Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome

Abstract

EXTL3 regulates the biosynthesis of heparan sulfate (HS), important for both skeletal development and hematopoiesis, through the formation of HS proteoglycans (HSPGs). By whole-exome sequencing, we identified homozygous missense mutations c.1382C>T, c.1537C>T, c.1970A>G, and c.2008T>G in EXTL3 in nine affected individuals from five unrelated families. Notably, we found the identical homozygous missense mutation c.1382C>T (p.Pro461Leu) in four affected individuals from two unrelated families. Affected individuals presented with variable skeletal abnormalities and neurodevelopmental defects. Severe combined immunodeficiency (SCID) with a complete absence of T cells was observed in three families. EXTL3 was most abundant in hematopoietic stem cells and early progenitor T cells, which is in line with a SCID phenotype at the level of early T cell development in the thymus. To provide further support for the hypothesis that mutations in EXTL3 cause a neuro-immuno-skeletal dysplasia syndrome, and to gain insight into the pathogenesis of the disorder, we analyzed the localization of EXTL3 in fibroblasts derived from affected individuals and determined glycosaminoglycan concentrations in these cells as well as in urine and blood. We observed abnormal glycosaminoglycan concentrations and increased concentrations of the non-sulfated chondroitin disaccharide D0a0 and the disaccharide D0a4 in serum and urine of all analyzed affected individuals. In summary, we show that biallelic mutations in EXTL3 disturb glycosaminoglycan synthesis and thus lead to a recognizable syndrome characterized by variable expression of skeletal, neurological, and immunological abnormalities.

CDG and immune response: From bedside to bench and back

“…However, in the context of periodontitis, ATP6AP1 silencing resulted in decreased inflammation with diminished infiltration of T cells, DCs, macrophages and expression of pro‐inflammatory cytokines in the periodontal lesion . Other variables influencing and influenced by glycans are the developmental stage, protein/cell activation status and aging …”

“…Accordingly, in an EXTL3‐CDG zebrafish model, significant alterations in thymus development were observed and rescued upon injection of wild‐type human EXTL3 mRNA . The absent EXTL3 expression in normal circulating T cells, while the protein is expressed in hemato‐ and/or lymphopoietic cells, further supports an early developmental defect . However, why some patients fail to present signs of immunodeficiency or progressively improve over time is still to be determined.…”

Bone and connective tissue disorders caused by defects in glycosaminoglycan biosynthesis: a panoramic view

“…This disorder is probably relatively common as approximately 20 patients have been identified in a short period of time. Consistent with its role in HS synthesis, low HS levels have been detected in fibroblasts from the patients . The skeletal finding similarities to FGFR3‐related chondrodysplasias underline the role of HS in FGFR‐related signalling; moreover, further experimental results have demonstrated the role of HS in thymus development and thus T lymphocyte maturation.…”

“…Altered synthesis of specific GAG species has also been observed in skeletal disorders such as in hereditary multiple exostosis with a specific defect in HS synthesis . In EXTL3 ‐deficient cells, reduced HS synthesis coupled to increased CS and DS synthesis have been observed . Moreover, in these patients HS chains were longer and abnormally sulfated compared to controls .…”

Hereditary Multiple Exostoses: Clinical, Molecular and Radiologic Survey in 9 Families

“…Recently, exostosinlike glycosyltransferase 3 (EXTL3) that regulates the biosynthesis of heparan sulphate (HS) and is important for both skeletal development and haematopoiesis has been linked to distinct clinical phenotype by two groups. EXTL3 mutations are responsible for "neuro-immuno-skeletal dysplasia syndrome" with variable skeletal abnormalities, neurodevelopmental defects and combined immunodeficiency (Oud et al, 2017;Volpi et al, 2017).…”