Takaaki Hayashi, Hiroko Maki, Natsuki Higa, Kei Mizobuchi, Takafumi Maruyama, Taiju Hayashi, Hirotomo Saitsu, Kei Shinoda
The molecular findings in this case support somatic mosaicism. Skewed XCI was also observed in buccal mucosa, but its contribution to the phenotype remains uncertain because the preferentially inactivated X chromosome could not be phased with the CHM variant and the HUMARA assay did not resolve XCI within variant-bearing cells. Choroideremia should be considered in the differential diagnosis of female patients with severe chorioretinal atrophy, even in the absence of a family history.
PURPOSE: To report a Japanese female patient with a male-pattern choroideremia phenotype associated with somatic mosaicism and skewed X-chromosome inactivation (XCI).
METHODS: A 24-year-old woman underwent comprehensive ophthalmic examinations, including fundus imaging and full-field electroretinography (ERG). Whole-exome sequencing (WES) was performed on leukocyte-derived DNA, with validation by Sanger sequencing of both leukocyte- and buccal mucosa-derived DNA. XCI status was assessed using the HUMARA assay on buccal mucosa-derived DNA.
RESULTS: Despite preserved visual acuity, fundus examination revealed diffuse chorioretinal atrophy with a residual stellate macular island of preserved autofluorescence. Full-field ERG demonstrated non-recordable responses under both dark- and light-adapted conditions. WES identified a pathogenic CHM frameshift variant (c.525_526del) at a low variant allele fraction (VAF: 10%) in leukocyte-derived DNA. The variant was undetectable by Sanger sequencing of leukocyte-derived DNA but was confirmed as a heterozygous allele in buccal mucosa-derived DNA. The 10% VAF was substantially lower than the approximately 50% expected for a constitutional heterozygous variant and was therefore consistent with somatic mosaicism. XCI analysis demonstrated skewing, with a methylated allelic ratio of 83.1:16.9; however, the preferentially inactivated X chromosome could not be phased with the CHM variant.
CONCLUSIONS: The molecular findings in this case support somatic mosaicism. Skewed XCI was also observed in buccal mucosa, but its contribution to the phenotype remains uncertain because the preferentially inactivated X chromosome could not be phased with the CHM variant and the HUMARA assay did not resolve XCI within variant-bearing cells. Choroideremia should be considered in the differential diagnosis of female patients with severe chorioretinal atrophy, even in the absence of a family history.