The paired domain is a conserved DNA binding motif which was first found in Drosophila segmentation gene products. This paired domain is encoded by a well conserved, paired box DNA sequence, also detected in the genomes of other species. The mouse paired box-containing genes are referred to as Pax genes and are expressed in a distinct spatiotemporal pattern during embryogenesis. Pax proteins are able to bind to specific DNA sequences and modulate transcriptional activity. Interestingly, three different Pax genes have already been shown to correspond to some mouse and human mutants, emphasizing their role as developmental control genes.

The paired box was found in Drosophila segmentation genes (Bopp et al., 1986; Frigerio et al., 1986; Coté et al., 1987; Baumgartner et al., 1987; Morrissey et al., 1991; Dambly-Chaudiere et al., 1992), and also in the genomes of mouse (Deutsch et al., 1988; Dressier et al., 1988; Walther et al., 1991), zebrafish (Krauss et al., 1991a,b,c; Püschel et al., 1992), chicken (M. Goulding and P. Gruss, unpublished), frog, turtle, nematode and man (Burri et al., 1989).

To date, eight murine paired box genes (Pax) have been isolated. Comparison of their sequence and genomic organization led to their classification into four classes (Fig. 1). Genes of the same group share similar intron-exon boundaries, similarities in the protein structure and display a related expression pattern during development (Walther et al., 1991; Deutsch and Gruss, 1991; Gruss and Walther, 1992). Pax genes are not organized in clusters and, although three Pax genes are found on chromosome 2, their loci are far apart (Fig. 1).

Fig. 1.

The Pax gene family. The chromosomal localization, protein structural features, size of the mRNA and proteins and the correlated mutants are shown. Paired and homeodomains are shown as boxes. The white vertical boxes represent a-helices. The paired domains represented with the same patterns belong to the same paired domain class as judged from protein sequence analysis, aa, amino acids; kb, kilobases; PD, paired domain; OP, octapeptide; HD, homeodomain; ?, not determined.

Fig. 1.

The Pax gene family. The chromosomal localization, protein structural features, size of the mRNA and proteins and the correlated mutants are shown. Paired and homeodomains are shown as boxes. The white vertical boxes represent a-helices. The paired domains represented with the same patterns belong to the same paired domain class as judged from protein sequence analysis, aa, amino acids; kb, kilobases; PD, paired domain; OP, octapeptide; HD, homeodomain; ?, not determined.

The proposed translation products of the Pax genes are between 361 (Pax-1) and 479 (Pax-3) amino acids long. The paired domain is 128 amino acids long and is located close to the amino terminus of the protein. Only Pax-3 has an extended amino terminus, containing 33 amino acids amino-terminal to the paired domain. In addition to the paired domain, Pax-3, Pax-7 and Pax-6 have an additional paired type homeodomain of 61 amino acids located car-boxy-terminal to the paired domain. Preliminary data also indicate the presence of a paired type, homeobox sequence in Pax-4 (M. Asano and P. Gruss, unpublished data). Pax-2, Pax-5 and Pax-8 have retained the amino-terminal part of the horneo domain including the first a-helix motif.

Computer structure predictions suggest that the paired domain contains three oc-helices. The first helix is located at the amino-terminal part of the paired domain while the second and third helices, separated by 8 amino acids, are located in the carboxy-terminal part of the paired domain (Bopp et al., 1989; Burri et al., 1989; Treisman et al., 1991). However, the most conserved residues lie outside of the helices and there is no homology to known helix-tum-helix or the helix-loop-helix motifs.

The paired type homeodomain contains a helix-tum-helix motif and the second helix appears to be responsible for specific DNA recognition. Treisman et al. (1989) have shown that amino acid 9 of this helix plays a critical role in sequence recognition.

