Portal de Periódicos da CAPES

SNPing at the Epidermal Barrier

2011; Elsevier BV; Volume: 131; Issue: 8 Linguagem: Inglês

10.1038/jid.2011.92

1523-1747

David P. Kelsell, Carolyn Byrne,

Food Allergy and Anaphylaxis Research

Filaggrin variants are well-established risk factors for atopic eczema (AE). Recent studies suggest additional epidermal differentiation complex (EDC) gene associations with AE. In this issue, Marenholz and colleagues confirm this prediction and show that a small proline-rich protein 3 (SPRR3) variant confers susceptibility to AE. This finding suggests that further genetic and functional characterization of SPRR3 should be performed in patients with AE. Filaggrin variants are well-established risk factors for atopic eczema (AE). Recent studies suggest additional epidermal differentiation complex (EDC) gene associations with AE. In this issue, Marenholz and colleagues confirm this prediction and show that a small proline-rich protein 3 (SPRR3) variant confers susceptibility to AE. This finding suggests that further genetic and functional characterization of SPRR3 should be performed in patients with AE. The chromosomal region 1q21.3 (the epidermal differentiation complex, or EDC) has been widely studied in epidermal biology and disease because it harbors a large number of genes (around 60) expressed in late keratinocyte differentiation (Mischke et al., 1996Mischke D. Korge B.P. Marenholz I. et al.Genes encoding structural proteins of epidermal cornification and S100 calcium-binding proteins form a gene complex ("epidermal differentiation complex") on human chromosome 1q21.J Invest Dermatol. 1996; 106: 989-992Crossref PubMed Scopus (396) Google Scholar). These include genes encoding proteins such as filaggrin (FLG), loricin, and gene family clusters, including the late cornified envelope genes, S100s, and the small proline-rich proteins (SPRRs). They are involved in various aspects of stratum corneum formation, including keratin filament aggregation (FLG) and cornified envelope formation (loricin), or are implicated in host defense (the S100s). Mutations in FLG and loricin are associated with ichthyosis vulgaris (Smith et al., 2006Smith F.J. Irvine A.D. Terron-Kwiatkowski A. et al.Loss-of-function mutations in the gene encoding filaggrin cause ichthyosis vulgaris.Nat Genet. 2006; 38 (42s): 337Crossref PubMed Scopus (755) Google Scholar) and a variant of Vohwinkel's syndrome (Maestrini et al., 1996Maestrini E. Monaco A.P. McGrath J.A. et al.A molecular defect in loricrin, the major component of the cornified cell envelope, underlies Vohwinkel's syndrome.Nat Genet. 1996; 13: 70-77Crossref PubMed Scopus (201) Google Scholar), respectively. However, the most striking finding has been that loss-of-function mutations in FLG are strongly associated with atopic eczema (AE), implicating an impaired epidermal barrier in the development of this chronic inflammatory skin disease that affects up to 20% of children in some countries (Palmer et al., 2006Palmer C.N. Irvine A.D. Terron-Kwiatkowski A. et al.Common loss-of-function variants of the epidermal barrier protein filaggrin are a major predisposing factor for atopic dermatitis.Nat Genet. 2006; 38: 441-446Crossref PubMed Scopus (2088) Google Scholar). It is thus intriguing that FLG appears to be the main culprit in the EDC for AE susceptibility, although other EDC genes could be predicted to have similar deleterious effects on epidermal barrier function. Three recent areas of genetic investigation suggest that FLG may not be solely responsible. First, genetic studies (Cascella et al., 2011Cascella R. Cuzzola V.F. Lepre T. et al.Full sequencing of the FLG gene in Italian patients with atopic eczema: evidence of new mutations, but lack of an association.J Invest Dermatol. 2011; 131: 982-984Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar; Sinclair et al., 2009Sinclair C. O'Toole E.A. Paige D. et al.Filaggrin mutations are associated with ichthyosis vulgaris in the Bangladeshi population.Br J Dermatol. 2009; 160: 1113-1115Crossref PubMed Scopus (8) Google Scholar) show that, although present, FLG mutations are not strongly implicated in AE susceptibility in Italian and Bangladeshi populations. Second, expression studies show dysregulation of a number of EDC proteins in AE skin (Sugiura et al., 2005Sugiura H. Ebise H. Tazawa T. et al.Large-scale DNA microarray analysis of atopic skin lesions shows overexpression of an epidermal differentiation gene cluster in the alternative pathway and lack of protective gene expression in the cornified envelope.Br J Dermatol. 