Alternative titles; symbols
HGNC Approved Gene Symbol: RFT1
SNOMEDCT: 733084000;
Cytogenetic location: 3p21.1 Genomic coordinates (GRCh38): 3:53,066,853-53,130,435 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 3p21.1 | Congenital disorder of glycosylation, type In | 612015 | Autosomal recessive | 3 |
N-glycosylation of proteins follows a highly conserved pathway that begins with the synthesis of a Man(5)GlcNAc(2)-dolichylpyrophosphate (PP-Dol) intermediate on the cytoplasmic side of the endoplasmic reticulum (ER) membrane followed by the translocation of Man(5)GlcNAc (2)-PP-Dol to the luminal side of the ER membrane. RFT1 is the flippase enzyme that catalyzes this translocation (Helenius et al., 2002).
By database searching with the sequence of S. cerevisiae Rft1 as query, followed by RT-PCR of human fibroblast total RNA, Haeuptle et al. (2008) cloned RFT1. The deduced 541-amino acid protein contains 11 transmembrane domains and has a potential N-glycosylation site in a hydrophilic loop within the ER lumen. RFT1 shares 22% sequence identity with the yeast protein.
Helenius et al. (2002) showed that the expression of yeast Rft1 complemented a defect in N-glycosylation in the delta-alg11 (613666) strain of yeast. Overexpression of Rft1 in delta-alg11 yeast suggested that Rft1 is the limiting component for the flipping of Man(5)GlcNAc(2)-PP-Dol from the cytoplasmic to luminal side of the ER membrane in vivo and that no additional factors are required. Rft1 depletion in yeast led to underglycosylation of the vacuolar N-linked glycoprotein carboxypeptidase Y. Haeuptle et al. (2008) found human RFT1 complemented Rft1-depletion in yeast and restored normal carboxypeptidase Y glycosylation.
Hartz (2008) mapped the RFT1 gene to chromosome 3p21.1 based on an alignment of the RFT1 sequence (GenBank AJ318099) with the genomic sequence (build 36.1).
A particular feature of N-glycans is that they are first assembled in the endoplasmic reticulum (ER) as lipid-linked oligosaccharides (LLO). This assembly proceeds through the sequential addition of monosaccharides to the growing LLO. The assembly of LLOs requires glycosyltransferases and their respective nucleotide- and dolicho-activated monosaccharide substrates, but it also requires several proteins that regulate the complex topology of the process. For example, in yeast the Rft1 protein is essential for translocation of the cytosolically oriented intermediate DolPP-GlcNAc(2)Man(5) into the ER lumen, where LLO assembly is completed. The identification of N-linked glycosylation disorders in humans, referred to as congenital disorders of glycosylation (see CDG1A, 212065), demonstrated the conservation of the LLO assembly pathway between yeast and humans. Haeuptle et al. (2008) identified a novel glycosylation defect in a theretofore untyped CDG case (CDG1N; 612015), thereby establishing the importance of the RFT1 protein in human N-linked glycosylation. The patient, who had been diagnosed with a disorder of N-linked glycosylation on the basis of detection of abnormal isoelectric focusing of serum transferrin, carried a homozygous point mutation in the RFT1 gene (R67C; 611908.0001).
In 3 unrelated children with CDG1N, Vleugels et al. (2009) identified 3 different homozygous missense mutations in the RFT1 gene (611908.0001-611908.0003). All mutations were located in 1 of the hydrophilic loops predicted to be within the ER lumen. Patient fibroblasts showed accumulation of Man(5)GlcNAc(2)-PP-dolichol and decreased DNase I secretion compared to controls, and these defects were restored by expression of wildtype RFT1.
In 2 unrelated children with CDG1N, Jaeken et al. (2009) identified biallelic mutations in the RFT1 gene (611908.0002; 611908.0004-611908.0005).
In 2 sibs, born of unrelated Czech parents, with CDG1N, Ondruskova et al. (2012) identified compound heterozygous missense mutations in the RFT1 gene (M408V, 611908.0006 and R442Q, 611908.0007). Functional studies were not performed, but both mutations occurred in the transmembrane domain, unlike previous RFT1 mutations that occurred in the luminal loops. The phenotype in the Czech sibs was somewhat milder compared to other patients, which Ondruskova et al. (2012) postulated may be due to the location of the mutation.
In a patient with a congenital disorder of glycosylation (CDG1N; 612015), Haeuptle et al. (2008) identified a homozygous C-to-T transition at nucleotide 199 of the RFT1 gene, resulting in a substitution of cysteine for arginine at codon 67 (R67C). The R67C substitution occurred in a 50-amino-acid hydrophilic stretch in the overall hydrophobic RFT1 protein. Despite the low sequence identity (22%) between yeast and human RFT1 protein, Haeuptle et al. (2008) demonstrated both their functional orthology and the pathologic effect of the human R67C mutation by complementation assay in Rft1-deficient yeast cells. The causality of the RFT1 R67C mutation was further established by restoration of normal glycosylation profiles in patient-derived fibroblasts after lentiviral expression of the normal RFT1 cDNA.
