Compound Heterozygosity for a Recessive Glycine Substitution and a Splice Site Mutation in the COL7A1 Gene Causes an Unusually Mild Form of Localized Recessive Dystrophic Epidermolysis Bullosa

Abstract

Type VII collagen is the major component of anchoring fibrils, adhesion structures of stratified epithelia that span the basement membrane region and papillary dermis. Mutations in the gene COL7A1 encoding type VII collagen cause dystrophic epidermolysis bullosa, a clinically heterogeneous autosomal dominant or recessive blistering disorder of the skin and mucous membranes. In this report, we investigate three siblings affected by an unusually mild form of localized recessive dystrophic epidermolysis bullosa who were shown to be compound heterozygotes for novel mutations affecting COL7A1. The maternally inherited mutation is a G-->C transversion that converts a codon for glycine to a codon for arginine (G1347R). The paternal mutation is a neutral G-->A transition at the last base of exon 70(5820G-->A) that alters the correct splicing of COL7A1 pre-mRNA, giving rise to an aberrant mRNA carrying the in-frame skipping of exon 70 in addition to the full-length RNA transcript carrying the G-->A substitution. Consistent with the normal levels of COL7A1 mRNA transcripts detected by northern analysis, immunoblotting and immunofluorescence studies evidenced that the patient keratinocytes synthesize and secrete normal amounts of stable type VII collagen, which is correctly deposited at the dermal-epidermal junction. In addition, mutated type VII collagen molecules assemble to form numerous, normally shaped anchoring fibrils, as shown by electron microscopic examination. The combination of a recessive glycine substitution with a splice site mutation that permits partially correct splicing therefore leads to a normal expression of mutated type VII collagen molecules with marginally altered biologic activity, and to the extremely mild phenotype observed in our patients.

The inversa type of recessive dystrophic epidermolysis bullosa is caused by specific arginine and glycine substitutions in type VII collagen

“…However, the four glycine codons involved in RDEB-I with limited acral involvement have been reported in the RDEB-sev gen or DDEB phenotypes. In contrast, of the glycine codons involved in the RDEB-I ‘mucosal only’ phenotype, only glycine codon 1347 was reported in other, milder RDEB phenotypes 10 29 31…”

Truncated Type XVII Collagen Expression in a Patient with Non-Herlitz Junctional Epidermolysis Bullosa Caused by a Homozygous Splice-Site Mutation

“…Donor splice-site mutations of genes other than COL17A1 have been observed in epidermolysis bullosa patients. In a number of cases this resulted in a similar effect on transcription, namely activation of cryptic splice sites and exon skipping (Gardella et al, 1996;Posteraro et al, 1998;Terracina et al, 1998;Weil et al, 1988). In some patients, this was accompanied by a milder phenotype.…”

Mutations in the hairless gene underlie APL in three families of Pakistani origin

“…The hairless gene product is a transcriptional co-regulator with a single zinc-finger domain, which is highly expressed in the skin and the brain [7][8][9]. Furthermore, mutations in the hairless (HR) gene were identified in many APL families and to date, more than 30 hairless mutations have been reported [10][11][12][13][14][15][16][17][18]. Although APL could potentially be diagnosed by its clinical features alone, or by skin biopsy, APL remains frequently misdiagnosed as the more common disorder, alopecia universalis (AU).…”

“…Therefore, many more mutations are likely to be discovered, since APL appears to be more common than previously considered [15]. To date, HR mutations have been found in APL patients from various ethnic backgrounds, including Pakistani, Mediterranean, Arab Palestinian, Caucasians, Japanese and Polish [10][11][12][13][14][15][16][17][18]. In this study, we identified three Pakistani families originating from different regions of Pakistan with clinical manifestations of APL and high degree of consanguinity.…”

“…The consequence of this mutation could result in the production of several mutant mRNA species. Exon skipping is a well-known potential consequence of mutations which disrupt consensus splicing sequences [18][19][20]. The presence of exon skipping in this case could not be assessed as tissue samples for analysis of mRNA species generated by this mutation were not available for study, however, exon 15 is 119 bp in size, predicting its skipping would be out-of-frame.…”