2005; 152: 146-149Crossref PubMed Scopus (137) Google Scholar; Saaf et al., 2008Saaf A.M. Tengvall-Linder M. Chang H.Y. et al.Global expression profiling in atopic eczema reveals reciprocal expression of inflammatory and lipid genes.PLoS One. 2008; 3: 24Crossref Scopus (62) Google Scholar; Guttman-Yassky et al., 2009Guttman-Yassky E. Suarez-Farinas M. Chiricozzi A. et al.Broad defects in epidermal cornification in atopic dermatitis identified through genomic analysis.J Allergy Clin Immunol. 2009; 124: 1235-1244Abstract Full Text Full Text PDF PubMed Scopus (177) Google Scholar). Third, a genome-wide association study supports another locus (or loci) associated with AE within the EDC (Esparza-Gordillo et al., 2009Esparza-Gordillo J. Weidinger S. Folster-Holst R. et al.A common variant on chromosome 11q13 is associated with atopic dermatitis.Nat Genet. 2009; 41: 596-601Crossref PubMed Scopus (237) Google Scholar). The latter, in particular, led Marenholz et al., 2011Marenholz I. Gimenez Rivera V.A. Esparza-Gordillo J. et al.Association screening in the epidermal differentiation complex (EDC) identifies an SPRR3 repeat number variant as a risk factor for eczema.J Invest Dermatol. 2011; 131: 1644-1649Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar, this issue) to investigate whether coding sequence variants in other EDC genes were associated with AE. The investigators examined the National Center for Biotechnology Information (NCBI) database to identify likely deleterious coding sequence variants in other EDC genes, primarily insertion/deletion, frameshift, and stop codon mutations. They selected 20 such variants and found that rs28989168 (variant allele is an in-frame 24-bp insertion in SPRR3) was associated with eczema after replication studies (odds ratio of 1.30). These data demonstrate two distinct proteins within the EDC that are linked to the impaired epidermal barrier function associated with AE susceptibility. SPRR family members are thought to have a structural role in the cornified envelope as linkers for transglutaminase-mediated protein cross-bridging (Steinert et al., 1998Steinert P.M. Candi E. Kartasova T. et al.Small proline-rich proteins are cross-bridging proteins in the cornified cell envelopes of stratified squamous epithelia.J Struct Biol. 1998; 122: 76-85Crossref PubMed Scopus (76) Google Scholar), and they have been proposed as reactive oxygen species quenchers, with an important function at the edges of wounds (Vermeij and Backendorf, 2010Vermeij W.P. Backendorf C. Skin cornification proteins provide global link between ROS detoxification and cell migration during wound healing.PLoS One. 2010; 5 (e11957)Google Scholar). SPRR proteins contain glutamine and lysine-rich N- and C-termini, which provide transglutaminase substrates. The termini flank 8- to 9-amino-acid proline-rich internal repeats, varying in sequence and number among family members, and act as flexible, cross-bridging spacers (Steinert et al., 1998Steinert P.M. Candi E. Kartasova T. et al.Small proline-rich proteins are cross-bridging proteins in the cornified cell envelopes of stratified squamous epithelia.J Struct Biol. 1998; 122: 76-85Crossref PubMed Scopus (76) Google Scholar). Repeat variability is thought to confer differences in flexibility, strength, and toughness to the stratum corneum. Accordingly, Marenholz et al., 2011Marenholz I. Gimenez Rivera V.A. Esparza-Gordillo J. et al.Association screening in the epidermal differentiation complex (EDC) identifies an SPRR3 repeat number variant as a risk factor for eczema.J Invest Dermatol. 2011; 131: 1644-1649Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar speculate that the eczema-associated SPRR3 variant, with an additional 8-amino-acid repeat, influences barrier function by altering the physical properties of the stratum corneum. SPRR3 is a surprise candidate for a functionally important skin barrier gene in that it is abundantly expressed in internal epithelia (for example, the tongue, esophagus, and tonsil) and undetectable in normal skin (Cabral et al., 2001Cabral A. Voskamp P. Cleton-Jansen A.M. et al.Structural organization and regulation of the small proline-rich family of cornified envelope precursors suggest a role in adaptive barrier function.J Biol Chem. 2001; 276: 19231-19237Crossref PubMed Scopus (126) Google Scholar; BioGPS, http://biogps.gnf.org). Its identification as a skin barrier component suggests that minor stratum corneum protein components can substantially affect barrier function. Alternatively, the SPRR3 variant may influence skin barrier activity as part of a skin stress response because the expression of SPRR proteins increases in response to diverse influences including UVR, aging, cancer, and wound healing (Vermeij and Backendorf, 2010Vermeij W.P. Backendorf C. Skin cornification proteins provide global link between ROS detoxification and cell migration during wound healing.PLoS One. 2010; 5 (e11957)Google Scholar; Cabral et al., 2001Cabral A. Voskamp P. Cleton-Jansen A.M. et al.Structural