A novel splice site variant in FOXN1 in a patient with abnormal newborn screening for severe combined immunodeficiency and congenital lymphopenia

Background: The Forkhead box protein N1 (FOXN1) is a key regulator of thymic epithelial development, and its complete deficiency leads to a nude-severe combined immunodeficiency (SCID) phenotype. More recently, heterozygous mutations in FOXN1 have been linked with a syndrome of congenital lymphopenia and a wide clinical spectrum, with most cases being caused by missense mutations. Aim: To broaden the genotypic and phenotypic spectrum of heterozygous FOXN1 deficiency. Methods: Case report of a patient with FOXN1 haploinsufficiency due to a novel splice-site mutation. Results: Our patient was identified at 3 weeks of life given an abnormal newborn screen (NBS) for SCID, and was found to have congenital lymphopenia preferentially affecting CD8 + T-cells. Her cellular and humoral function were both excellent, and she has remained entirely asymptomatic and thriving for the first 3 years of her life. The patient was found on whole exome sequencing to carry a heterozygous splice-site mutation in the FOXN1 gene, affecting the Forkhead domain. The mutation was also identified in her asymptomatic mother. Conclusion: Heterozygous FOXN1 mutations are an increasingly-recognized cause of congenital lymphopenia. Our experience suggests most patients remain clinically well, with main manifestation including T-lymphopenia, mostly affecting CD8 + cells. Identification of the same variant in an asymptomatic parent suggests age-dependent improvement in T-cell counts and an overall benign course, while provides impetus for diligent conservative management with regular follow-up. Statement of novelty: Heterozygous FOXN1 deficiency is a relatively new entity, attributed in most cases to missense mutations in FOXN1. To further expand the knowledge basis regarding this emerging disorder, as well as its genotypic repertoire, we herein report a case of heterozygous FOXN1 deficiency caused by a splice site mutation.