A novel nonsense and inactivating variant of ST3GAL3 in two infant siblings suffering severe epilepsy and expressing circulating CA19.9

Abstract

Three missense variants of ST3GAL3 are known to be responsible for a congenital disorder of glycosylation determining a neurodevelopmental disorder (intellectual disability/epileptic encephalopathy). Here we report a novel nonsense variant, p.Y220*, in two dichorionic infant twins presenting a picture of epileptic encephalopathy with impaired neuromotor development. Upon expression in HEK-293T cells, the variant appears totally devoid of enzymatic activity in vitro, apparently accumulated with respect to the wild-type or the missense variants, as detected by western blot, and in large part properly localized in the Golgi apparatus, as assessed by confocal microscopy. Both patients were found to efficiently express the CA19.9 antigen in the serum despite the total loss of ST3GAL3 activity, which thus appears replaceable from other ST3GALs in the synthesis of the sialyl-Lewis a epitope. Kinetic studies of ST3GAL3 revealed a strong preference for lactotetraosylceramide as acceptor and gangliotetraosylceramide was also efficiently utilized in vitro. Moreover, the p.A13D missense variant, the one maintaining residual sialyltransferase activity, was found to have much lower affinity for all suitable substrates than the wild-type enzyme with an overall catalytic efficiency almost negligible. Altogether the present data suggest that the apparent redundancy of ST3GALs deduced from knock-out mouse models only partially exists in humans. In fact, our patients lacking ST3GAL3 activity synthesize the CA19.9 epitope sialyl-Lewis a, but not all glycans necessary for fine brain functions, where the role of minor gangliosides deserves further attention.

Gangliosides in the Brain: Physiology, Pathophysiology and Therapeutic Applications

“…Decreased ganglioside levels, especially GM1 and GD1a were found in the cerebrospinal fluid (CSF) of a small number of patients with West syndrome (Izumi et al, 1993), an infantile epilepsy disorder caused by a variety of genetic, metabolic and developmental factors (D'Alonzo et al, 2018). More recently, loss-of-function mutations in ST3GAL3 -one of the two sialyltransferases involved in the synthesis of GD1a and GT1b (Sturgill et al, 2012; Figure 1) -were found in patients with West syndrome (Edvardson et al, 2013) and in two siblings with epileptic encephalopathy (Indellicato et al, 2019). In animal models, ST3GAL3 catalyses the sialylation of glycoproteins in addition to glycolipids (Sturgill et al, 2012).…”

Complementary Use of Carbohydrate Antigens Lewis a, Lewis b, and Sialyl-Lewis a (CA19.9 Epitope) in Gastrointestinal Cancers: Biological Rationale towards a Personalized Clinical Application

“…Detection of CA19.9 in the serum or other fluids or tissue homogenates is easily performed by a sandwich Enzyme-linked immunosorbent assay (ELISA) [44] using immobilized and labeled anti-CA19.9 antibody as the capture and detection antibody, respectively. We have set a similar procedure for Lea and Leb using the corresponding antibodies purified from available hybridomas (anti-Lea 151-6-A7-9, ATCC HB 8324, and anti-Leb 130-3-F7-5, ATCC HB 8326) and screened up to 20 sera collected from healthy volunteers, obtaining a range of values spanning from unmeasurable to an arbitrary maximum level, which was assumed as a reference putative normal range for each antigen.…”

“…However, the exclusive role of ST3GAL3 has been definitely ruled out very recently in a study of two young children harboring a recessive non-sense mutation of the gene. They carry a totally inactive ST3GAL3 variant but do express circulating CA19.9 at high levels, according to their age [44]. This indicates that multiple ST3GALs independently contribute to synthesize the epitope.…”

Metabolism of Glycosphingolipids and Their Role in the Pathophysiology of Lysosomal Storage Disorders

“…The first case was reported in 1974, when an infant presented with severe motor impairment and persistent seizures culminating in death by 3.5 months of age [73]. A variant of GM3 synthase was also found in a patient that suffered from severe auditory, visual, motor and cognitive impairments, as well as respiratory chain dysfunction [74]. Due to the absence of ST3Gal V activity, a post-mortem examination of the patient's brain revealed a complete absence of complex gangliosides.…”

“…In a patient with West syndrome (ST3Gal III-CDG), an illness characterized by epileptic spasms (ranging in severity), abnormal brain waves and intellectual disability [75], the α2,3-sialyltransferase III was found to be deficient, thus decreasing α2-3sialylation of Gal linked to GalNAc in GD1a, GT1b and GQ1c, which causes an accumulation of GM1. Missense variants of ST3Gal III have also been associated with epileptic encephalopathy and impaired neuromotor development in an infant [74]. In cortical neurons derived from induced pluripotent stem cells isolated from patients, a missense variant of ST3Gal III was associated with altered lectin binding patterns and enhanced adherence to poly-L-ornithine/laminin-coated surfaces [76].…”