A case of familial progressive hyperpigmentation with or without hypopigmentation presenting with hypopigmented striae along the lines of Blaschko

Familial progressive hyperpigmentation with or without hypopigmentation (FPHH) is an autosomal dominant disorder characterized by widespread skin hyperpigmentation, café‐au‐lait spots, and hypopigmented circular macules, resulting from KITLG variants. KITLG, expressed by keratinocytes, binds to KIT on melanocytes, stimulating melanogenesis. Disturbances in the KITLG‐KIT interaction result in diffuse hyperpigmentation in FPHH. However, the mechanisms behind hypopigmented macule formation remain unclear. This report presents a unique FPHH case in a patient with a novel KITLG mutation (Ser78Leu). Notably, the patient showed multiple hypopigmented macules and striae along the lines of Blaschko. Digital polymerase chain reaction analysis of the DNA from skin and blood tissues indicated a copy‐neutral loss of heterozygosity at the KITLG locus, only in the hypopigmented macule. These findings suggest that the hypopigmented macules might result from revertant mosaicism. Conversely, café‐au‐lait spots do not follow the lines of Blaschko and can superimpose on the hypopigmented striae, indicating a distinct pathogenesis. This case contributes to the understanding of the genetic mechanisms in FPHH.