Multiple transcripts of MUC3: evidence for two genes, MUC3A and MUC3B

The MUC3 gene encodes a transmembrane mucin-type glycoprotein. A number of consistent single nucleotide changes were observed in different MUC3 cDNAs from a single individual, suggesting the presence of at least three different transcripts per individual. This transcript heterogeneity is due both to the existence of a second copy of the MUC3 gene and to allelic changes. Sequencing of the second MUC3 shows that it has the same C-terminal domain and intron/exon structure as the previously described MUC3. The tandem repeat domain has the same amino acid consensus sequence but shows more substitutions. The unique exonic sequences range from 94 to 100% identity at the nucleotide level and correspondingly few amino acid changes have been identified. The introns show around 95% identity. We propose to name this second gene MUC3B, MUC3A being reserved for the first MUC3 gene. MUC3B, like MUC3A, is expressed in intestine and Caco-2 cells.

Genomic organization and structure of the 3' region of human MUC3: alternative splicing predicts membrane-bound and soluble forms of the mucin

The MUC3 gene encodes a large, glycosylated mucin produced by intestinal epithelial cells to form a protective barrier against the external environment. Recently published cDNA sequences for the carboxyl-terminal region of MUC3 polypeptide indicated that rodent Muc3 possesses two epidermal growth factor (EGF)-like domains, and putative transmembrane and cytoplasmic domains, whereas the sequence of human MUC3 predicted termination after the first EGF-like domain. Here we describe the complete genomic sequence encompassing the carboxyl terminal region of human MUC3, revealing the boundaries of 11 exons. RT-PCR and cDNA library cloning experiments indicate that the gene is alternatively spliced, yielding a major membrane-bound form as well as multiple soluble forms. Thus, this work indicates that both membrane-bound and soluble MUC3 mucin proteins are produced by alternative splicing of a single gene. A potentially important polymorphism involving a Tyr residue with a proposed role in signalling is described as well.

Variable number tandem repeat polymorphism of the mucin genes located in the complex on 11p15.5

A family of four genes that encode major secreted mucins (MUC6, MUC2, MUC5AC and MUC5B) map to within 400 kb on chromosome 11p15.5. These genes contain long stretches of tandem repeats of sequence that encode serine- and threonine-rich domains but that otherwise show no similarity from gene to gene, and regions of unique sequence domains that do show evidence of sequence homology. We have previously reported the existence of polymorphism in three of these genes but the extent and nature of this allelic variation is now described here in detail. Variable number tandem repeat polymorphisms of MUC6, MUC2 and MUC5AC are predicted to encode mucin polypeptides that differ in length. In the case of MUC2 and MUC6 these length differences are substantial (up to twofold). MUC5B in contrast does not show common allele length variation. Three MUC2 mutations are reported, none of which are associated with the meiotic recombinations previously observed in this region of chromosome 11.

MUC3 human intestinal mucin. Analysis of gene structure, the carboxyl terminus, and a novel upstream repetitive region

MUC3 is a large mucin glycoprotein expressed by the human intestine and gall bladder. In this manuscript, we present details of the deduced protein structure of MUC3. The MUC3 carboxyl-terminal domain is 617 residues in length, including 511 residues of a non-repetitive mucin-like domain (27% Thr, 22% Ser, and 11% Pro) and a 106-residue Cys-rich domain with homology to the epidermal growth factor (EGF) -like structural motifs found in many proteins. The region of MUC3 located upstream of the previously described 51-base pair (bp) tandem repeats, which encode a major Ser and Thr-rich domain, consists of a second type of repetitive structure with an imperfect periodicity of approximately 1125 bp. This domain is also mucin-like and appears to be considerably larger than 2000 residues (6000 bp). The MUC3 gene itself is large and complex. Using pulse field gel electrophoresis and blot analysis, the smallest fragment found that contained all human genomic DNA hybridizing to the 51-bp tandem repeat probe was 200 kilobases with restriction enzyme SwaI. Both PvuII and PstI produced two sets of hybridizing fragments that were hypervariable within the human population with a pattern suggestive of both a variation in the number of tandem repeats (VNTR) and sequence polymorphism. These fragments varied independently of each other, but no genetic recombination was detected in a study of 40 human families. Thus, the MUC3 gene encodes a very large glycoprotein with a structure very different from that of any mucin currently described.

