Expanding the Nude SCID/CID Phenotype Associated with FOXN1 Homozygous, Compound Heterozygous, or Heterozygous Mutations

Abstract

Human nude SCID is a rare autosomal recessive inborn error of immunity (IEI) characterized by congenital athymia, alopecia, and nail dystrophy. Few cases have been reported to date. However, the recent introduction of newborn screening for IEIs and high-throughput sequencing has led to the identification of novel and atypical cases. Moreover, immunological alterations have been recently described in patients carrying heterozygous mutations. The aim of this paper is to describe the extended phenotype associated with FOXN1 homozygous, compound heterozygous, or heterozygous mutations. We collected clinical and laboratory information of a cohort of 11 homozygous, 2 compound heterozygous, and 5 heterozygous patients with recurrent severe infections. All, except one heterozygous patient, had signs of CID or SCID. Nail dystrophy and alopecia, that represent the hallmarks of the syndrome, were not always present, while almost 50% of the patients developed Omenn syndrome. One patient with hypomorphic compound heterozygous mutations had a late-onset atypical phenotype. A SCID-like phenotype was observed in 4 heterozygous patients coming from the same family. A spectrum of clinical manifestations may be associated with different mutations. The severity of the clinical phenotype likely depends on the amount of residual activity of the gene product, as previously observed for other SCID-related genes. The severity of the manifestations in this heterozygous family may suggest a mechanism of negative dominance of the specific mutation or the presence of additional mutations in noncoding regions.

Keywords: EBV-related lymphoproliferative disease; FOXN1; Nude SCID; Omenn syndrome; alopecia; compound heterozygous; heterozygous; homozygous; nail dystrophy.

Fig. 1

Geographic distribution of the cases included in the study. Patients originated from 9 countries (Italy, Portugal, France, Lebanon, India, Turkey, Saudi Arabia, Slovakia, Belarus)

Fig. 2

Mutations identified in the patients and crystal structure of FOXN1/DNA complex. a Five mutations were identified in the N-terminal domain, three in the forkhead domain, and two in the C-terminal domain. Novel mutations are highlighted in red. b Structure of the complex with the protein represented as ribbons. The three mutated residues in the forkhead domain are represented as ball-and-stick. c Enlarged view of the protein-DNA interface with the mutated R320W reported in green. d Same as in c but for V294I, and with protein and DNA atoms represented as transparent spheres of van der Waals radius

Fig. 3

Heterozygous patients’ family pedigree and lymphocyte subpopulations at different ages. a Healthy subjects are shown in white; carriers are shown in black; deceased siblings are indicated by line crossing. b Lymphocytes. c B cells (CD19+). d NK cells (CD16/CD56+). e T cells (CD3+). f T helper (CD3+CD4+). g Cytotoxic T cells (CD3+CD8+). h Naïve T helper (CD3+CD4+CD45RA+). In all the panels, each line and symbol represents a single patient. The red dashed lines represent the 5th and 95th centile levels of the age-matched reference values

Fig. 4

Assessing the pathogenicity of the P350L FOXN1 variant found in P9. Luciferase reporter assay in 4D6 human cells transfected with either wild-type FOXN1 (WT) or P350L FOXN1 along with β5t-luc or β5t-mut-luc luciferase plasmids. RLU, relative light units; p values, calculated with unpaired t tests, *p = .0106, **p = .0080,***p = .0003

Fig. 5

Lymphocyte subpopulations at different ages. a Lymphocytes. b B cells (CD19+). c NK cells (CD16/CD56+). d T cells (CD3+). e T helper (CD3+CD4+). f Cytotoxic T cells (CD3+CD8+). g Naïve T helper (CD3+CD4+CD45RA+). h Proliferative response to PHA expressed as % of the healthy control. In all the panels, each line and symbol represents a single patient. Blue symbols represent compound heterozygous patients. The red dashed lines represent the 5th and 95th centile levels of the age-matched reference values