Do LINEs have a role in X-chromosome inactivation?

There is longstanding evidence that X-chromosome inactivation (XCI) travels less successfully in autosomal than in X-chromosomal chromatin. The interspersed repeat elements LINE1s (L1s) have been suggested as candidates for “boosters” which promote the spread of XCI in the X-chromosome. The present paper reviews the current evidence concerning the possible role of L1s in XCI. Recent evidence, accruing from the human genome sequencing project and other sources, confirms that mammalian X-chromosomes are indeed rich in L1s, except in regions where there are many genes escaping XCI. The density of L1s is the highest in the evolutionarily oldest regions. Recent work on X; autosome translocations in human and mouse suggested failure of stabilization of XCI in autosomal material, so that genes are reactivated, but resistance of autosomal genes to the original silencing is not excluded. The accumulation of L1s on the X-chromosome may have resulted from reduced recombination or late replication. Whether L1s are part of the mechanism of XCI or a result of it remains enigmatic.

Activating calcium-sensing receptor mutation in the mouse is associated with cataracts and ectopic calcification

The extracellular calcium-sensing receptor (CaSR) plays a pivotal role in the regulation of extracellular calcium such that abnormalities, which result in a loss or gain of function, lead to hypercalcemia or hypocalcemia, respectively, in patients. Mice carrying CaSR knockout alleles develop hypercalcemia that mimics the disorders observed in humans. To date, there is no mouse model for an activating CaSR mutation. Here, we describe such a mouse model, named Nuf, originally identified for having opaque flecks in the nucleus of the lens in a screen for eye mutants. Nuf mice also display ectopic calcification, hypocalcemia, hyperphosphatemia, and inappropriately reduced levels of plasma parathyroid hormone. These features are similar to those observed in patients with autosomal dominant hypocalcemia. Inheritance studies of Nuf mice revealed that the trait was transmitted in an autosomal-dominant manner, and mapping studies located the locus to chromosome 16, in the vicinity of the CaSR gene (Mouse Genome Database symbol Gprc2a). DNA sequence analysis revealed the presence of a Gprc2a missense mutation, Leu723Gln. Transient expression of wild-type and mutant CaSRs in human embryonic kidney 293 cells demonstrated that the mutation resulted in a gain of function of the CaSR, which had a significantly lower EC(50). Thus, our results have identified a mouse model for an activating CaSR mutation, and the development of ectopic calcification and cataract formation, which tended to be milder in the heterozygote Nuf mice, indicates that an evaluation for such abnormalities in autosomal dominant hypocalcemia patients who have activating CaSR mutations is required.

The Lyon and the LINE hypothesis

X-chromosome inactivation (XCI) was first suggested as an explanation for the variegated phenotypes in mice heterozygous for X-linked colour genes or for X-autosome translocations involving autosomal coat colour genes. The effects seen in X-autosome translocations led to the suggestion of an X-inactivation centre (Xic) from which the inactivation was initiated, and this suggestion has led to major advances in understanding. Another feature of X-autosome translocations is incomplete inactivation of the attached autosomal segment, implying that the X-chromosome is enriched in features favouring inactivation. Interspersed repeat elements, and in particular long interspersed elements (LINEs), have been suggested as the relevant enriching features. Recent evidence concerning this hypothesis is discussed.

Transmission Ratio Distortion in Mice

The most studied example of transmission ratio distortion (TRD) in mice is that of the t-complex. This is a variant region of Chromosome 17 which exists as a polymorphism in wild mice. Males heterozygous for a t-haplotype and a normal Chr 17 transmit the t-haplotype to >50% of their young, up to 99%. Homozygous males are sterile. The TRD produced by the t-complex is due to the action of three or more distorter genes (Tcd) on a responder gene (Tcr). t-Haplotypes are maintained intact by crossover suppression induced by four neighboring inversions, the Tcd and Tcr loci lying in different inversions. Sperm formation is normal in t/t males, but sperm function is impaired through gross defects in sperm motility. The responder gene has been identified as a fusion gene formed from a sperm motility kinase and a ribosomal S6 kinase. Three candidate distorter genes have also been identified as genes coding for dynein chains, and thus possibly involved in sperm flagellar function.

X-chromosome inactivation and human genetic disease

The inactivation of one X-chromosome in females in early development is the process by which the effective dosage of X-linked genes is equalized between XX females and XY males. The mechanism that brings this about is the subject of intense research. The X-linked gene Xist is a key player, which is necessary but not sufficient for the initiation of X-inactivation. It codes for an untranslated RNA that coats the inactive X-chromosome, which takes on properties characteristic of heterochromatin, but how this change in chromatin is brought about remains unknown. Because of X-inactivation, females heterozygous for X-linked genes are mixtures of two types of cells and show a variable phenotype. The proportions of the two types of cells can depart from equality due to cell selection either at the tissue or whole organism level. In rare cases, changes in the Xist gene can cause skewing of X-inactivation. A few genes escape from X-inactivation either wholly or partially.

