Three of these genes are clearly involved in the process of O-mannosylation (POMT1, POMT2, POMGnT1) (20, 24, 25), while the function of the remaining 3 genes, fukutin, FKRP and LARGE is still not clear (26–29). Of these 6 genes, the most frequently mutated in the Caucasian population is FKRP. While this was the first gene to be associated with an extremely wide range of clinical severity, more recent data suggests that this is also a common theme for mutations in other genes. The FKRP gene Our group originally described mutations in the fukutin-related protein gene (FKRP) in patients with a form of CMD (MDC1C) characterized by onset at
birth or in the first few months of life with profound weakness, markedly Inhibitors,research,lifescience,medical elevated Inhibitors,research,lifescience,medical serum CK and inability to achieve independent ambulation or standing (22). Intelligence was preserved and brain imaging normal. These patients had a significant reduction of the glycosylation of ADG both on immunocytochemistry and Western blot analysis (22). Shortly after, our group also identified involvement of the FKRP gene in a much milder variant of limb girdle muscular
dystrophy, LGMD2I, which had already been mapped to chromosome 19q13 where the FKRP gene lies (30). In contrast with MDC1C, the onset of the condition in LGMD2I varied from childhood to late adult life; typical patients Inhibitors,research,lifescience,medical with LGMD2I have a hypertrophic phenotype and a proximal Inhibitors,research,lifescience,medical distribution of weakness, limited or no contractures, markedly elevated serum CK and frequent cardiac complications (30–32). Both intelligence and brain imaging are entirely normal. Surprisingly, this form of LGMD was subsequently found to be the most common LGMD variant in the UK population,
due to the high frequency of a C826A mutation, with an estimated heterozygote frequency of ~1:400 (32). Inhibitors,research,lifescience,medical This particular mutation was also found at high frequency in other Caucasian populations, such as in Germany (33) and Scandinavian countries (34), while it was less common in Italians, and even less common in LGMD patients from Brazil (27, 35) and Japan. The expression of glycosylated ADG was only moderately reduced in LGMD2I, in keeping with the less severe muscle involvement compared to children with MDC1C (28). Subsequent studies clarified that nearly the originally described MDC1C phenotype did not represent the most severe end of the clinical spectrum, as we then identified FKRP mutations in patients with a CMD variant resembling MDC1C but with additional features such as mental retardation and cerebellar dysplasia and cysts on brain MRI (36), Tideglusib nmr followed by the identification of mutations in patients with severe supratentorial cortical dysplasia and structural eye involvement, mimicking classical Muscle-Eye Brain disease (MEB) and Walker Warburg syndrome (WWS) (37). The severity of loss of ADG glycosylation in these patients was more severe than previously found in MDC1C, in keeping with their more severe clinical phenotype.