Pax-1, Pax-2, Pax-3, Pax-5, Pax-7 and Pax-8, but not Pax-6 (Walther et al., 1991) contain a conserved octapeptide which was initially described for the human HuPl, HuP2 and the Drosophila gooseberry proteins (Burri et al., 1989). It is located carboxy-terminal to the paired domain and, in the case of Pax-3 and Pax-7, between the paired domain and the homeodomain. Although this short peptide is well conserved, no function has yet been assigned to it.

Expression pattern of Pax genes

The murine Pax genes are first expressed between days 8 and 9.5 of embryogenesis and some are also found to be active in adult tissues. They are expressed in axial structures along the entire antero-posterior axis in a tissuespecific manner. Pax-1, expressed predominantly in meso-dermally-derived tissues, is the only Pax gene not expressed in the developing central nervous system.

In general, Pax genes containing a homeobox appear to be expressed earlier and to be mainly restricted to mitotically active cells, whereas the others (Pax-2 and Pax-8) are rather restricted to differentiating cells. Pax-3, Pax-7 and Pax-6, which encode a homeodomain, in addition to the paired domain, are expressed in the neuroectoderm before neural differentiation starts (day-8.5 p.c.). On the other hand, Pax-2 and Pax-8, which lack a homeobox, are first expressed in the neural tube at day-10 p.c., at the time when neural differentiation begins. Although Pax genes, with the exception of Pax-6, are expressed in mesodermally-derived segmented structures, they cannot be considered to be primary segmentation genes as they appear after initial segmentation has been established.

Pax-1

Pax-1, unlike the other Pax genes, is not expressed in the developing central nervous system (Deutsch et al., 1988). Expression starts at day-9 p.c. in the segmented paraxial mesoderm of the developing vertebral column, specifically in the sclerotome. These Fax-7-expressing sclerotomal cells migrate and surround the notochord to form the perichordal condensations. Later on, Pax-7 expression is confined to the intervertebral disk anlagen. Thus, as suggested by its expression pattern, Pax-1 could be involved in the specification of the sclerotomal mesoderm.

Moreover, Pax-1 is expressed in the sternebrae of the segmented sternum and in the thymus. With the exception of Pax-1 expression in the thymus, all the other tissues expressing Pax-1 undergo chondrogenesis, indicating that Pax-1 might also be involved in certain chondrogenic events.

Pax-2, Pax-8 and Pax-5

Pax-2 and Pax-8 are similarly expressed in the nervous system (Nornes et al., 1990; Dressier et al., 1990; Plachov et al., 1990). Firstly, their transcripts have been identified in the neural tube and in the hindbrain. In the neural tube, expression has been shown to be restricted to a specific subset of postmitotic differentiating cells on both sides of the dorso-ventral midline, the sulcus limitans. However, while Pax-8 transcripts are undetectable after day-13.5 p.c., Pax-2 continues to be expressed throughout development and in adult tissues. Pax-2 is also active in the developing ear and eye.

Secondly, around days-9 and -10 p.c., both genes are expressed in the developing excretory system, more specifically in the condensed metanephric mesenchyme and in the resulting epithelial tubules. Pax-2 expression is also detected in the Wolffian duct and ureter which buds out from the Wolffian duct. Both Pax-2 and Pax-8 are also expressed in the Wilms tumor (Dressier and Douglas, 1992; Poleev et al., 1992). Finally, Pax-8 is expressed in the thyroid, from the earliest developmental stages to the adult stage (Zannini et al., 1992). Pax-8 thus constitutes one of the earliest markers of thyroid development.

Pax-5 is expressed in the developing brain, predominantly at the midbrain-hindbrain boundary, and in the neural tube. The expression pattern of Pax-5 in the developing brain is different from those of Pax-2 and Pax-8, while the expression pattern in the neural tube is very similar. Unlike Pax-2 and Pax-8, Pax-5 is not expressed in the developing excretory system or thyroid (Asano and Gruss, 1992; Adams et al., 1992). Pax-5 is also expressed in pre-B, pro-B and mature B-cells, but not in plasma cells. Transcripts of Pax-5 are also detected in the testis (Adams et al., 1992).

