Six cases of linear GVHD have occurred as an isotopic response along dermatomes previously affected by varicella-zoster virus. 4-8 These cases give credence to the idea that a cutaneous viral infection may alter the skin through unknown mechanisms, predisposing it to become affected by GVHD. Notably, this phenomenon occurred despite absence of a persistent viral genome when assessed using polymerase chain reaction analysis. 4
An additional 3 cases of GVHD occurring in a dermatomal distribution without any prior infections in those areas have been reported. 9,10 Of note, 2 of 3 patients did have episodes of zoster occur at other sites following transplantation and did not develop GVHD symptoms in any of those locations. 9 Interestingly, controversy exists as to whether the distribution of these lesions was dermatomal or followed Blaschko lines. 11
Two cases of linear GVHD have been reported in which lesions were identified as occurring along Blaschko lines. 12,13 The lines of Blaschko, first described in 1901, correspond to cellular migration patterns during embryological development. 14 Postzygotic mutations causing epidermal cell mosaicism may result in skin disorders occurring in segmental areas defined by the Blaschko lines. 15-17 Accordingly, the Blaschko-linear pattern in GVHD suggests cellular mosaicism as the etiology in this case. Although the host's immune system develops immunotolerance to both cellular lineages during maturation, transplanted lymphoid cells from a nongenetically identical sibling may identify just one of the cell lines as nonself, producing a selective pattern of GVHD 18 confined to the distribution of the genetically disparate cell line, which occurs along the lines of Blaschko in the skin. Candidate genes for mutations that would produce a mosaic following transplant GVHD include any of the 25 to 30 known minor histocompatibility antigens (or any of the several hundred yet to be found). 19 Although well established for monogenic dominant disorders, in 2007 it was recognized that a postzygotic mutation can cause many complex polygenetic disorders, including GVHD, to manifest in a limited segmental pattern. This understanding, along with retrospective case review, has brought into question previously reported "dermatomal" or "zosteriform" presentations of GVHD, asserting that the linear patterns were misidentified and thus inappropriately attributed to a postviral response. 20 Recognition of the Blaschko-linear distribution holds significance in both identifying lesion etiology and understanding disease pathogenesis and treatment.
Our patient illustrates a case of a Blaschko-linear GVHD. The distinctive pattern of her physical findings strongly favored epidermal cell mosaicism as the etiology of her disease. More than just a phenotypically unique case, it provided further insight into the complex etiology underlying GVHD and iterated the basic concepts of Blaschko lines and genetic alterations in development.