CONCLUSION

X-chromosome inactivation is a physiological mechanism that equalizes gene-dosage effects on the sex chromosomes. The occurrence of this normal process affects the phenotype seen in females carrying X-linked mutant genes or chromosome anomalies.

A dominant mutation within the DNA-binding domain of the bZIP transcription factor Maf causes murine cataract and results in selective alteration in DNA binding

The murine autosomal dominant cataract mutants created in mutagenesis experiments have proven to be a powerful resource for modelling the biological processes involved in cataractogenesis. We report a mutant which in the heterozygous state exhibits mild pulverulent cataract named 'opaque flecks in lens', symbol Ofl. By molecular mapping, followed by a candidate gene approach, the mutant was shown to be allelic with a knockout of the bZIP transcription factor, Maf. Homozygotes for Ofl and for Maf null mutations are similar but a new effect, renal tubular nephritis, was found in Ofl homozygotes surviving beyond 4 weeks, which may contribute to early lethality. Sequencing identified the mutation as a G-->A change, leading to the amino-acid substitution mutation R291Q in the basic region of the DNA-binding domain. Since mice heterozygous for knockouts of Maf show no cataracts, this suggests that the Ofl R291Q mutant protein has a dominant effect. We have demonstrated that this mutation results in a selective alteration in DNA binding affinities to target oligonucleotides containing variations in the core CRE and TRE elements. This implies that arginine 291 is important for core element binding and suggests that the mutant protein may exert a differential downstream effect amongst its binding targets. The cataracts seen in Ofl heterozygotes and human MAF mutations are similar to one another, implying that Ofl may be a model of human pulverulent cortical cataract. Furthermore, when bred onto a different genetic background Ofl heterozygotes also show anterior segment abnormalities. The Ofl mutant therefore provides a valuable model system for the study of Maf, and its interacting factors, in normal and abnormal lens and anterior segment development.

James Neel and the doubling dose concept

The doubling dose (DD) is a very valuable concept in attempts to assess the genetic risks of radiation in man. It was long thought that the value of the doubling dose obtained from specific locus experiments in mice could be applied to man. James Neel, as a result of his studies on the offspring of atomic bomb survivors, showed that this was not so, but that different doubling doses could be inferred from different endpoints.

A personal history of the mouse genome

The chapter describes some personal reminiscences of various stages in the growth of knowledge of the mouse genome in the past 50 years. Initially mapping was done by crossing new mutants with linkage testing stocks, a slow and laborious method. In the 1950s major mutagenesis experiments led to spin-offs in terms of new mutants, new knowledge of phenomena including sex determination and X-chromosome inactivation, and further understanding of the t-complex. The 1970s saw the development of recombinant inbred (RI) strains and the use of biochemical variants for mapping. In addition the linkage groups were assigned to chromosomes. Techniques of embryo surgery were developed, leading to work with embryonic stem (ES) cells and hence to the identification of gene functioning by knockouts and transgenesis. Another major advance in the 1970s and 1980s was the beginning of comparative mapping, which is now so important. With the advent of DNA technology, progress in mapping increased considerably. Progress became even faster with the use of interspecific backcrosses and with the development of microsatellite markers. The completion of the mouse DNA sequence is now imminent, opening fascinating prospects for the analysis of gene function.

A high-resolution genetic, physical, and comparative gene map of the doublefoot (Dbf) region of mouse chromosome 1 and the region of conserved synteny on human chromosome 2q35

The mouse doublefoot (Dbf) mutant exhibits preaxial polydactyly in association with craniofacial defects. This mutation has previously been mapped to mouse chromosome 1. We have used a positional cloning strategy, coupled with a comparative sequencing approach using available human draft sequence, to identify putative candidates for the Dbf gene in the mouse and in homologous human region. We have constructed a high-resolution genetic map of the region, localizing the mutation to a 0.4-cM (+/-0.0061) interval on mouse chromosome 1. Furthermore, we have constructed contiguous BAC/PAC clone maps across the mouse and human Dbf region. Using existing markers and additional sequence tagged sites, which we have generated, we have anchored the physical map to the genetic map. Through the comparative sequencing of these clones we have identified 35 genes within this interval, indicating that the region is gene-rich. From this we have identified several genes that are known to be differentially expressed in the developing mid-gestation mouse embryo, some in the developing embryonic limb buds. These genes include those encoding known developmental signaling molecules such as WNT proteins and IHH, and we provide evidence that these genes are candidates for the Dbf mutation.

