A Novel Clinical Syndrome Associating Severe Congenital Neutropenia and Complex Developmental Aberrations Caused by Deficiency of G6PC3

We here describe a previously unrecognized nosological entity in 12 patients from 8 unrelated pedigrees. All patients presented with severe congenital neutropenia and severe invasive bacterial infections. In addition, patients had a variety of additional syndromic features such as congenital heart disease (8/12), urogenital malformations (5/12), inner ear hearing loss (2/12), and myopathy (1/12). Furthermore, most patients (10/12) showed increased visibility/angiectasia of subcutaneous veins. The bone marrow smear was characterized by a typical “maturation arrest” due to premature apoptosis of mature neutrophils. Similar to Kostmann’s disease secondary to mutations in HAX1, myeloid cells from patients with this novel syndrome showed increased susceptibility to apoptosis. Myeloid progenitor cells revealed an abnormally enlarged rough endoplasmic reticulum and increased endoplasmic reticulum stress evidenced by increased expression of BiP. A genome-wide linkage study, performed in two consanguineous pedigrees, gave statistical evidene of a linkage interval on chromosome 17q21 (LOD score 5.74). We identified homozygous missense mutations in G6PC3, a ubiquitously expressed paralog of glucose-6-phosphatase. Biochemical studies confirmed deficient enzymatic activity. Using retroviral G6PC3-gene transfer into primary hematopoietic stem cells and in vitro differentiation into myeloid cells, the phenotype of increased susceptibility to apoptosis could be reverted. Eight distinct biallelic mutations were found, including missense and nonsense mutations. G6PC3-deficient myeloid cells showed a predominance of the unphosphorylated form of GSK3beta, a key molecule controlling cellular differentiation and apoptosis. As a consequence of increased GSK3beta activity, increased phosphorylation of the antiapoptotic molecule Mcl1 was detected, explaining increased susceptibility to apoptosis in neutrophils. In summary, our study describes a novel disease, determines its molecular etiology, and sheds light on the role of glucose-dependent pathways in controlling the homeostasis of the endoplasmic reticulum and control of apoptosis.