SGLT5 is the renal transporter for 1,5-anhydroglucitol, a major player in two rare forms of neutropenia

Neutropenia and neutrophil dysfunction in glycogen storage disease type 1b (GSD1b) and severe congenital neutropenia type 4 (SCN4), associated with deficiencies of the glucose-6-phosphate transporter (G6PT/SLC37A4) and the phosphatase G6PC3, respectively, are the result of the accumulation of 1,5-anhydroglucitol-6-phosphate in neutrophils. This is an inhibitor of hexokinase made from 1,5-anhydroglucitol (1,5-AG), an abundant polyol in blood. 1,5-AG is presumed to be reabsorbed in the kidney by a sodium-dependent-transporter of uncertain identity, possibly SGLT4/SLC5A9 or SGLT5/SLC5A10. Lowering blood 1,5-AG with an SGLT2-inhibitor greatly improved neutrophil counts and function in G6PC3-deficient and GSD1b patients. Yet, this effect is most likely mediated indirectly, through the inhibition of the renal 1,5-AG transporter by glucose, when its concentration rises in the renal tubule following inhibition of SGLT2. To identify the 1,5-AG transporter, both human and mouse SGLT4 and SGLT5 were expressed in HEK293T cells and transport measurements were performed with radiolabelled compounds. We found that SGLT5 is a better carrier for 1,5-AG than for mannose, while the opposite is true for human SGLT4. Heterozygous variants in SGLT5, associated with a low level of blood 1,5-AG in humans cause a 50-100% reduction in 1,5-AG transport activity tested in model cell lines, indicating that SGLT5 is the predominant kidney 1,5-AG transporter. These and other findings led to the conclusion that (1) SGLT5 is the main renal transporter of 1,5-AG; (2) frequent heterozygous mutations (allelic frequency > 1%) in SGLT5 lower blood 1,5-AG, favourably influencing neutropenia in G6PC3 or G6PT deficiency; (3) the effect of SGLT2-inhibitors on blood 1,5-AG level is largely indirect; (4) specific SGLT5-inhibitors would be more efficient to treat these neutropenias than SGLT2-inhibitors.

Novel G6PC3 Mutations in patients with Congenital Neutropenia: Case reports and Review of the Literature

BACKGROUND

Severe congenital neutropenia (SCN4) caused by mutations in glucose-6-phosphatase catalytic subunit 3 (G6PC3) is characterized by recurrent infections due to severe neutropenia, and it may be accompanied by other extra-hematopoietic manifestations; including structural heart defects, urogenital abnormalities, prominent superficial venous markings, growth retention, and inflammatory bowel diseases with rare incidence. The homozygous or compound heterozygous mutations of G6PC3 are responsible for most cases of autosomal recessive SCN4. Herein, we present two cases of SCN4 affected by novel mutations in the G6PC3, in addition to a summarized list of variants in G6PC3 gene that are reported as pathogenic and related to the SCN4 phenotype.

CASE PRESENTATION

Herein we present two cases of SCN4; the first case was a three-months old boy with severe neutropenia and prior history of hospitalization due to umbilical separation, umbilical herniation, omphalitis, and pyelonephritis; and the second case was an eight-year-old with a history of neutropenia, recurrent and severe episodes of intractable diarrhea, refractory rectovaginal and rectoperineal fistula, congenital inguinal hernia, and ASD type 2. Whole exome sequencing was performed for both cases and revealed two novel homozygous missense mutations in G6PC3 that were predicted to be deleterious; c.337G>A, p. Gly113Arg in the first case and c.479C>T; P. Ser160Leu in the second case. To our knowledge, both of these two mutations has not been reported in the G6PDC3 gene.

CONCLUSIONS

In patients with severe neutropenia with varying extra hematopoietic syndrome, mutation of G6PC3 should be suspected after ruling out other mutations related to neutropenia. This study pointed toward novel G6PC3 mutations, that should be considered in order to diagnose patients with severe congenital neutropenia.