Intronic hexanucleotide repeat expansion in TYMS in monozygotic twins with congenital progressive universal melanosis

Hyperpigmentation presents a diverse clinical spectrum, largely influenced by genetic factors that remain incompletely understood. The present study describes a case of monozygotic twin girls aged 15 years with congenital progressive universal melanosis (CPUM) born to non-consanguineous unaffected parents. CPUM represents a novel clinical entity characterized by progressive widespread hyperpigmentation beginning at birth, without other accompanying symptoms. Skin biopsy and histopathological analysis were performed, followed by long-read whole-genome sequencing and short tandem repeat analysis. Gene expression was evaluated using reverse transcription-PCR, and protein levels were assessed by western blotting in cultured skin fibroblasts from the twins and unaffected controls. Long-read genome sequencing revealed a biallelic GATGGT repeat expansion of 210-259 repeat units within the third intron of the thymidylate synthase (TYMS) gene in both twins, whereas their parents were heterozygous. Controls (n=236), derived from the in-house long-read sequencing database at Excellence Center for Genomics and Precision Medicine, King Chulalongkorn Memorial Hospital, Bangkok, Thailand, carried the GATGGT repeat in the 42-172 range. No single nucleotide or structural variants or copy number variations were present in both affected individuals and absent in the unaffected parents. RNA and protein levels of TYMS using cultured skin fibroblasts from both twins showed no discernible differences compared with controls. Fibroblasts were used due to their accessibility via skin biopsy and their role in skin pigmentation through paracrine signaling to melanocytes. More relevant cells, such as melanocytes or keratinocytes, may be required to exhibit such changes, as these cells are directly involved in melanin production and skin pigmentation. TYMS is implicated in skin pigmentation in normal physiological process and in disorders manifesting abnormal skin pigmentation, such as dyskeratosis congenita. The present findings imply a connection between the identified repeat expansion in TYMS and CPUM, underscoring the need for further investigations to elucidate its causal association.