Dyskeratosis Congenita with Pigmentary Mosaicism and Hematopoietic Trisomy 9 in a Female Associated with a de novo DKC1 Variant and Markedly Skewed X Chromosome Inactivation

Pathogenic germline variants (PGVs) in dyskerin pseudouridine synthase 1 ( DKC1 ) cause X-linked recessive dyskeratosis congenita (DC), a telomere biology disorder. Females with heterozygous DKC1 PGVs rarely exhibit DC phenotypes due to favorable X chromosome inactivation (XCI). We report a female with classic DC, pigmentary mosaicism and bone marrow failure associated with a novel de novo DKC1 variant (c.190 G > C, p.Val64Leu) with reduced expression of DKC1 and TERC along with downregulated signatures associated with aberrant telomere biology and ribosome function. Markedly skewed XCI was detected with expression of the mutated allele in skin fibroblasts and wild-type DKC1 expression in the bone marrow. We hypothesize that selective pressure in the bone marrow favored wild type expressing cells which acquired a trisomy 9 but were unable to resume normal hematopoiesis. This study demonstrates DKC1 c. 190 G > C as a likely PGV causing classic DC and highlights the molecular and clinical complexities associated with skewed XCI.