Abstract
Somatic mosaicism produces genetic differences between cells in an individual and is an underrecognized contributor to phenotypic variability. Precise understanding of the natural history of genetic diseases, therefore, requires detection and recognition of low-level mosaicism, which remains technically challenging, particularly for X-linked genes. Here, we identify six males with mosaic X-linked adrenoleukodystrophy (X-ALD), a neurometabolic peroxisomal disorder caused by pathogenic variants in ABCD1 that is currently included in 44 state newborn screening (NBS) programs, and estimate the incidence of somatic mosaicism. Of 227 males from 2 laboratories performing ABCD1 next-generation sequencing, 1.8% (4/227) had pathogenic or likely pathogenic ABCD1 variants that were mosaic. In one mosaic male individual, allele-specific measurements across multiple tissues demonstrated ABCD1 variant allele fractions ranging from 66 to 82%. Our findings have implications for the identification of X-ALD through NBS, and additional studies could provide insight into the pathogenesis and natural history of X-ALD.