Pigmentary Patterning as a Clinical Clue of Genetic Mosaicism

Abstract

GENETIC MOSAICISM, in which 2 or more genetically heterogeneous populations of cells arise within the same zygote, commonly manifests in the skin as patterned lesions.¹Particularly distinct among these patterns is that along the lines of Blaschko, thought to represent the pattern of embryologic migration of skin cells.^2,3In contrast to the linear distribution of the lines on the extremities, the S-shaped curves on the abdomen and V-shaped distribution on the central aspect of the back have been explained by the interplay of transverse clonal proliferation from the primitive streak and longitudinal growth and flexion of the embryo during development.¹ Based on the chromosomal localization of the underlying gene alteration, mosaic phenotypes may be divided into 2 major subgroups: functional mosaicism and genomic mosaicism. All female patients have functional mosaicism for X-linked genes, with approximately 50% of the cells expressing paternally derived genes and 50% expressing maternally derived genes. Happle4 originally proposed that skin lesions along the lines of Blaschko