGBβ. Mutation of fibrillin genes results in connective tissue phenotypes which reflect both the increased level of active TGBβ and the structural failure of the extracellular matrix due to the absence or abnormality of fibrillin protein. Fibrillinopathies include Marfan syndrome, familial ectopia lentis, familial thoracic aneurysm (mutations of FBN1) and congenital contractural arachnodactyly (mutation of FBN2). There are no diseases currently associated with mutation of FBN3 in humans, and this gene is no longer active in rodents. Expression patterns of fibrillin genes are consistent with their role in extracellular matrix structure of connective tissue. FBN1 expression is high in most cell types of mesenchymal origin, particularly bone. Human and mouse FBN2 expression is high in fetal cells and has more restricted expression in mesenchymal cell types postnatally. FBN3 is expressed early in development (embryonic and fetal tissues) in humans. The fibrillins are thus important in maintaining the structure and integrity of the extracellular matrix and, in combination with their sequence family members the LTBPs, also contribute to the regulation of the TGFβ family of major growth factors.

An N-ethyl-N-nitrosourea (ENU)-induced dominant negative mutation in the JAK3 kinase protects against cerebral malaria

Cerebral malaria (CM) is a lethal neurological complication of malaria. We implemented a genome-wide screen in mutagenized mice to identify host proteins involved in CM pathogenesis and whose inhibition may be of therapeutic value. One pedigree (P48) segregated a resistance trait whose CM-protective effect was fully penetrant, mapped to chromosome 8, and identified by genome sequencing as homozygosity for a mis-sense mutation (W81R) in the FERM domain of Janus-associated kinase 3 (Jak3). The causative effect of Jak3(W81R) was verified by complementation testing in Jak3(W81R/-) double heterozygotes that were fully protected against CM. Jak3(W81R) homozygotes showed defects in thymic development with depletion of CD8(+) T cell, B cell, and NK cell compartments, and defective T cell-dependent production of IFN-γ. Adoptive transfer of normal splenocytes abrogates CM resistance in Jak3(W81R) homozygotes, an effect attributed to the CD8(+) T cells. Jak3(W81R) behaves as a dominant negative variant, with significant CM resistance of Jak3(W81R/+) heterozygotes, compared to CM-susceptible Jak3(+/+) and Jak3(+/-) controls. CM resistance in Jak3(W81R/+) heterozygotes occurs in presence of normal T, B and NK cell numbers. These findings highlight the pathological role of CD8(+) T cells and Jak3-dependent IFN-γ-mediated Th1 responses in CM pathogenesis.

TB domain proteins: evolutionary insights into the multifaceted roles of fibrillins and LTBPs

Fibrillins and LTBPs [latent TGFβ (transforming growth factor β)-binding proteins] perform vital and complex roles in the extracellular matrix and are relevant to a wide range of human diseases. These proteins share a signature 'eight cysteine' or 'TB (TGFβ-binding protein-like)' domain that is found nowhere else in the human proteome, and which has been shown to mediate a variety of protein-protein interactions. These include covalent binding of the TGFβ propeptide, and RGD-directed interactions with a repertoire of integrins. TB domains are found interspersed with long arrays of EGF (epidermal growth factor)-like domains, which occur more widely in extracellular proteins, and also mediate binding to a large number of proteins and proteoglycans. In the present paper, newly available protein sequence information from a variety of sources is reviewed and related to published findings on the structure and function of fibrillins and LTBPs. These sequences give valuable insight into the evolution of TB domain proteins and suggest that the fibrillin domain organization emerged first, over 600 million years ago, prior to the divergence of Cnidaria and Bilateria, after which it has remained remarkably unchanged. Comparison of sequence features and domain organization in such a diverse group of organisms also provides important insights into how fibrillins and LTBPs might perform their roles in the extracellular matrix.

Random mutagenesis of the mouse genome: a strategy for discovering gene function and the molecular basis of disease

Mutagenesis of mice with N-ethyl-N-nitrosourea (ENU) is a phenotype-driven approach to unravel gene function and discover new biological pathways. Phenotype-driven approaches have the advantage of making no assumptions about the function of genes and their products and have been successfully applied to the discovery of novel gene-phenotype relationships in many physiological systems. ENU mutagenesis of mice is used in many large-scale and more focused projects to generate and identify novel mouse models for the study of gene functions and human disease. This review examines the strategies and tools used in ENU mutagenesis screens to efficiently generate and identify functional mutations.

Fibrillin-3 expression in human development

Fibrillin proteins are the major components of extracellular microfibrils found in many connective tissues. Fibrillin-1 and fibrillin-2 are well studied and mutations in these proteins cause a number of fibrillinopathies including Marfan syndrome and congenital contractural arachnodactyly, respectively. Fibrillin-3 was more recently discovered and is much less well characterized. Fibrillin-1 is expressed throughout life, whereas fibrillins-2 and -3 are thought to be primarily present during development. Here, we report detailed fibrillin-3 expression patterns in early human development. A polyclonal antiserum against a C-terminal recombinant half of human fibrillin-3 was produced in rabbit. Anti-fibrillin-3 antibodies were affinity-purified and antibodies cross-reacting with the other fibrillins were removed by absorption resulting in specific anti-fibrillin-3 antibodies. Immunohistochemical analyses with these purified antibodies demonstrate that fibrillin-3 is temporally expressed in numerous tissues relatively evenly from the 6th to the 12th gestational week. Fibrillin-3 was found spatially expressed in perichondrium, perineurium, perimysium, skin, developing bronchi, glomeruli, pancreas, kidney, heart and testis and at the prospective basement membranes in developing epithelia and endothelia. Double immunohistochemical analyses showed that all fibrillins are globally expressed in the same organs, with a number of differences on the tissue level in cartilage, perichondrium and developing bronchi. These results suggest that fibrillin-3, compared to the other fibrillins, fulfills both overlapping and distinct functions in human development.