Pax-3 and Pax-7

Pax-3 and Pax-7 exhibit a similar expression pattern (Goulding et al., 1991; Jostes et al., 1990). Transcripts are detected during early brain and neural tube development before the onset of neural differentiation. At day-8.5 p.c. Pax-3 is expressed in the dorsal ventricular zone of the neural tube, including the roof plate and neural crest cells and their derivatives. Pax-7 also presents a similar pattern, excluding, however, the roof plate and neural crest cells. In addition, its onset of expression is slightly later than Pax-3: while Pax-3 transcripts are detected in the neural folds at day-8.5 p.c., Pax-7 cannot be detected before closure of the neural tube. The expression of both genes in the neural tube and in the brain shows a correlation with mitotically active cells. Moreover, both Pax-3 and Pax-7 are expressed on the dorsal aspect of the somites, the dermomyotome, whereas Pax-3 transcripts are present earlier than Pax-7, detected between 8.5 and 11.5 days p.c., in the dorsolateral regions of the somites. While Pax-3 mesodermal expression is turned off at around day 11.5, Pax-7 is still expressed until day 14.5 in myotome-derived muscles, indicating that Pax-7 could be involved in myogenic differentiation. Finally, Pax-3 transcripts have been also detected in limb bud and in some craniofacial structures.

Pax-6

Pax-6 is the earliest Pax gene to be expressed during development (Walther and Gruss, 1991). Its expression is almost exclusively restricted to the developing central nervous system. Pax-6 transcripts have been detected in the ventral ventricular zone of the neural tube. In the brain, expression starts at day-8.5 p.c. in all vesicles, with a gap of expression in the roof of the mesencephalon and metencephalon. The expression in the brain is maintained until late in development. Pax-6 mRNA is also detected in the nose and in all developing structures of the eye starting from day-8.5 p.c. It is present in the epithelial layer of the optic vesicle, in the optic stalk and cup, as well as in the surface ectoderm which will later form the lens. At later stages, the transcripts are found in the inner layer of the optic cup, the neural retina and the overlying ectoderm from which the cornea will develop. The expression of Pax-6 in the developing eye indicates that Pax-6 could be involved in some inducing events leading to the formation of the eye.

Molecular function of Pax proteins

Many lines of evidence support the idea that Pax proteins act as transcription regulators. Firstly, the Drosophila proteins Pox meso and Pox neuro (Bopp et al., 1989) as well as the murine proteins Pax-1 (R. Fritsch and P. Gruss, unpublished) and Pax-2 (Dressier and Douglas, 1992) are localized in the nucleus. Secondly, various paired domain proteins have been shown to bind specific DNA sequences (the paired gene product from Drosophila, Prd: Treisman et al., 1989, 1991; Pax-3: Goulding et al., 1991; Pax-1: Chalepakis et al., 1991; Pax-2: Dressier and Douglas, 1992; Pax-8: Zannini et al., 1992; Pax-5: Adams et al., 1992; Pax-6: our unpublished data). The nuclear localization and the DNA binding activity of these genes is consistent with their function as transcriptional modulators capable of regulating developmental processes. This property of Pax genes has been further characterized.

Treisman et al. (1989, 1991) have shown that Prd can bind to a sequence present in the even-skipped promoter (e5). Prd contains both a paired domain and a paired type homeodomain. Each domain, as a separate entity, can bind to different parts of the e5 sequence: while the homeodomain recognizes the upstream half of the e5 site which contains the typical ATTA motif, identified in several other homeodomain recognition sequences (Treisman et al., 1992; Laughon, 1991), the paired domain contacts the downstream half of the e5 site.

