Genetic complementation reveals a novel human congenital disorder of glycosylation of type II, due to inactivation of the Golgi CMP–sialic acid transporter

We have identified a homozygous G>A substitution in the donor splice site of intron 6 (IVS6 + 1G>A) of the cytidine monophosphate (CMP)–sialic acid transporter gene of Lec2 cells as the mutation responsible for their asialo phenotype. These cells were used in complementation studies to test the activity of the 2 CMP–sialic acid transporter cDNA alleles of a patient devoid of sialyl-Lexexpression on polymorphonuclear cells. No complementation was obtained with either of the 2 patient alleles, whereas full restoration of the sialylated phenotype was obtained in the Lec2 cells transfected with the corresponding human wild-type transcript. The inactivation of one patient allele by a double microdeletion inducing a premature stop codon at position 327 and a splice mutation of the other allele inducing a 130–base pair (bp) deletion and a premature stop codon at position 684 are proposed to be the causal defects of this disease. A 4-base insertion in intron 6 was found in the mother and is proposed to be responsible for the splice mutation. We conclude that this defect is a new type of congenital disorder of glycosylation (CDG) of type IIf affecting the transport of CMP–sialic acid into the Golgi apparatus.