Human mucin genes assigned to 11p15.5: identification and organization of a cluster of genes

Four distinct genes that encode mucins have previously been mapped to chromosome 11p15.5. Three of these genes (MUC2, MUC5AC, and MUC6) show a high level of genetically determined polymorphism and were analyzed in the CEPH families. Linkage analysis placed all three genes on the genetic map in a cluster between HRAS and INS, and more detailed analysis of recombinant breakpoints revealed that MUC6 is telomeric to MUC2. Using these recombinants D11S150 was mapped close to MUC2. Ten of the 11 recombinant chromosomes studied in detail were paternal, and the recombinant events were distributed throughout the 11p15 region, suggesting that the high level of recombination observed in 11p15.5 is not due to a particular recombinational hot spot. Pulsed-field gel electrophoresis was used to make a detailed physical map of the MUC cluster and to integrate the physical and genetical maps. The gene order was determined to be HRAS-MUC6-MUC2-MUC5AC-MUC5B-IGF2. The MUC genes span a region of some 400 kb and the map extends 770 kb and contains 15 putative CpG islands. The order of the MUC genes on the map corresponds to the relative order of their expression along the anterior-posterior axis of the body, suggesting a possible functional significance to the gene order.

Two additional polymorphisms within the hypervariable MUC1 gene: association of alleles either side of the VNTR region

The gene MUC1 codes for a mucin-type glycoprotein and like most of the other mucin genes shows variable number tandem repeat (VNTR) polymorphism within the coding region. A polymorphism due to a G/A substitution in exon 2, responsible for a genetically determined variation in splicing of the MUC1 transcript, has also been reported (Ligtenberg et al. 1990, 1991). Here we describe the detection of this nucleotide substitution polymorphism by single stranded conformational analysis of genomic DNA and we also report a CA repeat polymorphism within intron 6 of the gene. Haplotypes were determined in a series of families and the common alleles of these two polymorphisms were found to be associated. These results support the notion that the VNTR polymorphism in the coding sequence of MUC1 is not caused by unequal reciprocal recombination at meiosis.

The MUC5AC gene: RFLP analysis with the Jer58 probe

We have recently obtained evidence that the locus corresponding to three groups of partial tracheobronchial cDNAs (A = Jer47, B = Jer57, C = Jer58) which mapped to chromosome 11p15 and was given the symbol MUC5 corresponds to two distinct genes which we have provisionally called MUC5B and MUC5AC. Here we describe the detection, using the Jer58 probe, which contains a 24-bp tandem repeat, of polymorphism in the MUC5AC gene with seven different restriction enzymes.

The lactase persistence/non-persistence polymorphism is controlled by a cis-acting element

Lactase activity is present at high levels in the small intestine of some human adults and not others. This is due to a genetically determined polymorphism which affects the developmental regulation of the expression of the lactase gene. This polymorphism is of considerable interest in relation to cultural differences in nutrition but despite exhaustive studies, the molecular basis has not yet been found. It has not even been shown whether the sequence differences reside within or adjacent to the lactase gene itself or in a trans-acting factor. We have therefore exploited known DNA 'marker' polymorphisms within the exons of the lactase gene to examine the expression of the individual lactase mRNA transcripts from persistent and non-persistent individuals in order to determine whether the regulation is in cis or trans. Our results show that in certain lactase persistent individuals one allele of the lactase gene is expressed at much lower levels than the other and these individuals tend to have intermediate lactase activities. It is proposed that these people are heterozygous for the lactase persistence and non-persistence alleles and that this means that the nucleotide substitutions responsible for the lactase persistence/non-persistence polymorphism are cis-acting. This narrows down considerably the area of the genome that needs to be searched for the relevant sequence differences.

DNA polymorphisms in the lactase gene. Linkage disequilibrium across the 70-kb region

The enzyme lactase, which is responsible for the digestion of dietary lactose, is present in the intestine of some adults but not others. As a means of providing a platform to explore the molecular basis of this nutritionally relevant genetic variation we have screened for polymorphism in several regions of the lactase gene. In each case simple polymerase chain reaction-based procedures (including single-strand conformation analysis and denaturing gradient gel electrophoresis) were used, combined with silver staining as a method of detection. Allelic variation was found at 6 different sites. One previously published polymorphism was also tested. The frequencies of the alleles were determined in more than 100 unrelated individuals of the Centre d'Etude du Polymorphisme Humain (CEPH) panel, and the haplotypes were deduced. A region of linkage disequilibrium was observed, which spans the whole coding region of the lactase gene (approximately 60-70 kb); there were only 3 common haplotypes in this population. When the CEPH sample was subdivided according to the population of origin (France or Utah) the haplotype frequencies were shown to be markedly different.