Pax-3 also contains a paired domain and a paired type homeodomain and binds, like Prd, to the e5 site. Binding interference experiments with Pax-3 on the e5 site revealed that Pax-3 contacts the upstream ATTA motif and the downstream sequences including the GTTCC motif. This GTTCC sequence was found to be required for efficient binding of all the Pax proteins tested so far. Interestingly, Pax-3 possesses a much lower DNA binding affinity for the isolated GTTCC motif and no binding is detected on the isolated ATTA motif of the e5 site. Thus, the paired domain and homeodomain bind cooperatively on the e5 site which includes both motifs. If the homeodomain is deleted, the resulting truncated protein binds with very high affinity to the GTTCC motif via its paired domain (our unpublished data). These data indicate that the paired domain might, in the native Pax-3 conformation, be masked by intra- or intermolecular interactions and be released when the homeodomain is deleted, thereby increasing its DNA binding affinity. If the homeodomain is also able to bind specific DNA sequences independently from the paired domain, Pax-3 would then exhibit three DNA binding activities: one mediated by the paired domain, one by the homeodomain and the third by both domains.

Pax-1, Pax-2 and Pax-8, which contain only a paired domain without a homeodomain, also bind to a modified e5 sequence. The nucleotide contacts of Pax-1 on the e5 site have been analyzed by binding interference experiments. Although Pax proteins bind to DNA as monomers, the contacted region is unusually large, encompassing 20 to 24 bp, making both minor and major groove contacts. A three-dimensional model of the contacted sites generated by computer graphics revealed that contact points are not confined to only one side but are rather scattered on all sides of the DNA helix (Chalepakis et al., 1991).

Pax-1, Pax-2 and Pax-8 are able to utilize the modified e5 site, subcloned in front of the thymidine kinase (TK) promoter, and to confer transcriptional activity to the TK promoter. The carboxy-terminal part of these proteins is rich in Pro, and also in Ser and Thr, resembling the transactivation domains of Oct-2 (Tanaka and Herr, 1990) and CTF-1 (Mermod et al., 1989). Nevertheless, it remains to be determined whether this region of Pax proteins encodes a functional transcriptional activation domain.

Pax-3 has been analyzed in more detail: it contains an autonomous transactivation domain encompassing the 80 carboxy-terminal amino acids and a transrepression domain located in the first 90 amino-terminal residues of the protein. Pax-3 is also able to utilize’the e5 sequence, in gene transfer experiments, and to confer promoter inducibility at low protein concentrations. At higher protein concentrations, the promoter activity is suppressed. This repression activity mediated by Pax-3 appears to be independent from specific DNA binding and to require the intact homeodomain (our unpublished data).

The DNA binding specificity of different paired domains was analyzed using 20 different sequences derived from the e5 site. The paired domains of Pax-1, Pax-3 and Pax-7 exhibit the same DNA binding specificity. Pax-2 and Pax-8 bind DNA with the same specificity and can be distinguished from Pax-1, Pax-3 and Pax-7 by their binding to a modified e5 sequence which is not bound by Pax-2 and Pax-8. On the other hand, the Pax-6 paired domain interacts preferentially with other sequences weakly recognized by other paired domains. Thus, based on the DNA binding specificity of the paired domains one can subgroup the Pax proteins into three families: the first includes Pax-1, Pax-3 and Pax-7, the second includes Pax-2 and Pax-8 and the third Pax-6. This classification closely resembles the previous classification based on protein conservation of the Pax genes (Fig. 1 and Walther et al., 1991).

Pax-8 has also been shown to bind a sequence in the proximal promoter region of thyroglobulin and thyroperoxidase and to enhance transcriptional activity of these genes (Zannini et al., 1992), reinforcing the idea that Pax-8 is involved in the differentiating events of the thyroid. Pax-1, Pax-6 and Pax-8 can activate the promoter of the c-fos oncogene in transient transfection experiments in NIH 3T3 cells. The Pax-mediated activation of 13 different c-fos promoter deletions was tested in serum-deprived cells and resembles the serum-mediated induction of these reporter constructs (our unpublished data). A Pax binding sequence was identified in the proximal promoter region. Further analysis will be required to decipher these molecular interactions. Finally, Pax binding sequences were also identified in the promoter region of the N-CAM gene which is, like cytotactin, activated by Pax-6 and Pax-8 in transient transfection experiments in NIH 3T3 cells.

