Alterations of Golgi Structural Proteins and Glycosylation Defects in Cancer

As the central hub in the secretory and endocytic pathways, the Golgi apparatus continually receives the flow of cargos and serves as a major processing station in the cell. Due to its dynamic nature, a sophisticated and constantly remodeling mechanism needs to be set up to maintain the Golgi architecture and function in the non-stop trafficking of proteins and lipids. Abundant evidence has been accumulated that a well-organized Golgi structure is required for its proper functions, especially protein glycosylation. Remarkably, altered glycosylation has been a hallmark of most cancer cells. To understand the causes of Golgi defects in cancer, efforts have been made to characterize Golgi structural proteins under physiological and pathological conditions. This review summarizes the current knowledge of crucial Golgi structural proteins and their connections with tumor progression. We foresee that understanding the Golgi structural and functional defects may help solve the puzzle of whether glycosylation defect is a cause or effect of oncogenesis.

P1803EFFICACY, SAFETY AND IMPACT ON RENAL FUNCTIONALITY OF ACETAZOLAMIDE TREATMENT IN PATIENTS WITH PMM2-CDG (AZATAX CLINICAL TRIAL)

Background and Aims Deficit of phosphomanomutase-(PMM2-CDG) is the most frequent congenital N-glycosylation defect, producing cerebellar syndrome with intellectual deficit and “stroke-like” episodes. It has been associated with renal alterations such as proteinuria, diffuse cortical hyperechogenicity and malformations. We evaluated efficacy, safety and renal repercussion of acetazolamide as a new therapeutic tool. Method First clinical trial in phase II including PMM2-CDG patients (5-21-years). First phase: 6 months treatment group with acetazolamide. Second phase: 5 weeks randomized withdrawal in responders (acetazolamide vs placebo), assessing renal functionality and effects of this medication. Dosing acetazolamide by 8 to 30mg/kg/day in 2-3 doses. Controls were performed at 3, 6, 14, 25 and 30 weeks determining acid-base balance, ionogram, renal function (creatinine) and Pr/Cr, Ca/Cr index and B2-microglobulin in first morning urine. All patients underwent in a bone densitometry study and renal ultrasound. Results 24 patients were included (mean age 12.3 ± 4.5 years). Bicarbonate levels and plasma pH were significantly lower at week 25(p <0.001). 13 patient needs to reduce acetazolamide dose due to excessive metabolic acidosis or asthenia. They showed a decrease in sodium (p=0.06), potassium (p<0.001) and serum calcium(p=0.030), although maintained in low normality limit with a decrease in protein loss(p=0.019) and increase in calcium/creatinine index(p=0.025) without B2-microglobulin alterations. The previously ultrasound renal described findings; the cortical hyperechogenicity was observed in 8.4% without renal dysfunction or associated nephrocalcinosis. One patient presented microlithiasis and another, symptomatic renal lithiasis. Densitometric study 69% of patients presented values in the range of osteopenia at the end of trial (-0.9 to -4.9SD, average of -2.36SD). Conclusion The efficacy of acetazolamide in the neurological symptoms of PMM2-CDG, due to an enzymatic stimulation mechanism mediated by acidosis, generates the possibility of chronic treatment with the drug in this group of patients, with possible renal adverse effects associated with long-term, overshadowing the skeletal and renal prognosis.

Unsuccessful intravenous D-mannose treatment in PMM2-CDG

Background PMM2-CDG (Phosphomannomutase 2 - Congenital disorder of glycosylation-Ia; CDG-Ia) is the most common glycosylation defect, often presenting as a severe multisystem disorder that can be fatal within the first years of life. While mannose treatment has been shown to correct glycosylation in vitro and in vivo in mice, no convincing effects have been observed in short-term treatment trials in single patients so far. Results We report on a boy with a severe PMM2-CDG who received a continuous intravenous mannose infusion over a period of 5 months during the first year of life in a dose of 0.8 g/kg/day. N-glycosylation of serum glycoproteins and mannose concentrations in serum were studied regularly. Unfortunately, no biochemical or clinical improvement was observed, and the therapy was terminated at age 9 months. Conclusion Postnatal intravenous D-mannose treatment seems to be ineffective in PMM2-CDG.

Mutations in MAGT1 lead to a glycosylation disorder with a variable phenotype

Congenital disorders of glycosylation (CDG) are a group of rare metabolic diseases, due to impaired protein and lipid glycosylation. We identified two patients with defective serum transferrin glycosylation and mutations in the MAGT1 gene. These patients present with a phenotype that is mainly characterized by intellectual and developmental disability. MAGT1 has been described to be a subunit of the oligosaccharyltransferase (OST) complex and more specifically of the STT3B complex. However, it was also claimed that MAGT1 is a magnesium (Mg2+) transporter. So far, patients with mutations in MAGT1 were linked to a primary immunodeficiency, characterized by chronic EBV infections attributed to a Mg2+ homeostasis defect (XMEN). We compared the clinical and cellular phenotype of our two patients to that of an XMEN patient that we recently identified. All three patients have an N-glycosylation defect, as was shown by the study of different substrates, such as GLUT1 and SHBG, demonstrating that the posttranslational glycosylation carried out by the STT3B complex is dysfunctional in all three patients. Moreover, MAGT1 deficiency is associated with an enhanced expression of TUSC3, the homolog protein of MAGT1, pointing toward a compensatory mechanism. Hence, we delineate MAGT1-CDG as a disorder associated with two different clinical phenotypes caused by defects in glycosylation.

Manganese-induced turnover of TMEM165

TMEM165 deficiencies lead to one of the congenital disorders of glycosylation (CDG), a group of inherited diseases where the glycosylation process is altered. We recently demonstrated that the Golgi glycosylation defect due to TMEM165 deficiency resulted from a Golgi manganese homeostasis defect and that Mn2+ supplementation was sufficient to rescue normal glycosylation. In the present paper, we highlight TMEM165 as a novel Golgi protein sensitive to manganese. When cells were exposed to high Mn2+ concentrations, TMEM165 was degraded in lysosomes. Remarkably, while the variant R126H was sensitive upon manganese exposure, the variant E108G, recently identified in a novel TMEM165-CDG patient, was found to be insensitive. We also showed that the E108G mutation did not abolish the function of TMEM165 in Golgi glycosylation. Altogether, the present study identified the Golgi protein TMEM165 as a novel Mn2+-sensitive protein in mammalian cells and pointed to the crucial importance of the glutamic acid (E108) in the cytosolic ELGDK motif in Mn2+-induced degradation of TMEM165.