Revertant somatic mosaicism is a recognized phenomenon in patients with epidermolysis bullosa (EB) and other inherited diseases.1 It occurs when spontaneous mutations result in correction of a germline mutation that underlies the genodermatosis, leading to phenotypic reversion and sometimes functional improvement.2 Revertant mosaicism occurs though several mechanisms, all causing a nonreciprocal transfer of genetic information from the parent cell to the daughter cells. Gene conversions, intragenic crossover, back mutation, and second-site mutation (eg, single-base substitution) have all been described as mechanisms, and multiple mechanisms may occur in different cell populations in the same individual. True forward somatic mosaicism, however, has not to our knowledge been described previously in EB. Forward, or nonrevertant, mosaicism occurs during embryogenesis, when a mutation occurs in mitosis affecting only that subsequent cell line and not the other dividing cells of the embryo. The later it occurs during embryogenesis, the fewer cells will be affected.
Dystrophic EB results from mutations in the COL7A1 gene that encodes type VII collagen, the major component of anchoring fibrils at the dermoepidermal junction. Blisters develop below the lamina densa clinically resulting in trauma-induced skin blistering, milia, and scarring, sometimes with nail dystrophy and mucosal involvement.
A woman in her 20s presented with lifelong skin blistering and clinical features consistent with a mild recessive dystrophic EB. On examination, she had normal hair and teeth but evidence of dystrophic toenails, milia, and scarring. Notably, however, there was segmental sparing of the left side of her trunk (Figure 1) and part of her left arm. There were no other affected family members and no consanguinity.
Clinical appearances of the abdomen (A and B), hand (C), and nails (D) showing localized erythema and scarring in contrast to the normal-appearing skin.
Following written informed consent, genomic DNA was extracted from peripheral blood leukocytes and used as a template to sequence COL7A1, as described elsewhere.3 We identified compound heterozygosity for a donor splice site mutation (IVS64 + 1G>A) and a frameshift mutation (c.7787delG; p.Gly2596fs*34). The heterozygous frameshift mutation was identified in her father’s DNA, but the splice site mutation was not present in either paternal or maternal DNA and therefore appeared to have arisen de novo.
Skin biopsy specimens were taken from the affected and unaffected abdominal skin following local anesthesia with 2% lidocaine. Immunofluorescence microscopy labeling with an antibody to type VII collagen (LH7.2; SeraLab) showed a marked reduction in type VII collagen immunostaining intensity in affected skin compared with unaffected skin, the latter resembling normal control skin (Figure 2). Genomic DNA was extracted from the biopsy specimens and used to assess the COL7A1 mutations: both were present in affected skin DNA, but in unaffected skin, although the frameshift mutation was present, the splice site mutation was barely detected.
The photographic images across the top row represent immunofluorescence (IF) studies (scale bar = 50 μm) in affected (A), unaffected (C), and control skin (D) and a clinical image of the patient’s abdomen (B). Evident in the IF images is a reduction in type VII collagen immunoreactivity at the dermoepidermal junction in affected skin (A) compared with bright, linear labeling in unaffected skin (C) and control skin (D). A-C, The Sanger sequencing graphs of genomic DNA (gDNA) reveal compound heterozygosity for splice site/frameshift mutations in COL7A1, with both mutations clearly evident in DNA from affected skin (A) and blood (B), but the splice site mutation barely detectable in unaffected skin (C), although the heterozygous frameshift mutation is still evident. D, The Sanger sequencing graphs of gDNA from control blood are provided for comparison.
We have described a case of recessive dystrophic EB with clinical, immunohistochemical, and molecular data supporting somatic nonrevertant mosaicism. In this patient, the frameshift mutation was inherited from the paternal gamete, and the splice site mutation occurred spontaneously and affected only some areas of skin.
Mosaicism has important connotations for treatment. In revertant mosaicism, punch grafting of small areas of reverted skin to unreverted sites has been successful,4 although the much larger areas of somatic mosaicism in our patient makes this a more scalable therapy. Other options include culturing the mosaic keratinocytes for grafting or using these cells to generate inducible pluripotent stem cells, as has been attempted for revertant keratinocytes.5
Corresponding Author: Alexa Rose Shipman, MA (Hons Oxon), MRCP(UK), Department of Dermatology, Warwick Hospital, Lakin Road, Warwick CV34 5BW, England (firstname.lastname@example.org).
Published Online: July 2, 2014. doi:10.1001/jamadermatol.2014.281.
Conflict of Interest Disclosures: None reported.
Shipman AR, Liu L, Lai-Cheong JE, McGrath JA, Heagerty A. Somatic Forward (Nonrevertant) Mosaicism in Recessive Dystrophic Epidermolysis Bullosa. JAMA Dermatol. 2014;150(9):1025-1027. doi:10.1001/jamadermatol.2014.281