Recently, it has been reported that Pax-5 encodes for the transcription factor BSAP which is expressed at the early stages of B-cell differentiation (Adams et al., 1992). Pax-5 is involved in the regulation of the CD 19 gene which encodes a B-lymphoid specific transmembrane receptor involved in signal transduction.

Pax mutants

One of the most surprising features of the Pax family is the identification of an unusually large number of both murine and human mutants which have been assigned to some of the Pax family members: Pax-1, Pax-3 and Pax-6. The semidominant nature of most of these mutations has facilitated the isolation and characterization of these mutants and has also brought some insight into the molecular action of the genes.

After its initial identification, mapping studies localized the Pax-1 gene on chromosome 2 between the (32-microglobulin and agouti loci, closely linked to the naturally occurring mutation undulated (un). Initially documented by Wright (1947) and Grtineberg (1950, 1954), this recessive mutation is characterized by a kinky-tail phenotype. Animals affected suffer from severe vertebral malformations along the entire vertebral axis: intervertebral disks are larger while the vertebral centers are reduced in size, leading to a distorted vertebral column. The primary defect appears to reside in the intrinsic inability of the scle-rotomal cells to form perichordal condensations, resulting in the improper allocation of these cells to intervertebral disks and vertebral bodies.

The pattern of Pax-1 expression correlates with these defects, being first observed in the ventral mesoderm at day-9 p.c., in the condensing sclerotomal cells at day-12 p.c. and, later on, in developing intervertebral disks (Deutsch et al., 1988). This evidence indicated that Pax-1 could be the gene altered in the un mutant. Indeed, sequencing of the Pax-1 paired box isolated from un mice, led to the identification of a single nucleotide mutation within the paired box, resulting in a glycine-to-serine exchange (Balling et al., 1988). Although these results provide only circumstantial evidence that Pax-1 and un are allelic, recent observations have reinforced this idea. First, two other undulated mutants, un-extensive (unex) and Un-short tail (Un’), also have alterations within the Pax-1 gene (Balling et al., 1992, Dietrich et al., unpublished data). With the recessive unex mutation, phenotypically stronger than un (Wallace, 1985), embryos contain fewer of the Pax-1 transcripts. Interestingly, Uns, the only un semidominant allele, harbours a complete deletion of Pax-1 (Balling et al., 1992). While heterozygous Uns animals present a kinky tail phenotype, homozygosity leads to late embryonal or neonatal death (Blandova and Egerov, 1975).

Secondly, gel shift experiments performed with Pax-1 have defined a synthetic core motif recognized by the Pax-1 protein. The point mutation found within the paired domain has similarly been shown to reduce the affinity of the Pax-lun protein for its target consensus sequence. In addition, this mutation enables the Pax-1 protein to bind to previously unrecognized sequences (Chalepakis et al., 1991). These data provide a molecular basis for the effect observed in the un mutant, strongly suggesting that this mutation results in the loss of Pax-1 protein’s ability to bind to its natural targets and to modulate their expression. Moreover, the demonstration that the Pax-lun mutant has acquired the ability to bind to new target sequences may indicate that some of the effects observed in the un mutant might be mediated by some dominant gain of function. Although the final proof of the identity of Pax-1 and un depends on rescue experiments of the undulated phenotype, these results taken together leave little doubt about the identity of the un mutation, clearly indicating that Pax-1 is a crucial determinant for the specification of sclerotomal mesoderm.

