Sialic acids (Sias) are acidic 9‑carbon monosaccharides. Both free Sias and conjugated Sias (sialylation) exist in the human body and have decisive impacts on human health and disease. Cellular free Sias are made via de novo biosynthesis, recycled from lysosomal salvage, and even by uptake of extracellular Sias, respectively. Sialylation of glycoproteins and glycolipids are catalyzed by sialyltransferases using CMP-Sia as the donor in the Golgi apparatus. In addition, free Sia can be degraded/catabolized into ManNAc and pyruvate in the cytosol. Overall, cellular free Sia and sialylation are kept at certain levels for normal cell functions. However, Sias deficiency and overproduction (accumulation), hyposialylation (undersialylation) and hypersialylation all cause disorders in the human body through a variety of mechanisms, but most of them are still not fully clarified. This review discusses recent understanding of disorders in Sia biosynthesis, salvage, catabolism, and sialylation pathways and therapeutic explorations for these disorders as well.

Modulation of various nucleotide sugar levels in cells has been demonstrated as an effective way to alter the composition of N-glycans. Previous studies have demonstrated the ability to impact CMP-Neu5Ac levels by the addition of N-acetylated mannosamine (ManNAc) to culture media. In this study, the relationship between adding varying levels of ManNAc to cell cultures and the impact on both CMP-Neu5Ac levels and cell growth were examined. Increasing the concentration of ManNAc added resulted in higher levels of CMP-Neu5Ac, but negatively impacted cell growth. Through cellular genetic engineering, we sought to devise an alternative method of increasing ManNAc levels without impacting cell growth. The UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine-kinase (GNE) gene is the rate-limiting enzyme in which congenital mutations can cause Sialuria, a rare metabolic disorder characterized by cytoplasmic accumulation and urinary excretion of free sialic acid. A mutant form of the GNE gene, harboring three mutations (D53H, R263I, R266Q), was site-specifically integrated (SSI) into one locus in CHO cells. This mutant protein dramatically increased the intracellular concentrations of CMP-Neu5Ac, reaching the maximal level as with the addition of ManNAc. These data together indicate that the GNE mutants could provide an effective way for substituting the high-cost supplementation of ManNAc without impacting cell growth. The investigation has also demonstrated the feasibility of the dual-landing-pad SSI cell line engineering approach for improving product qualities of biotherapeutics.

Salla disease (SD) is a rare lysosomal storage disorder characterised by intellectual disability ataxia, athetosis, nystagmus, and central nervous system demyelination. Although the neurological spectrum of SD's clinical phenotype is well defined, psychotic symptoms in SD remain unreported. We reviewed the presence of psychiatric symptoms in patients diagnosed with SD. Medical records of all SD patients at Oulu University Hospital during the years 1982-2015 were systematically reviewed to evaluate the presence of psychiatric symptoms. Psychiatric symptoms were frequently associated with SD (10/24, 42%), and two patients were described as developing psychosis as adolescents. We reported their clinical characteristics in detail and assessed the prevalence of psychiatric symptoms in a cohort of 24 patients. Other psychiatric factors associated with SD were sleeping disorders (8/24, 32%), aggressive behaviour disorders or restlessness (6/24, 25%), and off-label antipsychotic medication (4/24, 17%). This report expands the knowledge of the phenotypic spectrum of SD and demonstrates the importance of recognising the possibility of psychiatric symptoms, including psychosis, in persons with SD.

Urine free sialic acid (UFSA) is an important diagnostic biomarker for sialuria (GNE variants) and infantile sialic acid storage disease/Salla disease (SLC17A5 variants). Traditionally, UFSA has been measured using specific single-plex methodology in relatively small cohorts of patients with clinical symptoms suggestive of these disorders. The use of multiplex tandem mass spectrometry urine screening (UMSMS) has meant that UFSA can be measured semi-quantitatively in a much larger cohort of patients being investigated for suspected metabolic disorders. We hypothesised that the neuraminidase of Streptococcus pneumoniae may release free sialic acid from endogenous sialylated glycoconjugates and result in increased UFSA levels. We conducted a retrospective review of clinical records of patients who were identified as having S. pneumoniae infection and who also had UMSMS at the time of their acute infection. We identified three cases of increased UFSA detected by UMSMS screening that were secondary to S. pneumoniae sepsis. Additional testing ruled out genetic causes of increased UFSA in the first patient. All three patients had overwhelming sepsis with multiorgan dysfunction which was fatal. Glycosylation abnormalities consistent with the removal of sialic acid were demonstrated in serum transferrin patterns in one patient. We have demonstrated in a retrospective cohort that elevation of UFSA levels have been observed in cases of S. pneumoniae sepsis. This expands our knowledge of UFSA as a biomarker in human disease. This research demonstrates that infection with organisms with neuraminidase activity should be considered in patients with unexplained increases in UFSA.

N-acetylneuraminic acid (sialic acid) is an abundantly found carbohydrate moiety covering the surface of all vertebrate cells and secreted glycoproteins. The human N-acetylneuraminate pyruvate lyase (NPL) interconverts sialic acid to N-acetylmannosamine and pyruvate, and mutations of the NPL gene were found to cause sialuria and impair the functionality of muscles. Here we report the soluble and functional expression of human NPL in Escherichia coli, which allowed us to study the biochemical properties of two clinically relevant NLP mutations (Asn45Asp and Arg63Cys). The Asn45Asp mutant variant was enzymatically active, but had lower expression levels and showed reduced stability when compared to the wild-type NPL variant. Expression trials of the Arg63Cys mutant did not yield any recombinant protein and consequently, no enzymatic activity was detected. The locations of these clinically relevant amino acid substitutions are also discussed by using a human NPL homology model.