EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay

Area of Science:

• Genetics
• Immunology
• Developmental Biology

Background:

• Severe skeletal dysplasia, T cell immunodeficiency, and developmental delay are debilitating conditions.
• Heparan sulfate (HS) biosynthesis is crucial for various biological processes.
• The role of exostosin-like 3 (EXTL3) in human disease was previously unknown.

Purpose of the Study:

• To identify the genetic cause of severe skeletal dysplasia, T cell immunodeficiency, and developmental delay in three patients.
• To investigate the functional consequences of identified mutations on HS biosynthesis and cellular signaling.
• To elucidate the role of EXTL3 in immune development and skeletal/brain formation.

Main Methods:

• Whole-exome sequencing to identify genetic mutations.
• Analysis of heparan sulfate composition and fibroblast growth factor 2 signaling in patient-derived fibroblasts.
• Assessment of STAT5 phosphorylation in patient lymphocytes.
• Zebrafish (Danio rerio) model to study thymopoiesis.
• Differentiation of patient-derived induced pluripotent stem cells.

Main Results:

• Homozygous missense mutations in the EXTL3 gene were identified in all three patients.
• Patient fibroblasts exhibited abnormal HS composition and altered FGF2 signaling, which were rescued by wild-type EXTL3.
• Reduced STAT5 phosphorylation was observed in patient lymphocytes.
• EXTL3 deficiency in zebrafish led to defective thymopoiesis, which was rescued by human EXTL3.
• Patient-derived iPSCs showed impaired lymphohematopoietic and thymic epithelial progenitor cell differentiation.

Conclusions:

• Mutations in EXTL3 represent a novel genetic cause of severe immune deficiency, skeletal dysplasia, and developmental delay.
• Heparan sulfate biosynthesis mediated by EXTL3 is essential for normal thymopoiesis, skeletal development, and brain development.