Charles Edmund Ford

Charles Edmund Ford was distinguished for his outstanding contributions to mammalian cytogenetics, particularly human cytogenetics. He was especially renowned for his part in establishing the number of human chromosomes as 46, rather than 48 as previously believed. However, his contributions to the use of chromosome variants as cell markers in tracing cell lineages, particularly of haemopoietic cells, were of equal importance. He had a great mastery of cytological techniques and his ability to devise suitable methods for mammalian cells was a major factor in his contribution to the explosive advance of human and other mammalian genetics in the 1960s. Equally important were his superb observational powers in interpreting chromosome aberrations under the microscope, and his scrupulous adherence to scientific method.

Further genetic analysis of two autosomal dominant mouse eye defects, Ccw and Pax6(coop)

PURPOSE

The work forms part of a major project to study the genetics of mouse cataract mutants found during the course of mutagenesis experiments. The long-term aim is to find the underlying gene mutation in each cataract mutant. Here we report further studies of the mutant cataract and curly whiskers (Ccw), previously mapped to Chromosome 4, and also investigations of the corneal opacity (Coop) mutant, which is shown to involve a mutation in the Pax6 gene.

METHODS

For Ccw, the methods included mapping relative to microsatellite markers and histological studies. For the Coop mutant, breeding methods were used to show that Coop was allelic with Pax6. The Pax6 coding region in the mutant was then sequenced.

RESULTS

The Ccw locus was mapped to approximately position 45cM on the consensus map of Chr 4. Histologically, progressive degeneration of the lens was seen. In the Coop mutant, a base-pair change C->T was found at position 1033 in the Pax6 gene, which created a stop codon leading to premature termination of translation, and to a truncated Pax6 protein.

CONCLUSIONS

The phenotype in Ccw/+ heterozygotes involves a new type of lens degeneration in the mouse. On the basis of the phenotype and the locus position, no candidate gene has yet been identified. The Pax6coop mutant differs in phenotype from known null alleles of Pax6, implying that it is a hypomorph.

X-chromosome inactivation: a repeat hypothesis

Recent work has shown that X-chromosome inactivation is brought about by Xist mRNA, which coats the inactive X-chromosome. This paper presents a hypothesis on the function of this RNA. It is suggested that interspersed repetitive elements of the LINE type, in which the X-chromosome is particularly rich, act as booster elements to promote the spread of Xist mRNA. Contact with this RNA causes the LINE elements to be sensed as repeated elements by the cell's system for repeat-induced gene silencing. This leads to the silencing of these elements and the intervening unique sequences by their conversion to heterochromatin.

An answer to a complex problem: cloning the mouse t-complex responder

The t-complex is maintained in wild mouse populations by its high transmission (up to 99%) from heterozygous males and provides an example of "meiotic drive". Its molecular basis has remained obscure despite long and intensive study. In a major advance, the t-complex responder gene, thought to be the key gene on which several distorters act, has now been cloned.

Narrowing the critical regions for mouse t complex transmission ratio distortion factors by use of deletions

Previously a deletion in mouse chromosome 17, T(22H), was shown to behave like a t allele of the t complex distorter gene Tcd1, and this was attributed to deletion of this locus. Seven further deletions are studied here, with the aim of narrowing the critical region in which Tcd1 must lie. One deletion, T(30H), together with three others, T(31H), T(33H), and T(36H), which extended more proximally, caused male sterility when heterozygous with a complete t haplotype and also enhanced transmission ratio of the partial t haplotype t(6), and this was attributed to deletion of the Tcd1 locus. The deletions T(29H), T(32H), and T(34H) that extended less proximally than T(30H) permitted male fertility when opposite a complete t haplotype. These results enabled narrowing of the critical interval for Tcd1 to between the markers D17Mit164 and D17Leh48. In addition, T(29H) and T(32H) enhanced the transmission ratio of t(6), but significantly less so than T(30H). T(34H) had no effect on transmission ratio. These results could be explained by a new distorter located between the breakpoints of T(29H) and T(34H) (between T and D17Leh66E). It is suggested that the original distorter Tcd1 in fact consists of two loci: Tcd1a, lying between D17Mit164 and D17Leh48, and Tcd1b, lying between T and D17Leh66E.

Sox6 is a candidate gene for p100H myopathy, heart block, and sudden neonatal death

The mouse p locus encodes a gene that functions in normal pigmentation. We have characterized a radiation-induced mutant allele of the mouse p locus that is associated with a failure-to-thrive syndrome, in addition to diminished pigmentation. Mice homozygous for this mutant allele, p(100H), show delayed growth and die within 2 wk after birth. We have discovered that the mutant mice develop progressive atrioventricular heart block and significant ultrastructural changes in both cardiac and skeletal muscle cells. These observations are common characteristics described in human myopathies. The karyotype of p(100H) chromosomes indicated that the mutation is associated with a chromosome 7 inversion. We demonstrate here that the p(100H) chromosomal inversion disrupts both the p gene and the Sox6 gene. Normal Sox6 gene expression has been examined by Northern blot analysis and was found most abundantly expressed in skeletal muscle in adult mouse tissues, suggesting an involvement of Sox6 in muscle maintenance. The p(100H) mutant is thus a useful animal model in the elucidation of myopathies at the molecular level.

