Breakdown of the reciprocal stabilization of QBRICK/Frem1, Fras1, and Frem2 at the basement membrane provokes Fraser syndrome-like defects

Abstract

An emerging family of extracellular matrix proteins characterized by 12 consecutive CSPG repeats and the presence of Calx-␤ motif(s) includes Fras1, QBRICK͞Frem1, and Frem2. Mutations in the genes encoding these proteins have been associated with mouse models of Fraser syndrome, which is characterized by subepidermal blistering, cryptophthalmos, syndactyly, and renal dysmorphogenesis. Here, we report that all of these proteins are localized to the basement membrane, and that their basement membrane localization is simultaneously impaired in Fraser syndrome model mice. In Frem2 mutant mice, not only Frem2 but Fras1 and QBRICK͞Frem1 were depleted from the basement membrane zone. This coordinated reduction in basement membrane deposition was also observed in another Fraser syndrome model mouse, in which GRIP1, a Fras1-and Frem2-interacting adaptor protein, is primarily affected. Targeted disruption of Qbrick͞Frem1 also resulted in diminished expression of Fras1 and Frem2 at the epidermal basement membrane, confirming the reciprocal stabilization of QBRICK͞ Frem1, Fras1, and Frem2 in this location. When expressed and secreted by transfected cells, these proteins formed a ternary complex, raising the possibility that their reciprocal stabilization at the basement membrane is due to complex formation. Given the close association of Fraser syndrome phenotypes with defective epidermal-dermal interactions, the coordinated assembly of three Fraser syndrome-associated proteins at the basement membrane appears to be instrumental in epidermal-dermal interactions during morphogenetic processes.epithelial-mesenchymal interaction ͉ gene targeting ͉ morphogenesis T he extracellular matrix (ECM) is an insoluble supramolecular complex surrounding metazoan cells that is often fibrous or sheet-like. The ECM functions in the control of cellular behaviors, including migration, proliferation, and differentiation, and mediates intercellular communication, as in epithelialmesenchymal interactions; both of these functions are critical during development. Because individual ECM components often function in combination with other ECM components and soluble factors, genetic disorders of the ECM are often linked to severe developmental abnormalities.

p63 induces key target genes required for epidermal morphogenesis

“…Among these was the gene encoding Fras1, a keratinocyte-produced extracellular matrix protein which forms a protein complex with its family members Frem1 (Fras1-related ECM protein 1) and Frem2 (Fras1-related ECM protein 2) (37). The absence of any one of these proteins results in a failure of this protein complex to form, causing a severe embryonic blistering phenotype in mice (37)(38)(39)(40). Indeed, real-time RT-PCR analysis confirmed that Fras1 was downregulated in ⌬Np63 i-kd skin (Fig.…”

Expression of the fras1/frem gene family during zebrafish development and fin morphogenesis

“…The most notable of these is the difference in gene structure encoding orthologous Frem3 proteins, in which a "complete" C-terminal domain comprising 5 Calx␤, a transmembrane and a PDZ interaction domain are present in fish but are "lost" in higher eutherians. This difference is significant, because in mouse Fras1 and Frem2, these domains mediate their interaction and subsequent traffic with the intracellular Grip1 protein, as evidenced by the mislocalization of these proteins in its absence (Takamiya et al, 2004;Kiyozumi et al, 2006). Consequently, the absence of these domains in the mouse essentially renders Frem3 independent of Grip1, notionally allowing the protein to assume functions independent of the Frem2-Frem1-Fras1 complex.…”

“…These observations, along with the domain structure of the proteins, led us to speculate that the proteins interact directly to mediate epidermal adhesion (Smyth et al, 2004). Elegant biochemical studies have subsequently shown that Frem1, Frem2, and Fras1 form a ternary complex in vitro and that this complex is most probably required for normal epidermal adhesion in utero (Kiyozumi et al, 2006). It is unclear whether Frem3 is involved in this complex, but the gene's divergent expression and the normal localization of the protein in mice carrying mutations in the other three proteins suggest that it acts in an independent manner (Chiotaki et al, 2007;Petrou et al, 2007b).…”

Fraser Syndrome: Case Report With Review of Literature.

“…It is caused by mutation in FRAS 1 gene located on the long arm of chromosome 4 (4q21), FREM2 gene on chromosome 13, FREN1 and GPIP1. 5,6,12 It is inherited in autosomal recessive fashion. The term cryptophthalmos was introduced by Zehender et al 10 in 1872 who described a child whose eyes were covered by continuous sheets of skin from forehead to cheek, associated with additional malformations including hypertelorism, syndactyly, abnormal genitalia, umbilical hernia, anal stenosis and hoarse voice.…”

“…Fraser syndrome (FS) is an autosomal recessive condition with incidence of 0.043 per 10,000 live born infants and 1.1 in 10,000 stillbirths, making it a rare syndrome 3,4 . Mutations in FRAS1, FREN1, FREM2 and GPIP1 gene have been reported to underlie FS, indicating genetic heterogeneity 5,6 . These genes encode the extracellular matrix proteins that are necessary for the adhesion between basement membrane of epidermis and connective tissues of dermis during embryological development.…”