Loss of desmoplakin tail causes lethal acantholytic epidermolysis bullosa

Abstract

The cytoplasmic plaque protein desmoplakin (DP), which is located in desmosomes, plays a major role in epithelial and muscle cell adhesion by linking the transmembrane cadherins to the cytoplasmic intermediate filament network. Mutations of DP may cause striate palmoplantar keratoderma, arrhythmogenic right ventricular dysplasia, skin fragility/woolly hair syndrome, Naxos-like disease, and Carvajal syndrome. DP must be indispensable, because DP-/- mice are early abortive. Here, we report a patient with severe fragility of skin and mucous membranes caused by genetic truncation of the DP tail. The new phenotype is lethal in the neonatal period because of immense transcutaneous fluid loss. The phenotype also comprised universal alopecia, neonatal teeth, and nail loss. Histology showed suprabasal clefting and acantholysis throughout the spinous layer, mimicking pemphigus. Electron microscopy revealed disconnection of keratin intermediate filaments from desmosomes. Immunofluorescence staining of DP showed a distinct punctate intercellular pattern in the patient's skin. Protein analysis revealed expression of truncated DP polypeptides. Mutational analysis of the patient demonstrated compound heterozygosity for two DP mutations, 6079C-->T (R1934X) and 6370delTT, respectively. Aberrant mRNA transcripts that predict premature termination of translation with loss of the three intermediate filament-binding subdomains in the DP tail were detected by RT-PCR. The new dramatic phenotype, which we named "lethal acantholytic epidermolysis bullosa," underscores the paramount role of DP in epidermal integrity.

Figure 1

Clinical presentation of day 1. A, B, Generalized skin erosions covering 70% of the body surface area. B, C, Universal alopecia. D, neonatal teeth. E and F, Eroded skin of the hands and feet, respectively, with complete nail loss.

Figure 2

A, Histopathology of skin showing suprabasal clefting and acantholysis throughout the spinous layer, leaving a row of basal keratinocytes (“tombstones”) on the (B) blister floor (objective 40 times). C, Electron microscopy revealed that the desmosomal connection remained intact while the plasma membrane was stretched to its limits until intracellular cleavage occurred behind the desmosomal plaque, which was torn from the cell body (arrow). D, Desmosome, lacking bound intermediate filaments, contained an inner (IDP) and outer (ODP) dense plaque of normal thickness. E and F, Cultures of patient’s keratinocytes show loosened cell-to-cell contacts (spongiosis representing early acantholysis) compared with the close apposition between control human keratinocytes.

Figure 3

Immunofluorescence with monoclonal antibodies of patient’s skin (A, C, and E) and age-matched human control skin (B, D, and F). Desmoplakin I/II in the patient shows a more punctate intercellular staining (A) than in human control skin (B). Note the suprabasal clefting in the patient (A). The suprabasal keratin 10 is retracted around nuclei in the patient (C) as compared with a control subject (D). The adherens junction component β-catenin (9G2) has a similar fine granular intercellular staining pattern in patient skin (E) and control skin (F).

Figure 4

Mutations in exon 24 of the DSP gene in the patient and his parents. A, Automated sequence analysis of the patient’s and his father’s DNA revealed a heterozygous C→T transition at nucleotide position 6079 of the cDNA (lower panel). The mutation was not present in the mother and a control (upper panel). B, A heterozygous frame-shift mutation, 6370delTT, was disclosed by nucleotide sequencing in the patient’s and his mother’s DNA (lower panel). This deletion was not present in the father and a control (upper panel). C, The DP protein comprises an N-terminus, a central rod-domain and a C-terminus implicated in the binding with the intermediate filaments by the subdomains A, B, and C. The paternal allele results in a DP protein of 1933 amino acids lacking all three subdomains. The maternal allele leads to a truncated protein of 2,058 amino acids, from which the last 28 amino acids are aberrant from the wild-type protein sequence (red).

Figure 5

Immunoblot staining for desmoplakin I demonstrating two different truncated DP I forms in the patient’s cells. Extracts of keratinocytes cultured from the patient (Pt), the mother (Mo), the father (Fa), and a normal human control (Ctr) were separated on 5% SDS-PAGE slab gels, electroblotted, and stained with DP I-specific monoclonal 2.17. Left, molecular markers 150 and 200 kDa. The mRNA-computed molecular weights for the truncated forms are 228 kDa and 243 kDa, versus 332 kDa for the wild-type protein. Note that all proteins run slightly faster than deduced from the nucleotide sequences in line with earlier observations (Green et al. 1990).