The major brain isoform of kif1b lacks the putative mitochondria-binding domain

Kinesin and kinesin superfamily proteins are molecular motors involved in important intracellular functions such as organelle transport and cell division. They are microtubule-activated ATPases composed of a motor domain that binds to microtubules and a cargo-binding domain that binds to specific organelles. While searching for the slow Wallerian degeneration mutation (WldS) on distal mouse Chromosome (Chr) 4, we have identified a member of the kinesin superfamily whose predicted gene product has the N-terminal motor domain of Kif1b and a novel C-terminal cargo-binding domain homologous to Kif1a. Kif1b is responsible for the movement of mitochondria along the axon, but the novel isoform containing the alternative C-terminal domain is likely to have a different cargo-binding specificity. cDNA library screening and Northern blot analysis indicate that the alternatively spliced form of Kif1b containing the novel 3'end accounts for the most part of Kif1b expression. We also found more alternatively spliced exons that can give rise to heterogeneous transcripts. Therefore, alternative splicing, as well as multiple genes, may contribute to the selective movement of diverse organelles by anterograde axonal transport. Kif1b maps on distal mouse Chr 4, within the Wld genetic candidate interval, but outside the recently identified triplication. There is, however, no evidence that Kif1b is the Wld gene.

Deletion mapping of the head tilt (het) gene in mice: a vestibular mutation causing specific absence of otoliths

Head tilt (het) is a recessive mutation in mice causing vestibular dysfunction. Homozygotes display abnormal responses to position change and linear acceleration and cannot swim. However, they are not deaf. het was mapped to the proximal region of mouse chromosome 17, near the T locus. Here we report anatomical characterization of het mutants and high resolution mapping using a set of chromosome deletions. The defect in het mutants is limited to the utricle and saccule of the inner ear, which completely lack otoliths. The unique specificity of the het mutation provides an opportunity to better understand the development of the vestibular system. Complementation analyses with a collection of embryonic stem (ES)- and germ cell-induced deletions localized het to an interval near the centromere of chromosome 17 that was indivisible by recombination mapping. This approach demonstrates the utility of chromosome deletions as reagents for mapping and characterizing mutations, particularly in situations where recombinational mapping is inadequate.

A mutation in the connexin 50 (Cx50) gene is a candidate for the No2 mouse cataract

PURPOSE

The No2 cataractous mouse mutant displays a bilateral, congenital, hereditary nuclear opacity of the ocular lens. The aim of this work was to identify and subsequently screen an optimal candidate gene for a mutation correlated and consistent with the observed phenotype.

METHODS

The No2 cataract was mapped in relation to genes and microsatellite markers by crossing to the wild mouse strain Mus spretus and then backcrossing to the inbred strain C3H/ HeH. The Cx50 (MP70) protein coding region and flanking sequences were amplified from normal parental as well as heterozygous and homozygous mutant genomic DNAs. These PCR products were then sequenced directly. Sequence data was corroborated by restriction analysis of PCR products.

RESULTS

Mapping of the No2 cataract placed it in the vicinity of Gja8, the gene encoding connexin 50 (MP70), a major component of lens fiber gap junctions. Amplification and subsequent sequencing of the Cx50 protein coding regions revealed a single A-->C transversion within codon 47. This sequence change resulted in the creation of an HhaI restriction endonuclease restriction site, allowing for corroboration of the sequence data via restriction analysis using this enzyme. The sequence alteration is also predicted to result in the nonconservative substitution of alanine (Ala) for the normally encoded aspartic acid (Asp) at this position within the polypeptide.

CONCLUSIONS

The identified mutation in Gja8 is both correlated and consistent with the cataract observed in the No2 mouse mutant, making it an ideal candidate for the cataract. This study provides the first evidence that a mutation in a lens connexin can result in congenital hereditary cataract, highlighting the importance of lens connexins in maintaining lens transparency.

An 85-kb tandem triplication in the slow Wallerian degeneration (Wlds) mouse

Wallerian degeneration is the degeneration of the distal stump of an injured axon. It normally occurs over a time course of around 24 hr but it is delayed in the slow Wallerian degeneration mutant mouse (C57BL/Wlds) for up to 3 weeks. The gene, which protects from rapid Wallerian degeneration, Wld, previously has been mapped to distal chromosome 4. This paper reports the fine genetic mapping of the Wld locus, the generation of a 1.4-Mb bacterial artificial chromosome and P1 artificial chromosome contig, and the identification of an 85-kb tandem triplication mapping within the candidate region. The mutation is unique to C57BL/Wlds among 36 strains tested and therefore is a strong candidate for the mutation that leads to delayed Wallerian degeneration. There are very few reports of tandem triplications in a vertebrate and no evidence for a mutation mechanism so this unusual mutation was characterized in more detail. Sequence analysis of the boundaries of the repeat unit revealed a minisatellite array at the distal boundary and a matching 8-bp sequence at the proximal boundary. This finding suggests that recombination between short homologous sequences ("illegitimate" or "nonhomologous" recombination) was involved in the rearrangement. In addition, a duplication allele was identified in two Wlds mice, indicating some instability in the repeat copy number and suggesting that the triplication arose from a duplication by unequal crossing over.

