L-fucose supplementation in a patient with global hypofucosylation and a mono-allelic variant in SLC35C1: Clinical improvement and assessment of biomarkers.

Fucosylation disorders are rare types of congenital disorders of glycosylation (CDG), the most common being SLC35C1-CDG, which is classically described as a leukocyte adhesion deficiency (hence the previous name of "leukocyte adhesion deficiency type II") with dysmorphic features, short stature, and moderate-to-severe developmental and intellectual disabilities. In more recent years, several cases have been described of individuals with bi-allelic SLC35C1 variants and biochemical proof of hypofucosylation who had short stature, dysmorphic features, and intellectual disability, but no hematological abnormalities. In this article, we report a patient with growth faltering, neuroirritability, nystagmus, developmental delays, microcephaly, dysmorphic features, and hypogammaglobulinemia G. Biochemical investigations including serum N-glycan profiling, fucosylation-focused whole serum glycoprotein profiling, and serum lectin blots, all of which showed significant global hypofucosylation. Exome sequencing revealed a single likely pathogenic variant, SLC35C1 (NM_018389.4):c.503_505delTCT, p.(Phe168del), which was inherited from an unaffected mother. Whole genome sequencing with manual review of raw data did not reveal any second pathogenic variants; SLC35C1 mRNA sequencing was negative for changes in the second allele or allelic imbalance. The patient was started on L-fucose supplementation, with subsequent improvements in weight and head circumference, normalization of IgG levels, and remarkable developmental catch-up. Biochemically, there was an increase in abundance of previously decreased fucosylated glycan species in serum, especially Fuc1GlcNAc2Man3 (a glycan that is known to be enriched in neutrophils). In summary, we present here further evidence for the role of L-fucose supplementation in treating hypofucosylation disorders and suggest that IgG and fucosylated glycan species may be useful as biomarkers in this scenario, although further research is needed to validate them as such. It is likely that the early introduction of L-fucose in this patient may have led to the excellent developmental outcomes observed.