The Pax-3 gene (Goulding et al., 1991) has recently been shown to correspond to the genetic locus Splotch (Sp, Epstein et al., 1991; Goulding et al., unpublished data). This spontaneous mutant (Russell, 1947) exhibits, with several other alleles (Sp-retarded: Spr, Sp-delayed: Spd, SpIH, Sp2H), a semidominant phenotype (Dickie, 1964; Beechey and Searle, 1986). In the homozygous state, Sp is lethal. Abnormalities are observed in various Pax-3-expressing structures (Goulding et al., 1991). Within the central nervous system, exencephaly, spina bifida and meningocele are often associated with excessive neural growth (Auerbach, 1954; Franz, 1989, 1990). Moreover, these animals manifest defects associated with neural crest cell deficiency: absence or reduction of dorsal root ganglia, white spotting, Schwann cell and heart defects. Heterozygous animals present only pigmentation defects, detectable as white spotting mostly on the belly, but also on the back, the tail and limbs. Both Pax-3 and the various Sp alleles have been mapped to the proximal end of chromosome 1 (Goulding et al., 1991; Snell et al., 1954; Skew et al., 1988) and various alleles of Sp have now been characterized molecularly.

Firstly, the Spr allele has been shown to harbour a large cytogenetically detectable deletion encompassing the Pax-3 locus (Epstein et al., 1991). Secondly, the Sp2H allele contains a 32-bp deletion within the Pax-3 homeobox, introducing a nonsense mutation and resulting in premature translation termination (Epstein et al., 1991). This results in a Pax-3 protein deprived of part of its homeodomain and of its entire carboxy-terminal domain potentially involved in transactivation (our unpublished data). Thirdly, the Sp4H allele has been shown to harbour a large deletion encompassing most of the Pax-3 coding region (Goulding et al., unpublished data). Finally, the Sp allele generates, in addition to the normal full size transcript, a shortened version of the Pax-3 messenger, devoid of an exon which normally encodes the octapeptide (Goulding et al., unpublished data). The molecular defect underlying this splicing aberration has not been characterized.

A human syndrome has also been associated with alterations within the PAX3 gene. The Waardenburg syndromes (type 1 and type 2) present a combination of various defects such as deafness, pigmentary deficiency (heterochromia irides, white forelock and eyelash) as well as, in VES1 type I, dystopia canthorum (lateral displacement of the inner corner of the eye). The effects of these dominant syndromes most probably arise from cranial neural crest deficiencies. In five different families presenting WS syndromes, various alterations have been documented within the PAX3 gene: one allele with an 18-bp deletion within the paired box (WS.05; Tassabehji et al., 1992), one allele with a single bp mutation affecting an invariant residue preceding the first helix of the paired domain (Brazil; Baldwin et al., 1992), one allele harbouring a single bp deletion causing a frame shift within the paired domain (WS.06; A. Read, personal communication), another allele containing a 2-bp deletion causing a frameshift within the octapeptide motif (WS.l 1; A. Read, personal communication) and a last allele where a single bp mutation results in a glycine-to-alanine substitution within the paired domain (WS15; A. Read, personal communication).

The deficiencies associated with the various Sp and WS mutants together with the semidominant nature of these mutations indicate that Pax-3 constitutes a crucial factor in the development of the brain, the neural tube and neural crest derived structures.

Pax-6 is the last member of the Pax gene family shown to be mutated in mouse mutants. The semidominant mutant, Small eye (Sey) which was isolated by breeding of small body weight mice, is lethal at birth in the homozygous state. This condition is characterized by the complete absence of eyes and nose, two structures already shown to express Pax-6. In the less dramatic heterozygous state, animals present underdevelopment of the eye to various degrees.