Vleugels et al. (2009) identified a homozygous R67C mutation (c.199C-T, NM_052859.2) in a female child of Scottish descent (patient 1) with CDG1N. The patient was severely affected, with dysmorphic features, seizures, severe mental retardation, and virtually no development; she died at age 8 months.
In a 5.5-year-old boy (patient 2), born of consanguineous Italian parents, with congenital disorder of glycosylation type In (CDG1N; 612015), Vleugels et al. (2009) identified a homozygous c.454A-G transition (c.454A-G, NM_052859.2) in the RFT1 gene, resulting in a lys152-to-glu (K152E) substitution at a conserved residue in the second luminally oriented hydrophilic stretch of the protein. The mutation segregated with the disorder in the family.
Jaeken et al. (2009) identified a homozygous K152E mutation in a boy of Moroccan descent (patient 1) with CDG1N.
In a 2.2-year-old boy (patient 3), born of consanguineous Algerian parents, with congenital disorder of glycosylation type In (CDG1N; 612015), Vleugels et al. (2009) identified a homozygous c.892G-A transition in the RFT1 gene (c.892G-A, NM_052859.2), resulting in a glu298-to-lys (E298K) substitution at a conserved residue in the largest luminal loop. No tissue from the parents or sibs was available for segregation analysis.
In an Italian girl (patient 2) with congenital disorder of glycosylation type In (CDG1N; 612015), Jaeken et al. (2009) identified compound heterozygous mutations in the RFT1 gene: a c.887T-A transversion, resulting in an ile296-to-lys (I296K) substitution, and a c.887T-G transversion, resulting in an ile296-to-arg (I296R; 611908.0005) substitution. Both mutations affected the same highly conserved residue in the luminal loop of the protein. Each unaffected parent was heterozygous for 1 of the mutations. Functional studies of the variants were not performed.
For discussion of the ile296-to-arg (I296R) mutation in the RFT1 gene that was found in compound heterozygous state in a patient with congenital disorder of glycosylation type In (CDG1N; 612015) by Jaeken et al. (2009), see 611908.0004.
In 2 sibs, born of unrelated Czech parents, with congenital disorder of glycosylation type In (CDG1N; 612015), Ondruskova et al. (2012) identified compound heterozygous mutations in exon 12 of the RFT1 gene: a c.1222A-G transition, resulting in a met408-to-val (M408V) substitution, and a c.1325G-A transition, resulting in an arg442-to-gln (R442Q; 611908.0007) substitution. Each unaffected parent was heterozygous for 1 of the mutations, neither of which were found in 200 controls. Functional studies were not performed, but both mutations occurred in the transmembrane domain, unlike previous RFT1 mutations that occurred in the luminal loops. The phenotype in the Czech sibs was somewhat milder compared to other patients, which Ondruskova et al. (2012) postulated may be due to the location of the mutation.
For discussion of the arg442-to-gln (R442Q) mutation in the RFT1 gene that was found in compound heterozygous state in 2 sibs with congenital disorder of glycosylation type In (CDG1N; 612015) by Ondruskova et al. (2012), see 611908.0006.
Haeuptle, M. A., Pujol, F. M., Neupert, C., Winchester, B., Kastaniotis, A. J., Aebi, M., Hennet, T. Human RFT1 deficiency leads to a disorder of N-linked glycosylation. Am. J. Hum. Genet. 82: 600-606, 2008. [PubMed: 18313027] [Full Text: https://doi.org/10.1016/j.ajhg.2007.12.021]
Hartz, P. A. Personal Communication. Baltimore, Md. 3/19/2008.
Helenius, J., Ng, D. T. W., Marolda, C. L., Walter, P., Valvano, M. A., Aebi, M. Translocation of lipid-linked oligosaccharides across the ER membrane requires Rft1 protein. Nature 415: 447-450, 2002. [PubMed: 11807558] [Full Text: https://doi.org/10.1038/415447a]
Jaeken, J., Vleugels, W., Regal, L., Corchia, C., Goemans, N., Haeuptle, M. A., Foulquier, F., Hennet, T., Matthijs, G., Dionisi-Vici, C. RFT1-CDG: deafness as a novel feature of congenital disorders of glycosylation. J. Inherit. Metab. Dis. 32 (suppl. 1): S335-S338, 2009. [PubMed: 19856127] [Full Text: https://doi.org/10.1007/s10545-009-1297-3]
Ondruskova, N., Vesela, K., Hansikova, H., Magner, M., Zeman, J., Honzik, T. RFT1-CDG in adult siblings with novel mutations. Molec. Genet. Metab. 107: 760-762, 2012. [PubMed: 23111317] [Full Text: https://doi.org/10.1016/j.ymgme.2012.10.002]
Vleugels, W., Haeuptle, M. A., Ng, B. G., Michalski, J.-C., Battini, R., Dionisi-Vici, C., Ludman, M. D., Jaeken, J., Foulquier, F., Freeze, H. H., Matthijs, G., Hennet, T. RFT1 deficiency in three novel CDG patients. Hum. Mutat. 30: 1428-1434, 2009. [PubMed: 19701946] [Full Text: https://doi.org/10.1002/humu.21085]