Differential Proteomics of Urinary Exovesicles from Classical Galactosemic Patients Reveals Subclinical Kidney Insufficiency

Classical galactosemia is caused
by a nearly complete deficiency
of galactose-1-phosphate uridyltransferase (GALT; EC 2.7.712), resulting
in a severely impaired galactose metabolism with galactose-1-phosphate
and galactitol accumulation. Even on a galactose-restricted diet,
patients develop serious long-term complications of the central nervous
system and ovaries that may result from chronic cell-toxic effects
exerted by endogenous galactose. To address the question of whether
disease-associated cellular perturbations could affect the kidney
function of the patients, we performed differential proteomics of
detergent-resistant membranes from urinary exovesicles. Galactosemic
samples (showing drastic shifts from high-mannose to complex-type N-glycosylation on exosomal N-glycoproteins)
and healthy, sex-matched controls were analyzed in quadruplex iTRAQ
experiments performed in biological and technical replicates. Particularly
in the female patient group, the most striking finding was a drastic
increase of abundant serum (glyco)­proteins, like albumin, leucine-rich
α-2-glycoprotein, fetuin, immunoglobulins, prostaglandin H2 d-isomerase, and α-1-microglobulin protein (AMBP), pointing
to a subclinical failure of kidney filter function in galactosemic
patients and resulting in a heavy overload of exosomal membranes with
adsorbed serum (glyco)­proteins. Several of these proteins are connected
to TBMN and IgAN, proteinuria, and renal damage. The impairment of
renal protein filtration was also indicated by increased protein contents
derived from extracellular matrices and lysosomes.