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SRP54 discovered as a novel cause of congenital neutropenia
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ATLANTA — Researchers discovered SRP54 mutations as a novel pathological pathway in the development of severe primary neutropenia, according to study results presented during the late-breaking abstract session of the ASH Annual Meeting and Exposition.
This new genetic subtype of congenital neutropenia is characterized by promyelocytic maturation arrest with dysgranulopoiesis, leading to profound neutropenia and a poor response to granulocyte colony-stimulating factor. Further, some patients also had severe neurodevelopmental delay and exocrine pancreatic insufficiency.
Patients with congenital neutropenia, a heterogenous group of diseases, often have low neutrophil count, severe bacterial infections and various extra-hematopoietic organ dysfunction.
“To date, about 25 genes have been identified in congenital neutropenias, and some of the genetic subtypes are associated with an increased risk for leukemic transformation,” Christine Bellanné-Chantelot, PhD, PharmD, assistant professor in département de génétique at Hôpital Pitié- Salpêtrière in Paris, France, said during her presentation. “In addition to the hematological defect, some patients are associated with various organ dysfunctions. About 30% of patients remain without genetic cause.”
To identify additional genetic causes of congenital neutropenia, Bellanné-Chantelot and colleagues conducted whole-exome sequencing on a trio-based approach for eight sporadic cases and six multiplex families.
Results of this analysis revealed a heterozygous mutation in the SRP54 gene — which encodes the signal recognition particle (SRP) 54 GTPase protein — in three patients with sporadic congenital neutropenia and in an autosomal dominant family.
Based on these findings, researchers then conducted Sanger sequencing to screen for the SPR54 gene among 66 patients from the French congenital neutropenia registry. This analysis identified 13 additional sporadic cases and two multiplex families.
Thus, the analysis included 23 patients from 19 families who harbored a germline SRP54 mutation. The mutational spectrum consisted of six missense mutations, and 14 probands also had Thr1117del in-frame deletion.
All of these patients had profound neutropenia, with a mean neutrophil absolute count of 0.23 x 109/L. They had been diagnosed during the neonate period or during early childhood (mean age, 4.2 months).
When researchers evaluated bone marrow, they found maturation arrest at the promyelocytic stage.
All patients required long-term G-CSF therapy but were poor responders, requiring a mean dose of 9 µg/kg per day. Despite these high doses, after a median follow-up of 14.8 years, no patient had disease that evolved into acute myeloid leukemia.
Unlike patients with congenital neutropenia caused by ELANE mutations, patients who harbored SRP54 mutations had a high degree of dysgranulopoiesis — characterized by abnormal localization and number of mature granules — enlarged endoplasmic reticulum in promyelocytes and dystrophic neutrophils.
Six of the patients had extra-hematopoietic features, including severe neurodevelopmental delay (n = 5), exocrine pancreatic insufficiency (n = 3) and bone abnormalities (n = 2).
“A few patients, 23%, presented with Shwachman-Diamond-like features, particularly with neurodevelopmental delay and exocrine pancreatic insufficiency,” Bellanné-Chantelot said.
Results of structural and functional studies conducted using primary cells from patients, including fibroblasts and hematopoietic cells, showed the SRP54 protein is key component of the ribonucleoprotein complex SRP that mediates co-translational targeting and the insertion of secretory and membrane proteins to the endoplasmic reticulum. Researchers identified seven distinct mutations that impact highly conserved residues associated with G motifs involved with GTPase activity of SRP54.
Seventeen of 18 patients with mutations located within the genomic island element that affects the structure and stability of the NG domain only had severe neutropenia. Conversely, five patients with mutations that affected the magnesium-binding or the guanine-binding site, or that are implied in the heterodimeric association with the SPR54 receptor, had Shwachman-Diamond-like features.
Researchers then conducted in vitro granulocytic differentiation by culturing CD34-positive cells in serum-free medium with stem cell factor, interleukin-3 and G-CSF for 21 days.
These analyses showed a strong increase in SPR54 mRNA expression levels associated with a slight decrease in the protein level among patients vs. controls. SRP54 mutations also had a major deleterious effect on proliferations and led to delay in differentiation associated with increased apoptosis.
“This study identified a novel pathway in congenital neutropenia related to the co-translational process of protein targeting,” Bellanné-Chantelot said. “This novel genetic subtype represents the second cause of congenital neutropenia with maturational arrest and extra-hematopoietic features in the French congenital neutropenia registry.” – by Alexandra Todak
Reference:
Bellanné-Chantelot C, et al. Abstract LBA-3. Presented at: ASH Annual Meeting and Exposition; Dec. 9-12, 2017; Atlanta.
Disclosures: The authors report no relevant financial disclosures.