Evidence that preaxial polydactyly in the Doublefoot mutant is due to ectopic Indian Hedgehog signaling

Patterning of the vertebrate limb along the anterior-posterior axis is controlled by the zone of polarizing activity (ZPA) located at the posterior limb margin. One of the vertebrate Hh family members, Shh, has been shown to be able to mediate the function of the ZPA. Several naturally occurring mouse mutations with the phenotype of preaxial polydactyly exhibit ectopic Shh expression at the anterior limb margin. In this study, we report the molecular characterization of a spontaneous mouse mutation, Doublefoot (Dbf). Dbf is a dominant mutation which maps to chromosome 1. Heterozygous and homozygous embryos display a severe polydactyly with 6 to 8 digits on each limb. We show here that Shh is expressed normally in Dbf mutants. In contrast, a second Hh family member, Indian hedgehog (Ihh) which maps close to Dbf, is ectopically expressed in the distal limb bud. Ectopic Ihh expression in the distal and anterior limb bud results in the ectopic activation of several genes associated with anterior-posterior and proximal-distal patterning (Fgf4, Hoxd13, Bmp2). In addition, specific components in the Hedgehog pathway are either ectopically activated (Ptc, Ptc-2, Gli1) or repressed (Gli2). We propose that misexpression of Ihh, and not a novel Smoothened ligand as recently suggested (Hayes et al., 1998), is responsible for the Dbf phenotype. We consider that Ihh has a similar activity to Shh when expressed in the early Shh-responsive limb bud. To determine whether Dbf maps to the Ihh locus, which is also on chromosome 1, we performed an interspecific backcross. These results demonstrate that Dbf and Ihh are genetically separated by approximately 1.3 centimorgans, suggesting that Dbf mutation may cause an exceptionally long-range disruption of Ihh regulation. Although this leads to ectopic activation of Ihh, normal expression of Ihh in the cartilaginous elements is retained.

A very large protein with diverse functional motifs is deficient in rjs (runty, jerky, sterile) mice

Three radiation-induced alleles of the mouse p locus, p6H, p25H, and pbs, cause defects in growth, coordination, fertility, and maternal behavior in addition to p gene-related hypopigmentation. These alleles are associated with disruption of the p gene plus an adjacent gene involved in the disorders listed. We have identified this adjacent gene, previously named rjs (runty jerky sterile), by positional cloning. The rjs cDNA is very large, covering 15,264 nucleotides. The predicted rjs-encoded protein (4,836 amino acids) contains several sequence motifs, including three RCC1 repeats, a structural motif in common with cytochrome b5, and a HECT domain in common with E6-AP ubiquitin ligase. On the basis of sequence homology and conserved synteny, the rjs gene is the single mouse homolog of a previously described five- or six-member human gene family. This family is represented by at least two genes, HSC7541 and KIAA0393, from human chromosome 15q11-q13. HSC7541 and KIAA0393 lie close to, or within, a region commonly deleted in most Prader-Willi syndrome patients. Previous work has suggested that the multiple phenotypes in rjs mice might be due to a common neuroendocrine defect. In addition to this proposed mode of action, alternative functions of the rjs gene are evaluated in light of its known protein homologies.

Morphogenesis of Doublefoot (Dbf), a mouse mutant with polydactyly and craniofacial defects

We report the morphogenesis of a new mouse mutant, Doublefoot (Dbf). The major phenotypic features involve the limb and craniofacial regions. There is polydactyly of all 4 limbs, with typically 6-8 digits per limb. All of the digits are triphalangeal; some show bifurcations and some are not attached to the carpus/tarsus. The carpus and tarsus are broader than normal, and their elements are partially fused. There are also tibial defects. Mutant embryos show a diencephalic bulge on d 10.0, with older animals exhibiting broadened and bulbous skulls sometimes with an additional midline skeletal element, shortened snouts and bulging eyes. Homozygotes, which do not survive beyond d 15, show midline facial clefting. In this study of the embryonic and fetal development of Dbf animals, we focus on the morphogenesis of the limbs and head, and discuss the possible molecular developmental mechanisms.

