A missense mutation in the Na~+/glucose cotransporter gene SGLT1 in a patient with congenital glucose-galactose malabsorption: normal trafficking but inactivation of the mutant protein

摘要

The Na~+/glucose cotransporter gene SGLT1 was analyzed in a Japanese patient with congenital glucose-galactose malabsorption. Genomic DNA was used as a template for amplification by the polymerase chain reaction of each of the 15 exons of SGLT1. The amplification products were cloned and sequenced. About half of the exon 5 clones of the patient contained a C → T transition, resulting in an Arg~(135) → Trp mutation, whereas the remaining clones contained the normal exon 5 sequence. In addition, whereas some exon 12 clones exhibited the normal sequence, others showed a CAgtaggtatcatc → CAgacc mutation at the splice donor site of intron 12 that may result either in the skipping of exon 12 or in read-through of intron 12. Neither the Arg~(135) → Trp mutant nor either of the possible intron 12 mutant proteins exhibited Na~+-dependent glucose transport activity when expressed in Xenopus oocytes. Immunocytochemical analysis indicated, however, that the Arg~(135) → Trp mutant was localized to the oocyte plasma membrane. DNA sequence analysis revealed that the missense mutation in exon 5 and the splice site mutation in intron 12 were inherited from the proband's father and mother, respectively. These results indicate that the patient is a compound heterozygote for this disease, and that the Arg~(135) → Trp mutant of SGLT1 undergoes normal trafficking to the plasma membrane but is non-functional.