Mutation in Mpzl3, a novel [corrected] gene encoding a predicted [corrected] adhesion protein, in the rough coat (rc) mice with severe skin and hair abnormalities

Abstract

The rough coat (rc), an autosomal-recessive mutation, arose spontaneously in C57BL/6J mice. Homozygous rc mice develop severe skin and hair abnormalities, including cyclic and progressive hair loss and sebaceous gland hypertrophy. The rc locus was previously mapped to Chromosome 9. To elucidate the genetic basis underlying the rc phenotype development, we carried out positional cloning, and mapped the rc locus to a 246-kb interval. We identified a missense mutation within a novel open reading frame in the rc/rc mice, which is predicted to encode a cell adhesion molecule with the highest homology to myelin protein zero (MPZ) and myelin protein zero-like 2 (MPZL2, also called epithelial V-like antigen). We therefore named this gene Mpzl3 (myelin protein zero-like 3). The mutation in the rc/rc mice occurred at a highly conserved residue within the conserved Ig-like V-type domain, thus likely altering the MPZL3 protein function. Reverse transcriptase-PCR and Western blot analyses revealed expression of the Mpzl3 gene in various adult organs, including the skin. Using indirect immunofluorescence, we detected MPZL3 protein in the keratinocytes and sebocytes in the skin. Results from this study identified a novel gene encoding a predicted adhesion protein whose mutation in the rc/rc mice likely caused the rc phenotype.

Figure 1. Gross phenotype of the rc/rc mice

(a) Hair loss is apparent on the dorsal trunk of an rc/rc mouse on day 25 after birth. (b) New hair coat retains the rough coat phenotype, sometimes with reduced pigmentation. (c) Hair loss is progressive in older rc/rc mice. (d) In more than 50% of rc/rc mice older than 1 year, ulcerated wounds develop spontaneously in the ventral neck region (arrow).

Figure 2. Sebaceous gland hypertrophy in the rc/rc mice

(a, b) Hematoxylin and eosin staining and (c, d) oil red O staining of lipids in back skin sections from (a, c) a +/rc and (b, d) an rc/rc mouse. Arrowheads point to the sebaceous glands. (b, d) Notice the hypertrophic sebaceous glands in the rc/rc mouse. The sebocytes (d) in the rc/rc mouse are functional lipid-secreting cells as (c) in the normal mouse. Bar = 100 μm.

Figure 3. High-resolution genetic map of the rc interval and the candidate genes

(a) High-resolution genetic map of the rc interval based on linkage analysis in 200 C57BL/6J-BALB/cJ and 361 C57BL/6J-CAST/Ei F₂ hybrid offspring. Microsatellite markers in the Ensembl Mouse Genome Database (www.ensembl.org/Mus_musculus) are shown on the left with their chromosomal locations in megabases (Mb). Novel microsatellite polymorphisms between the C57BL/6 and CAST/Ei strains identified in this study are shown on the right. Marker #10 was concordant with rc in all the F₂ hybrids we analyzed, and its location relative to the rc locus could not be determined in our linkage analysis. (b) Haplotype analysis of the three recombinants with D9Mit191 or marker #3 among the 361 C57BL/6J-CAST/Ei F₂ hybrid offspring. These results show that the rc locus lies within a 246-kb interval. (c) Candidate genes in the 246 kb mapping interval for the rc locus (Ensembl Mouse Genome Database, www.ensembl.org/Mus_musculus).

Figure 4. Gene structure and mutation analysis of a novel gene in which a mutation was identified in the rc/rc DNA

(a) Sequence analysis identified a point mutation in a predicted open reading frame in the rc/rc genomic DNA, which was confirmed in rc/+ genomic DNA and rc/rc cDNA. We have named this gene “Mpzl3” based on the similarity of the predicted domain structure of its encoded protein to MPZ and MPZL2 (also called EVA1). (b) The Mpzl3 gene (ENSMUSG00000070305), predicted on the basis of multiple Ensembl transcripts based on ESTs, in its genomic context. Eva1 (Mpzl2) gene is upstream, and Amica1 gene is downstream. (c) Our prediction of the murine Mpzl3 gene that is homologous to the human gene ENSG00000160588 and rat gene ENSRNOESTG00000015598. Exon and EST information was based on the Ensembl Genome Database (www.ensembl.org) and our prediction by sequence comparison between species. Numbers underneath each exon indicate the length of the coding sequence in that exon (filled portion). Exon 2 has 167 bp of coding sequence, and exon 2′ has 215 bp of coding sequence (48 bp longer than exon 2) as well as a 3′ UTR. Block arrow points to the splice site in exon 2′ used to generate the six-exon transcript. The arrows denote primers 2F, 2R, 4F, and 4R used for RT-PCR analysis in Figure 6. *The position of the mutation detected in rc/rc DNA. (d) Exon–intron information of the predicted murine Mpzl3 gene.

Figure 5. Sequence comparison of the human, murine, and rat MPZL3 protein and murine MPZ and MPZL2 (EVA1)

The signal peptides and transmembrane domains are underlined, flanking the Ig-like V-type domains (in box), with conserved residues highlighted bold. N-linked glycosylation sites and the Arg100 → Gln substitution caused by the G → A mutation in the rc/rc mice are shown in small boxes. Conserved “signature” cysteines for disulfide bond formation in the Ig-like domains are marked by arrowheads. The GenBank accession number for murine Mpzl3 gene/mRNA is EF102773.

Figure 6. RT-PCR analysis of Mpzl3 gene expression

(a) Expression of the six-exon Mpzl3 transcripts (2F-4R) and the two-exon Mpzl3 transcripts (2F-2R) in normal adult mouse organs. The positions of the primers are shown in Figure 4c. The two transcripts had similar tissue distribution. (b) Expression of the six-exon Mpzl3 transcripts (4F-4R to amplify exons 3–6) and the two-exon Mpzl3 transcripts (2F-2R) in wild-type (WT) and rc/rc mouse skin.

Figure 7. Characterization of the anti-MPZL3 antibodies and expression of MPZL3 in adult mouse organs

(a–d) Western blot analysis of total cell extracts from control NIH/3T3 cells (lane 1) and NIH/3T3 cells transfected with pEF/myc/cyto containing wild-type murine Mpzl3 cDNA (lane 2). The primary antibodies used were: (a) rabbit anti-MPZL3 peptide DKLTIDWTYRPPSSSRT affinity-purified antibodies, (b) mouse anti-Myc tag antibody, rabbit anti-MPZL3 affinity-purified antibodies preabsorbed with (c) 1:3 or (d) 1:10 molar ratio of peptide DKLTIDWTYRPPSSSRT. Blot c showed very faint bands of 27 and 29 kDa after a much longer exposure (data not shown). (e) Western blot analysis of total protein extracts from adult mouse organs detected by rabbit anti-MPZL3 affinity-purified antibodies. Sp: spleen; Mu: muscle; Lu: lung; B: brain; Li: liver; T: testis; I: intestine; St: stomach; K: kidney; H: heart; Sk: skin. M: MagicMark XP.

Figure 8. Expression of MPZL3 in mouse skin sections detected by indirect immunofluorescence

(a–f) MPZL3 (green) was detected in the keratinocytes of the epidermis and hair follicles as well as (a–d, g) the sebocytes. (b, d–g) Sections were counterstained with propidium iodide (red). (a, b): +/+ skin; (c–g): rc/rc skin. Bar = 50 μm in (a–d), 20 μm in (e–g).