Abnormal protein glycoforms in Prader-Willi syndrome

Background: Prader-Willi syndrome (PWS) is a contiguous gene disorder characterized by a range of neurological and endocrine abnormalities. Genetically, PWS arises from a loss of paternal chromosome 15q11-q13 with associated maternal imprinting of genes in this region. The most common molecular mechanisms are either paternal interstitial deletions or maternal uniparental disomy leading to haploinsufficiency. A number of genes involved in protein processing are present in the PWS region. After synthesis, proteins are frequently glycosylated as a post-translational modification. Proteins with incorrect processing or other cellular damage are postulated to be cleared by certain putative genes within the common PWS deletion breakpoints. Method: We undertook investigations into protein post-translational modifications in our PWS cohort (n = 23, age range = 1–18 years) using isoelectric focusing (IEF) of the serum O-linked glycoprotein, apolipoprotein C-III (apoC-III). Through apoC-III IEF, three isoforms differing in sialic acid content may be resolved. Non affected individuals typically contain relatively equal amounts of the disialylate d and monosialylated isoforms with low levels of the asialylated isoform. The glycans observed in PWS were further characterized by mass spectrometry. Results: Our study revealed nine PWS patients (∼40%) had an abnormal apoC-III IEF profile with increases in the asialylated isoform (9.1–16.9%; normal range 0–8.0%) and decreases in the disialylated isoform (22–39%; normal range 26–59%). The abnormal apoC-III IEF patterns observed in our study were independent of genotype. Mass spectrometry confirmed the abnormal structures. Conclusion: Although no genes involved in glycosylation are situated in 15q11–q13, there are two genes that have been given putative roles as ubiquitin E3 enzymes. We propose the increase in asialylated apoC-III isoforms in PWS is a reflection of an abnormality in the clearance of abnormal proteins rather than from a defect in glycan biosynthesis. Accumulation of other abnormal proteins in PWS is hypothesized.