Splice-site mutations: a novel genetic mechanism of Crigler-Najjar syndrome type 1.
Crigler-Najjar syndrome type 1 (CN-1) is a recessively inherited, potentially lethal disorder characterized by severe unconjugated hyperbilirubinemia resulting from deficiency of the hepatic enzyme bilirubin-UDP-glucuronosyltransferase. In all CN-1 patients studied, structural mutations in one of the five exons of the gene (UGT1A1) encoding the uridinediphosphoglucuronate glucuronosyltransferase (UGT) isoform bilirubin-UGT1 were implicated in the absence or inactivation of the enzyme. We report two patients in whom CN-1 is caused, instead, by mutations in the noncoding intronic region of the UGT1A1 gene. One patient (A) was homozygous for a G-->C mutation at the splice-donor site in the intron, between exon 1 and exon 2. The other patient (B) was heterozygous for an A-->G shift at the splice-acceptor site in intron 3, and in the second allele a premature translation-termination codon in exon 1 was identified. Bilirubin-UGT1 mRNA is difficult to obtain, since it is expressed in the liver only. To determine the effects of these splice-junction mutations, we amplified genomic DNA of the relevant splice junctions. The amplicons were expressed in COS-7 cells, and the expressed mRNAs were analyzed. In both cases, splice-site mutations led to the use of cryptic splice sites, with consequent deletions in the processed mRNA. This is the first report of intronic mutations causing CN-1 and of the determination of the consequences of these mutations on mRNA structure, by ex vivo expression.
Gantla
S
S
Department of Medicine, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
Bakker
C T
CT
Deocharan
B
B
Thummala
N R
NR
Zweiner
J
J
Sinaasappel
M
M
Roy Chowdhury
J
J
Bosma
P J
PJ
Roy Chowdhury
N
N
D000328:Adult; D003414:Crigler-Najjar Syndrome / Q000235:genetics*; D014453:Glucuronosyltransferase / Q000235:genetics*; D006801:Humans; D007223:Infant; D007438:Introns; D008099:Liver / Q000201:enzymology; D008297:Male; D009154:Mutation; D012323:RNA Processing, Post-Transcriptional; D012326:RNA Splicing / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012342:RNA, Small Nuclear / Q000235:genetics
D012333:RNA, Messenger; D012342:RNA, Small Nuclear; C019787:bilirubin glucuronoside glucuronosyltransferase; D014453:Glucuronosyltransferase