Analysis of different alleles of small eye (Sey, SeyH, Seyneu) has led to the identification of various alterations within the Pax-6 gene. First, analysis of the Pax-6 gene of Sey embryos identified a single bp mutation which introduces a stop codon, truncating the protein between the paired domain and the homeodomain (Hill et al., 1991). Secondly, Seyneu, obtained by chemical mutagenesis (Favor et al., 1988) and presenting a phenotype similar to Sey, harbours a 116-nucleotide insert within the Pax-6 cDNA sequence (Hill et al., 1991). This insert originates from an unspliced intron, consequence of a point mutation within the splice acceptor consensus sequence. It introduces a nonsense mutation, resulting in truncation of the protein down-stream from its homeobox. The resulting protein lacks its 115 carboxy-terminal residues, a domain which contains a high serine/threonine content (25%), possibly involved in transcription activation. The SeyH and SeyDey allele appears to have the entire Pax-6 gene deleted (Hill et al., 1991; Ton et al., 1991).

The human condition known as anridia (AN) has already been proposed to be the counterpart of the murine Sey mutant (Glaser et al., 1990; van der Meer et al., 1990). This dominant inherited disorder presents phenotypic variability: partial or complete absence of iris is often accompanied by impaired vision, cataracts, foveal and optic nerve hypoplasia, corneal opacification and glaucoma (Shaw et al., 1960; Nelson et al., 1984). A putative cDNA candidate for the AN locus was recently cloned by chromosome walking and shown to be partially or totally deleted in two patients with aniridia (Ton et al., 1991). The corresponding 2.7-kb message is present in various tissues, most importantly the brain, the eyes and the olfactory bulb. The protein encoded by this cDNA contains a paired domain and a paired-type homeodomain and corresponds to the human counterpart of the murine Pax-6 cDNA. The drastic phenotype as well as the semidominant nature of the Sey and AN alleles, underlines the importance of Pax-6 in the development of the eye and nasal placodes.

The phenotypes observed with these various mutants most probably arise from the lost ability of the Pax gene products to modulate the expression of their normal target genes. However, one cannot exclude the possibility that, for some of these mutants, some particular activities of these proteins remain or that some gain-of-function might result from some of these mutations. This is well exemplified by some of the binding studies performed with Pax-1 and Pax-lun proteins in vitro (see above, Chalepakis et al., 1991).

The identification of five different mutants (un, Sey, Sp, AN, ITS) of murine or human origin corresponding to members of the Pax gene family emphasizes the importance of these genes as critical determinants of developmental processes. On the other hand, the nature of the phenotypes observed also raises important questions about the mechanism of action of these genes.

Firstly, an intriguing aspect of some of the Pax mutants resides in the absence of apparent phenotype in some structures expressing these genes during development. For instance, Pax-1 is present at high levels in the thymus and Pax-6 is present in the spinal cord, where no phenotypes have been yet documented. Although careful examination of these mutants using various markers might lead to the identification of subtle alterations within these compartments, this phenomenon might reflect some functional redundancies masking the effect of Pax gene deficiency. Such arguments have already been put forward for gene knock-out experiments where a phenotype was observed only in a subset of tissues normally expressing the knocked-out protein (Hox-1.5:Chisaka and Capecchi, 1991; Hox-1.6:Lufkin et al., 1991; Hox-3.1:Le Mouellic et al., 1992; Wnt-1:McMahon et al., 1992; En-2:Joyner et al., 1989).

Secondly, the semidominant nature of most of these mutants indicates that, according to the tissue, some considerable gene dosage effect may intervene. The gene product dosage may be, in certain instances, too low to allow proper tissue development to occur, while complete absence of the gene product completely annihilates the development of this same tissue. This is the case for the Sey mutation which results in the small eye phenotype in heterozygous animals and in the total absence of eye structures in homozygotes. The apparent normal development of the olfactory placode in the heterozygous animals, and its absence in homozygotes, also signifies that this tissue might require a lower threshold of Pax-6 protein (as compared to the eye) for proper development, or that alternatively it possesses a higher level of expression of Pax-6. Such an argument could also be made with Sp mutants where only pigmentation is affected in heterozygous animals. These observations underline the importance of the precise regulatory mechanisms controlling the expression of these genes during development as well as the precise balancing of the subordinate transcription processes regulated by the Pax proteins.

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