| Before the late 1980s, the likely targets
of genetic mutation in EBS were unknown. A variety of studies suggested conflicting
mechanisms of disease. Consistent with the prevailing theory about the role of collagenase
in RDEB, for example, in 1983 Takamori and colleagues suggested that intraepidermal
blisters arose in EBS as the result of the release of one or more epidermal derived
proteases that had specificity for targets in basilar keratinocytes. Their in vitro model
using EBS blister fluid and normal human skin organ explants tended to confirm that
hypothesis, although subsequent work by Fine and co-workers demonstrated the
nonspecificity of these findings. In 1983, Sanchez and colleagues demonstrated reduced in
vitro activity of a fibroblast-derived enzyme, gelatinolytic protease, which was produced
by fibroblast cultures established from patients who had generalized and localized
forms of EBS. Unfortunately, these findings could not be later reproduced by other
investigators. An alternate hypothesis, proposed in 1985 by Fine and Griffith, was that
the mechanical fragility observed in EBS skin reflected the presence of an abnormality in
a glycosylated cell membrane component of the keratinocyte, a mechanism suggested in part
by the altered staining of EBS epidermis by selective lectins and the lack of apparent
concurrent abnormal staining of EBS skin by the only antikeratin monoclonal antibodies
available for study at that time. On the other hand, early
electron microscopy studies repeatedly demonstrated disruption of keratin filaments in EBS
keratinocytes in lesional skin, and the clumping keratin tonofilaments in Dowling-Meara
EBS led Anton- Lamprecht and Schnyder to suggest as early as 1982 that keratins might be
the target of mutation or injury in this disease. This hypothesis was further echoed in
1991 by two independent groups of investigators from London and Japan, based on their
studies using immunohistochemical and ultrastructural techniques. One of these groups, led
by Eady and Leigh, further postulated that the EBS keratinocyte was characterized by the
presence of a structurally abnormal intracytoplasmic network of filaments of keratins 5
and 14. In that same year, two different laboratories reported dramatic findings in EBS
using molecular biologic approaches. Bonifas, Rothman, and Epstein used linkage analysis
to demonstrate the presence of keratin gene mutations in two families with the Koebner
variant of generalized EBS. Fuchs and colleagues independently mapped mutations in the
keratin 14 gene in several patients with Dowling-Meara EBS and elegantly demonstrated the
pathogenicity of these mutations by way of keratinocyte cell culture and transgenic mouse
models. These investigators and others have subsequently demonstrated that mutations in
the genes for keratin 5 and keratin 14 form the molecular basis for disease in the
Koebner, Weber-Cockayne, Dowling-Meara, and mottled pigmentation subtypes of EBS, and that
the site of mutation in the keratin gene correlates well with severity of disease and EBS
subtype. A much more de- tailed discussion of these keratin gene mutations may be found in
chapter 14.
In 1996, McLean and colleagues demonstrated that mutations in the
gene for plectin, a constituent of skeletal muscle that is also present just inside the
inferiormost aspect of the basilar keratinocyte plasma membrane, forms the molecular basis
of the subtype of generalized EBS that is associated with congenital muscular dystrophy.
In 1997, Koss-Harnes and colleagues found that skin from patients with the Ogna subtype of
EBS exhibited strongly reduced binding of some anti-rat plectin antibodies to the basal
keratinocytes, consistent with the co-assignment of the EBS1-GPT loci and the plectin
PLEC1 locus to the same 8q24 chromosome band. |