organization and regulation of the small proline-rich family of cornified envelope precursors suggest a role in adaptive barrier function.J Biol Chem. 2001; 276: 19231-19237Crossref PubMed Scopus (126) Google Scholar, and references therein). A role for SPRR3 in events triggering eczema outbreaks (for example, repair of barrier microlesions after scratching or in response to infection) is plausible, and its function in skin requires further elucidation. Another surprising finding reported by Marenholz et al., 2011Marenholz I. Gimenez Rivera V.A. Esparza-Gordillo J. et al.Association screening in the epidermal differentiation complex (EDC) identifies an SPRR3 repeat number variant as a risk factor for eczema.J Invest Dermatol. 2011; 131: 1644-1649Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar is the large number of sequence variants annotated in the NCBI database that they could not find in their patient samples, with only 4 of 20 being experimentally validated. The investigators suggested this may be attributable to the rarity of these alleles (although they tested 94 individuals), their population specificity, or their being sequence artifacts. We have revisited these original 20 sequence variants in a newer build of the human genome sequence; we also looked for their presence in the 1,000-genome database accessed via Ensembl, and we suggest that the 16 unvalidated variants were likely to be artifacts. For example, of the four FLG2 single-nucleotide polymorphisms (SNPs) genotyped by Marenholz et al., only rs12568784 (the only FLG2 SNP they experimentally validated by PCR) was found in the 1,000-genome dataset. In light of this, we revisited SPRR3 to determine what other potentially deleterious sequence variants or SNPs have been annotated in the current sequence build. Table 1 shows that further SPRR3 coding variants warrant investigation as risk factors linked to AE susceptibility. These include two 8-amino-acid insertion/deletion sequence variants (rs67156933 and rs66616552) in addition to the AE-associated rs28989168, suggesting that these should also be assessed for association with AE.Table 1Nonsynonymous sequence variation at the SPRR3 locusVariation IDVariation typeEffect on proteinrs67156933DeletionIn-frame 8-amino-acid insertion/deletionrs28989168DeletionIn-frame 8-amino-acid insertion/deletionrs66616552DeletionIn-frame 8-amino-acid insertion/deletionrs17845559Nonsynonymous codingAmino acid substitution (valine or leucine)rs1055935Nonsynonymous codingAmino acid substitution (leucine or valine)rs2075740Nonsynonymous codingAmino acid substitution (threonine or methionine)rs28989168 (in bold) is the insertion polymorphism associated with atopic eczema in the study by Marenholz et al. (2011). The other five coding polymorphisms were not genotyped. Open table in a new tab rs28989168 (in bold) is the insertion polymorphism associated with atopic eczema in the study by Marenholz et al. (2011). The other five coding polymorphisms were not genotyped. The study by Marenholz et al., 2011Marenholz I. Gimenez Rivera V.A. Esparza-Gordillo J. et al.Association screening in the epidermal differentiation complex (EDC) identifies an SPRR3 repeat number variant as a risk factor for eczema.J Invest Dermatol. 2011; 131: 1644-1649Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar highlights how complex the genetic makeup of AE susceptibility is likely to be. On the other hand, it emphasizes that the epidermal barrier is a major factor in AE susceptibility. From a mechanistic point of view, developing topical applications that improve the stratum corneum should therefore result in improvement for many patients with AE despite a distinct genetic contribution to their etiology. In addition, the current and future impact of high-throughput sequencing in understanding genetic disease cannot be ignored. Since the completion of Marenholz et al., 2011Marenholz I. Gimenez Rivera V.A. Esparza-Gordillo J. et al.Association screening in the epidermal differentiation complex (EDC) identifies an SPRR3 repeat number variant as a risk factor for eczema.J Invest Dermatol. 2011; 131: 1644-1649Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar, the 1,000-genome project is revealing new sequence variants and providing allele frequency data. The decreasing costs of high-throughput sequencing, including exome sequencing, allow geneticists to return to family-based studies (as opposed to the recent focus on identifying "low"-risk variants through large-scale genome-wide association studies). The exome sequencing of AE families is likely to reveal "private" genetic variation linked to AE susceptibility, which may be family specific—akin to that revealed for many monogenic skin diseases. The question is whether the majority will still be found within the EDC.