On 25 March at 15 Mari-Anne Vals will defend her doctoral thesis „Congenital N-glycosylation disorders in Estonia“.
Professor Katrin Õunap (dr. med.), Institute of Clinical Medicine, University of Tartu
Associate professor of Experimental Paediatrics Erik A. Eklund (MD, PhD), Clinical Sciences, Lund University, Sweden
Congenital disorders of glycosylation (CDG) are an expanding group of inherited metabolic diseases caused by impaired synthesis and attachment of glycans on proteins and lipids. Disorders affecting the N-glycosylation pathway form the most common CDG subgroup, and the most common N-glycosylation disorder is PMM2-CDG. The symptoms of different CDG are often non-specific and multisystem. Serum transferrin isoelectric focusing (Tf IEF) is a routine method to screen CDG.
The aim of this study was to implement Tf IEF in Estonian clinical practice and to study the presence of N-glycosylation defects among Estonian patients in a three-year screening period. Altogether, positive CDG screening with subsequent molecular confirmation was detected in six patients among 1230 subjects screened. First, the most frequent CDG in Estonia is PMM2-CDG as we diagnosed this disorder in four patients from two families. In one family, the siblings show a mild neurological phenotype with normal-borderline cognitive development, which has previously been seldom described. Among PMM2-CDG patients, the most common variant in PMM2 gene is p.Val131Met. Second, we reported the expected frequency of PMM2-CDG based on the Estonian population data. In this cohort, we identified five different heterozygous variants in PMM2 gene. The most frequent variant is p.Arg141His with carrier frequency 1/224. The carrier frequency for p.Val131Met based on the Estonian population data is 1/449. The expected frequency of PMM2-CDG is 1/77,000. Third, we described a patient with SLC35A2-CDG and compared his phenotype-genotype with 14 international SLC35A2-CDG patients. This type of CDG presents as a non-specific neurological syndrome with global developmental delay, hypotonia, seizures and epileptic encephalopathy, together with dysmorphic features and short stature. In addition, Tf IEF can show a normal profile. Fourth, we presented a patient with multisystem clinical CDG features and a novel type II CDG likely caused by homozygous variant in STX5. In conclusion, Tf IEF proved to be an effective method to detect CDG among Estonian patients. Our results led to many findings, which have helped to add new clinical and epidemiological data about different known types of CDG, but also to expand the group of CDG by the discovery of a new type of CDG.