The MUC5AC gene: RFLP analysis with the Jer58 probe

Mucin methods: genes encoding mucins and their genetic variation with a focus on gel-forming mucins

Mucin genes encode the polypeptide backbone of the mucin glycoproteins which are expressed on all epithelial surfaces and are major constituents of the mucus layer. Mucins are, thus, expressed at the interface between the external and the internal environment of the organism, and represent the first line of defence of our body. These genes often have an extensive region of repetitive exonic sequence which codes for the heavily glycosylated domain, whose roles include bacterial interactions and gel hydration. This region shows, in several of the genes, considerable inter-individual variation in repeat number and sequence. Because of their site of expression and their high variability in this important domain, mucin genes are good candidates for conferring differences in genetic susceptibility to multifactorial epithelial and inflammatory disease. However, progress in characterizing the genes has been considerably slower than the rest of the genome because of their size and the GC-rich content of the large, repetitive variable region. Some of the issues relating to the study of these genes are discussed in this chapter. In addition, methods and approaches that have been used successfully are described.

Mucin apoprotein expression in COPD

Mucins, which are complex glycoproteins that provide the viscoelastic properties of mucus that are essential for the protection of the airways, are characterized by a variable-number tandem repeats (VNTR) region that may undergo alternate splicing during transcription. Such transcripts may yield multiple proteins via diverse post-translational modifications involving glycosylation (within each VNTR). Fifteen distinct mucin genes have been identified, with several mapping to chromosomal clusters (ie, 7q22 and 11p15.5), possibly having evolved by gene duplication. The deduced protein sequences can be subdivided into both membrane-associated mucins and secreted mucins. Membrane-associated mucins consist of cytoplasmic, transmembrane, and extracellular domains. The membrane-associated mucins MUC1, MUC4, and MUC11 have been localized to the lung. In addition to VNTRs, secreted mucins possess repeated cysteine-rich D-domains (which are important in polymerization). Secreted mucins that are localized to the lung include MUC2 (in cells with and without secretory granules), MUC5AC (in surface and submucosal mucous cells), MUC5B and MUC8 (in submucosal mucous cells), and MUC7 (in submucosal serous cells). Currently, little is known about the regulation of mucins in COPD patients. Recent studies with acrolein and cigarette smoke have suggested that MUC5AC is inducible (accompanied by epidermal growth factor [EGF] ligand formation and the activation of EGF receptor-dependent pathways), whereas MUC5B is constitutively expressed (increasing through gland enlargement). Similarly, little is known about the genetic determinants that control mucus hypersecretion, but preliminary findings in animal models suggest that intrastrain differences in acrolein-induced mucin formation are amenable to genetic analysis. As our understanding of the functional genomics of mucin biology increases, further clinical targets and therapeutic strategies are likely to emerge.

Human mucin gene MUC5AC: organization of its 5'-region and central repetitive region

Human mucin gene MUC5AC is clustered with MUC2, MUC5B and MUC6 on chromosome 11p15.5. We report here the full length cDNA sequence upstream of the repetitive region of human MUC5AC. We have also determined the sequence of its large central tandem repeat array. The 5'-region reveals high degree of sequence similarity with MUC2 and MUC5B and codes for 1336 amino acids organized into a signal peptide, four pro-von Willebrand factor-like D domains (D1, D2, D' and D3) and a short domain which connects to the central repetitive region. In the central region, 17 major domains have been identified. Nine code for cysteine-rich domains (Cys-domains 1-9) and exhibit high sequence similarity to the cysteine-rich domains described in the central region of MUC2 and MUC5B. Cys-domains 1-5 are interspersed by domains enriched with serine, threonine, and proline residues. Cys-domains 1-9 are interspersed by four domains (TR1-TR4) composed of various numbers of MUC5AC-type repeats. Southern-blot analyses reveal allelic variations both in length and nucleotide sequence. The length polymorphism which is due to variable numbers of tandem repeats is located in TR1 and TR4, whereas a mutation polymorphism detected with TaqI is located in Cys-domain 6. In this study, the organization of MUC5AC has been entirely elucidated showing extensive similarity to the other chromosome 11p15 MUC genes, particularly MUC5B, and providing additional arguments for common evolution from a single ancestral gene.

Cysteine-rich regions of pig gastric mucin contain von willebrand factor and cystine knot domains at the carboxyl terminal(1)

In order to sequence the cysteine-rich regions of pig gastric mucin (PGM), we used our previously identified pig gastric mucin clone PGM-2A to screen a pig stomach cDNA library and perform rapid amplification of cDNA ends to obtain two cysteine-rich clones, PGM-2X and PGM-Z13. PGM-2X has 1071 base pairs (bp) encoding 357 amino acids containing five serine-threonine-rich 16 amino acid tandem repeats, downstream from a cysteine-rich region similar to human and mouse MUC5AC. PGM-Z13 encodes the complete 3'-terminus of PGM and is composed of 3336 bp with a 2964 bp open reading frame encoding 988 amino acids with four serine-threonine-rich tandem repeats upstream from a cysteine-rich region similar to the carboxyl terminal regions of human and rat MUC5AC and human MUC5B. This region is homologous to von Willebrand factor C and D domains involved in acid induced polymerization, and to the carboxyl terminal cystine-knot domain of various mucins, TGF-beta, vWF and norrin, which is involved in dimerization. These newly sequenced cysteine-rich regions of pig gastric mucin may be critical for its gelation and for its observed increased viscosity induced by low pH.

Genomic organization of the 3'-region of the human MUC5AC mucin gene: additional evidence for a common ancestral gene for the 11p15.5 mucin gene family

The human mucin gene MUC5AC is mapped clustered with MUC2, MUC5B and MUC6 on chromosome 11p15.5. We report here the isolation and characterization of a genomic cosmid clone, designated ELO9, spanning the 3'-region of MUC5AC and the 5'-region of MUC5B, allowing us to conclude that MUC5AC and MUC5B have the same transcriptional orientation. We determined the genomic organization and the entire sequence of the 3'-region of MUC5AC. The comparative molecular analysis of MUC5AC and MUC5B points to a remarkable similarity in the size and the distribution of exons, and in the type of splice sites, supporting the notion that MUC5AC and MUC5B have evolved from a single common ancestral gene. The derivation of the four genes of the 11p15.5 mucin gene family from a single ancestral gene is discussed.