Moderation of phenotypic severity in dystrophic and junctional forms of epidermolysis bullosa through in-frame skipping of exons containing non-sense or frameshift mutations

A homozygous nonsense mutation in type XVII collagen gene (COL17A1) uncovers an alternatively spliced mRNA accounting for an unusually mild form of non-Herlitz junctional epidermolysis bullosa

In this study we describe six Italian patients presenting an unusually mild variant of non-Herlitz junctional epidermolysis bullosa associated with a reduced expression of type XVII collagen. All patients are homozygous for a novel nonsense mutation (R795X) within exon 33 of COL17A1 and show a common haplotype, attesting propagation of an ancestral allele within the Italian population. Analysis of patients' COL17A1 transcripts showed the presence of two mRNA species: a normal-sized mRNA carrying mutation R795X that undergoes rapid decay, and a transcript generated by in-frame skipping of exon 33. Patients keratinocytes were shown to synthesize minute amounts of type XVII collagen, which appeared correctly localized along the cutaneous basement membrane. We therefore suggest that the exon 33-deleted COL17A1 splice variant encodes for type XVII collagen molecules that maintain a functional role and account for the mild phenotype of our patients.

Laminins and human disease

The laminin protein family has diverse tissue expression patterns and is involved in the pathology of a number of organs, including skin, muscle, and nerve. In the skin, laminins 5 and 6 contribute to dermal-epidermal cohesion, and mutations in the constituent chains result in the blistering phenotype observed in patients with junctional epidermolysis bullosa (JEB). Allelic heterogeneity is observed in patients with JEB: mutations that results in premature stop codons produce a more severe phenotype than do missense mutations. Gene therapy approaches are currently being studied in the treatment of this disease. A blistering phenotype is also observed in patients with acquired cicatricial pemphigoid (CP). Autoantibodies targeted against laminins 5 and 6 destabilize epithelial adhesion and are pathogenic. In muscle cells, laminin alpha 2 is a component of the bridge that links the actin cytoskeleton to the extracellular matrix. In patients with laminin alpha 2 mutations, the bridge is disrupted and mature muscle cells apoptose. Congenital muscular dystrophy (CMD) results. The role of laminin in diseases of the nervous system is less well defined, but the extracellular protein has been shown to serve an important role in peripheral nerve regeneration. The adhesive molecule influences neurite outgrowth, neural differentiation, and synapse formation. The broad spatial distribution of laminin gene products suggests that laminin may be involved in a number of diseases for which pathogenic mechanisms are still being unraveled.

Herlitz junctional epidermolysis bullosa: novel and recurrent mutations in the LAMB3 gene and the population carrier frequency

Herlitz junctional epidermolysis bullosa is a heritable bullous disease caused by mutations found primarily in the b3 chain of laminin 5 (LAMB3). In this study, we examined the LAMB3 gene for mutations in 22 Herlitz junctional epidermolysis bullosa families, and identified 15 distinct mutations, eight of them previously unreported, bringing the total number of distinct Herlitz junctional epidermolysis bullosa mutations in LAMB3 to 35. Examination of the mutation database revealed several recurrent mutations that have been reported, as well as six previously unreported. All recurrent mutations may be readily detected by polymerase chain reaction of genomic DNA and restriction endonuclease digestion. Mutation screening and prenatal diagnosis of families at risk may be expedited by molecular testing for these recurrent mutations prior to screening the entire gene. Finally, the U.S. population carrier risk for Herlitz junctional epidermolysis bullosa and all variants of junctional epidermolysis bullosa was calculated to be one in 781 and one in 350, respectively, while the overall epidermolysis bullosa carrier frequency was calculated to be one in 113. These data allow accurate testing, counseling, and risk calculation for nuclear families, as well as extended family members at risk for junctional epidermolysis bullosa.

Mutation report: complete paternal uniparental isodisomy of chromosome 1: a novel mechanism for Herlitz junctional epidermolysis bullosa

Uniparental disomy denotes a situation when an individual has inherited two copies of a specific chromosome from a single parent. Uniparental disomy has been demonstrated to be involved in the pathogenesis of recessively inherited diseases in rare cases. Here we report a patient of Japanese origin with Herlitz junctional epidermolysis bullosa (OMIM no. 226700), who died at the age of 8 mo from complications of the disease. The mutation analysis revealed that the proband was homozygous for a nonsense mutation C553X in the LAMC2 gene encoding the gamma2 chain of laminin 5. The father was a heterozygous carrier of this mutation whereas the mother had two normal alleles of this gene. The patient showed homozygosity for 15 known intragenic polymorphisms in the LAMC2 gene. Furthermore, genotype analysis, performed from the parents and the proband, using 16 microsatellite markers spanning the entire chromosome 1, revealed that the patient was homozygous for all markers tested, and that these alleles originated from the father. Among the 16 markers, eight were fully informative for the absence of the maternal chromosome 1 in the proband, suggesting that the patient had complete paternal isodisomy of this chromosome. Thus, the Herlitz junctional epidermolysis bullosa phenotype in this patient is caused by homozygous LAMC2 mutation C553X that is of paternal origin and results from nondisjunction and uniparental disomy involving monosomy rescue. This is a novel mechanism resulting in Herlitz junctional epidermolysis bullosa and has implications for assessment of the risk in subsequent pregnancies.

The molecular basis of dystrophic epidermolysis bullosa in Mexico

BACKGROUND

Type VII collagen gene (COL7A1) mutations are the cause of dystrophic epidermolysis bullosa (DEB), but most mutations are specific to individual families, and there are limited data on the nature of COL7A1 mutations in certain ethnic populations.

OBJECTIVE

To determine the molecular basis of DEB in Hispanic Mexican patients.

METHODS

Patients were recruited through a newly established support group, Fundacion DEBRA Mexico. Molecular analysis was performed by polymerase chain reaction (PCR) of genomic DNA using COL7A1-specific primers, heteroduplex analysis, and direct nucleotide sequencing.

RESULTS

Fifty-nine of a possible 67 COL7A1 mutations (88%) were identified in 36 affected individuals (31 recessive, five dominant) in 21 families. Recessive mutations included six frameshift mutations, four silent glycine substitutions, and two splice-site mutations. Dominant mutations comprised a de novo glycine substitution and an internal deletion. Conclusions This study establishes the molecular basis of DEB in a group of Mexican patients. Only two of the mutations have been identified previously in other ethnic groups; the remainder are specific to this population. These new data are helpful in facilitating the accurate diagnosis of DEB subtype, in improving genetic counseling, and in providing further insight into the pathophysiology of this mechanobullous disease.