Sonic hedgehog is not required for polarising activity in the Doublefoot mutant mouse limb bud

The mouse mutant Doublefoot (Dbf) shows preaxial polydactyly of all four limbs. We have analysed limb development in this mutant with respect to morphogenesis, gene expression patterns and ectopic polarising activity. The results reveal a gain-of-function mutation at a locus that mediates pattern formation in the developing limb. Shh expression is identical with that of wild-type embryos, i.e. there is no ectopic expression. However, mesenchyme from the anterior aspects of Dbf/+ mutant limb buds, when transplanted to the anterior side of chick wing buds, induces duplication of the distal skeletal elements. Mid-distal mesenchymal transplants from early, but not later, Dbf/+ limb buds are also able to induce duplication. This demonstration of polarising activity in the absence of Shh expression identifies the gene at the Dbf locus as a new genetic component of the Shh signalling pathway, which (at least in its mutated form) is able to activate signal transduction independently of Shh. The mutant gene product is sufficient to fulfil the signalling properties of Shh including upregulation of the direct Shh target genes Ptc and Gli, and induction of the downstream target genes Bmp2, Fgf4 and Hoxd13. The expression domains of all these genes extend from their normal posterior domains into the anterior part of the limb bud without being focused on a discrete ectopic site. These observations dissociate polarising activity from Shh gene expression in the Dbf/+ limb bud. We suggest that the product of the normal Dbf gene is a key active constituent of the polarising region, possibly acting in the extracellular compartment.

Identification of a mutation in the MP19 gene, Lim2, in the cataractous mouse mutant To3

PURPOSE

Lim2, the gene encoding the second most abundant lens specific integral membrane protein, MP19, has recently been proposed as an ideal candidate gene for the cataractous mouse mutant, To3. The aim of this study was to screen the Lim2 gene in the To3 mutant for a genetic lesion that was correlated and consistent with the mutant phenotype.

METHODS

Genomic DNA was isolated from both normal mouse parental strains as well as the heterozygous and homozygous To3 cataract mutant. PCR was used to generate overlapping fragments of the entire Lim2 gene from these DNAs. The coding regions, including splice junctions and the translational termination site, of these fragments were then sequenced.

RESULTS

A single G -> T transversion was identified within the first coding exon of the Lim2 gene in the To3 mutant DNA. This DNA change results in the nonconservative substitution of a valine for the normally encoded glycine at amino acid 15 of the MP19 polypeptide.

CONCLUSIONS

The identified genetic lesion in the Lim2 gene of the cataractous mouse mutant, To3, confirms Lim2 as an ideal candidate gene. Future transgenic experiments should provide proof or disproof of a causative relationship between the identified mutation and the cataractous phenotype. These studies indicate that MP19 may play an important role in both normal lens development and cataractogenesis, and warrants more intense investigation of its role within the ocular lens.

Doublefoot: a new mouse mutant affecting development of limbs and head

The mutant doublefoot, Dbf, of the mouse arose spontaneously, and was shown to be inherited as an autosomal dominant, mapping 9-13 cM proximal to leaden, In, on chromosome 1 and showing no recombination with the microsatellite markers D1Mit24 and D1Mit77. In heterozygotes the phenotype includes many extra toes on all four feet, and the tibia and fibula may be reduced and bowed. The head is shortened and broad and the eyes are held half-closed, and some animals develop hydrocephalus. The tail is kinked and abnormally thick, and the soles of the feet are swollen. Growth is retarded, viability is reduced, and reproduction is impaired in both sexes. Only about 30% of males are normally fertile, and testis weights and sperm counts may be reduced, although this appears not to be the main cause of poor fertility. In females vaginal opening is delayed and oestrous cycles are irregular, although the animals appear to respond to gonadotrophic hormones. Crosses of Dbf/+ x Dbf/+ are very poorly fertile. Prenatally, Dbf/+ heterozygotes can first be recognized at 11 1/2 days gestation by abnormally broad fore limb buds. Putative Dbf/Dbf homozygotes at 12 1/2 days have similar limbs defects and also split face, due to failure of the maxillae to fuse in the midline. Some homozygotes and a few putative heterozygotes have cranioschisis. At 13 1/2 days, the heads of homozygotes tend to bulge in the frontal region and a bleb of clear fluid is visible medially. At 14 1/2 days Dbf/Dbf fetuses may have oedema and some are dead. From 15 1/2 days onwards no live Dbf/Dbf fetuses have been found. The gene maps close to the locus of Pax3, but crossovers between Dbf and Pax3 have been found, ruling out the possibility that a gain-of-function mutation in Pax3 might be involved.

