Uniparental Disomy Leads to a Novel Cause of MC2R-Related Familial Glucocorticoid Deficiency Type 1

Abstract Familial Glucocorticoid Deficiency Type 1 (FGD1) is a rare autosomal recessive disease caused by pathogenic variants in the MC2R gene. This case report presents the first documented instance of FGD1 caused by a homozygous 1.421 kb deletion affecting the non-coding promoter region of MC2R. The patient, a 9-year-old girl, presented with severe cortisol insufficiency and hyperpigmentation starting at birth. Genetic testing initially missed the deletion, as standard whole-exome sequencing in 2016 did not include a copy number variation analysis. However, a whole-genome sequencing analysis in 2024 identified the deletion. The variant was inherited through paternal uniparental disomy (UPD), a rare genetic mechanism that caused the homozygous state. This case underscores the value of utilizing current genetic testing approaches, especially in cases where clinical features strongly suggest a genetic etiology despite inconclusive initial genetic testing results. Additionally, it highlights the need to consider non-coding regions and UPD in genetic diagnostics.