Identification of a Novel Nonsense Mutation in the IGSF1 Gene Reveals Sex-Specific Phenotypic Variability Within a Single Family

Background: The immunoglobulin superfamily member 1 (IGSF1) gene encodes for a transmembrane glycoprotein involved in crucial processes such as growth, metabolism, and reproductive function. Loss-of-Function (LOF) mutations in the IGSF1 gene have been reported to cause the X-linked IGSF1 deficiency syndrome, a rare genetic condition that primarily affects males, characterized by hypothyroidism, macroorchidism, delayed puberty, obesity, and infertility. Case Report: In this study, we identified a novel hemizygous nonsense IGSF1 variant c.1989G>A (p.Trp663Ter) in a male patient who initially presented with growth impairment and growth hormone deficiency (GHD), with a positive family history on the maternal lineage. Notably, the proband does not present with macroorchidism, a feature typically associated with IGSF1 deficiency. The variant was also found in his heterozygous sister, who presented with isolated growth hormone deficiency, and in his mother, who displayed hypertension and thyroid dysfunction but no significant growth impairment. Discussion: This phenotypic variability suggests a differential expression of IGSF1-related symptoms depending on zygosity and sex within the same family, probably explained by X-chromosome inactivation (XCI) in females, which can lead to varying degrees of functional IGSF1 expression in different tissues. Conclusions: This case highlights the intrafamilial phenotypic variability associated with IGSF1 mutations, illustrating differences between male and female carriers and highlighting the importance of genetic testing in patients with similar clinical presentations.