Two new cataract loci, Ccw and To3, and further mapping of the Npp and Opj cataracts in the mouse

Many types of inherited early onset cataract are known in both human and mouse. Here we describe the mapping of two novel dominant cataract loci in the mouse genome. Cataract and curly whiskers, Ccw, maps to Chromosome 4, 3.1 +/- 1.1 cM distal to the b (brown) locus. Total opacity 3, To3, maps to Chromosome 7, 7.1 +/- 1.8 cM proximal to p (pink-eyed dilution). The map positions of two other dominant cataract mutants have now been refined by three-point crosses. Nuclear and posterior polar cataract, Npp, maps to the central part of Chromosome 5, 1.4 +/- 0.5 cM distal to We (dominant spotting-extreme, an allele at the Kit locus), and Opaque secondary fiber cell junctions, Opj, maps to the proximal region of Chromosome 16, 9.1 +/- 1.5 cM distal to the marker md (mahoganoid). While there are no obvious candidate genes in the vicinity of the Ccw, Npp, and Opj mutations, To3 lies remarkably close to the recently mapped Lim2 locus, which encodes lens intrinsic membrane protein 2, also called MP19.

An additional type of male sterility and inherited urinary obstruction in mice with the t-haplotype th7

The t-complex on mouse chromosome 17 results in transmission ratio distortion in males heterozygous for complete haplotypes, and sterility in those homozygous for semi-lethal or doubly heterozygous for complementing lethal haplotypes. This sterility is due to inability of spermatozoa to fertilize. The haplotype th7 is an unusual laboratory-derived haplotype, postulated to carry a small duplication of t chromatin. Males heterozygous for th7 show a new form of sterility, apparently due to failure to form copulation plugs during mating. This is accompanied by a strong propensity to acute urinary obstruction. It is suggested that both the failure to form copulation plugs and the urinary obstruction are due to some abnormality in function of the accessory sex glands, and are the result of incorrect dosage of a gene in the postulated duplication. The symbol Msu for male sterility and urinary obstruction is suggested for the locus concerned. Previously a recessive form of abnormal behaviour had also been attributed to this duplication.

Mapping of four mouse genes encoding eye lens-specific structural, gap junction, and integral membrane proteins: Cryba1 (crystallin beta A3/A1), Crybb2 (crystallin beta B2), Gja8 (MP70), and Lim2 (MP19)

Four genes encoding eye lens-specific proteins, potential candidate genes for congenital cataract (CC) mutations, were mapped in the mouse genome using a panel of somatic cell hybrids and DNAs from the EU-CIB (European Collaborative Interspecific Backcross). Two of them are lens fiber cell structural proteins: the Cryba1 locus encoding crystallinbetaA3/A1 maps to chromosome 11, 2.5 +/- 2.5 cM distal to D11Mit31, and the Crybb2 locus encoding crystallinbetaB2 maps to chromosome 5, 9.1 +/- 4.3 cM distal to D5Mit88. The other two genes encode lens-specific gap junction and integral membrane proteins, respectively: The Gja8 locus encoding gap juction membrane channel protein alpha8, also called connexin50 or MP70, maps to chromosome 3, 11.9 +/- 5.0 cM distal to D3Mit22, and the Lim2 locus encoding lens intrinsic membrane protein 2, also called MP19, maps to chromosome 7, 2.5 +/- 2.5 cM proximal to Ngfg. All four map positions, when compared with the corresponding positions in human, lie within known regions of conserved synteny between mouse and human chromosomes.

The history of X-chromosome inactivation and relation of recent findings to understanding of human X-linked conditions

This paper represents the text of two lectures given on the occasion of a Workshop on the X-Chromosome held at Hacettepe University, Ankara, in September 1994. The history of the development of ideas concerning the mechanism of X-chromosome inactivation is traced from the finding of the sex chromatin body in 1949. Important recent findings concern the Xist gene, which is a candidate gene for the X-inactivation center, from which inactivation is initiated. These recent findings shed new light on the abnormalities seen in human patients with X-linked genetic diseases or with aneuploidy of the X-chromosome.

Close linkage of the dominant cataract mutations (Cat-2) with Idh-1 and cryge on mouse chromosome 1

The murine dominant gene Cat-2 was located on chromosome 1 between the loci of fuzzy and leaden. Subsequent linkage analysis revealed one recombinant between Cat-2t and isocitrate dehydrogenase-1, and one between Cat-2t and gamma E-crystallin among 338 offspring in three-point backcrosses. The resulting genetic distance between the loci is 0.3 +/- 0.3 cM. The very close linkage between the Cat-2 and the gamma-crystallin gene cluster together with the finding of reduced gamma-crystallin transcripts in mutant lenses suggest strongly that the gamma-crystallin genes may be candidate genes for the Cat-2 mutations.

Mapping of six dominant cataract genes in the mouse

The mapping of six mouse autosomal dominant cataract mutations that were induced by mutagenic treatment with radiation or ethylnitrosourea is described. Three, with differing phenotypes, mapped on Chromosome 1 between the loci of fuzzy (fz) and leaden (ln) and close to the locus of the gamma-crystallin gene cluster. One of these, Cat-2t, had previously been shown to be a member of a group of five allelic mutants. In addition, the previously known mutant eye lens obsolescence, Elo, maps to the same point. There are thus now eight mutants that map to this point and that may involve mutations in one of the gamma-crystallin genes. In addition, one of these mutants may be a homologue of Coppock cataract in man, which also maps close to the gamma-crystallin locus. Of the three remaining mutants, one, with the suggested symbol Cat-5, mapped to the proximal region of Chromosome 10, 23.4 +/- 4.0 cM from downless (dl), a region with homology to human 6q. A second mutant, provisionally designated Opj, mapped on Chromosome 16, 8.2 +/- 3.9 cM from the marker mahoganoid (md). Thus, it possibly has a homologue on human 22q, a region in which one of the beta-crystallin loci is sited. A third mutant, provisionally designated Npp, mapped to Chromosome 5, 1.3 +/- 0.9 cM from the locus of W, and thus probably has a homologue on human Chromosome 4.

The X inactivation centre and X chromosome imprinting

Genetic imprinting is an important component of X chromosome inactivation, since in marsupials and extraembryonic cell lineages of mice and rats, the paternally derived X chromosome is preferentially inactivated. This imprinting is thought to be mediated via the X inactivation centre. The gene symbolized Xist is a strong candidate for a role in the function of the X inactivation centre and the paper reviews the evidence that Xist shows imprinted behaviour and that differential methylation is the possible basis of the imprint. This paper is the text of the speech given by Dr. Mary Lyon after the awarding of the Mauro Baschirotto prize at the meeting of the European Society of Human Genetics in Paris, June 1994 (see page 305).

A gene affecting Wallerian nerve degeneration maps distally on mouse chromosome 4

When a nerve axon is cut or crushed, the nerve fibers in the distal part of the axon, separated from the cell body, undergo a form of spontaneous degeneration, known as Wallerian degeneration. A substrain of the mouse inbred strain C57BL, known as C57BL/Ola, carries a mutant form of a gene involved in Wallerian degeneration in the peripheral and central nervous systems, and in retrograde degeneration of retinal ganglion cells. Wallerian degeneration in this substrain is abnormally slow. Previously the defect had been shown to be due to an autosomal dominant gene. The locus has been given the name and symbol Wallerian degeneration Wld, with the mutant allele Wlds (Wallerian degeneration-slow). The Wld locus has now been mapped, by using conventional and molecular markers, to the distal end of chromosome 4, near the locus of pronatriodilatin (Pnd). The order of loci (with recombination distances in centimorgans, cM) is cen-D4Mit11-8.9 +/- 1.7 cM-Fuca-2.5 +/- 0.93 cM-Akp-2-3.2 +/- 1.1 cM-D4Mit48-3.5 +/- 1.1 cM-(Wld, Pnd, D4Mit49)-0.71 +/- 0.50 cM-(Eno-1, D4Mit33)-1.4 +/- 0.70 cM-D4Mit42-2.5 +/- 0.93 cM-D4Smh6b. The information on the position of the Wld locus should be valuable in further characterization of this gene involved in nerve degeneration and regeneration.

Epigenetic inheritance in mammals

The epigenetic phenomena of genome imprinting and X-chromosome inactivation, found in mammals, both entail homologous genes or chromosomes behaving differently within the same cell. Although both have consequences for genic balance in the whole genome, in imprinting the control seems mainly at the single gene level, whereas in X-chromosome inactivation there is coordinated regulation of the whole chromosome, and single gene effects are relatively minor.

The mouse pink-eyed dilution gene: association with human Prader-Willi and Angelman syndromes

Complementary DNA clones from the pink-eyed dilution (p) locus of mouse chromosome 7 were isolated from murine melanoma and melanocyte libraries. The transcript from this gene is missing or altered in six independent mutant alleles of the p locus, suggesting that disruption of this gene results in the hypopigmentation phenotype that defines mutant p alleles. Characterization of the human homolog revealed that it is localized to human chromosome 15 at q11.2-q12, a region associated with Prader-Willi and Angelman syndromes, suggesting that altered expression of this gene may be responsible for the hypopigmentation phenotype exhibited by certain individuals with these disorders.

Genetic and molecular analysis of recessive alleles at the pink-eyed dilution (p) locus of the mouse

Recessive mutant alleles at the pink-eyed dilution (p) locus on mouse chromosome 7 reduce pigmentation of both the coat and eyes. Here we describe the properties and complementation interactions of 10 p alleles, including 6 not previously reported. Several alleles that cause additional phenotypes affecting development, reproduction, and behavior were shown to be deletions by using DNA probes derived from the p region. An alignment of functional and marker-defined units is proposed, giving a linear complementation map that orders at least four functional loci. The characterization of a nested set of deletions around p will facilitate detailed molecular analyses of the genes and developmental functions associated with this part of the mouse genome.