Distinct evolution of the human carcinoma-associated transmembrane mucins, MUC1, MUC4 AND MUC16

Abstract

The MUC family of mucins consists of secreted and membrane-bound forms. Overexpression of the membrane-bound family members, MUC1 (CA15-3), MUC4 and MUC16 (CA125), is found in diverse human carcinomas. However, despite being classified in the same family, little is known about the genetic origins of the carcinoma-associated mucins. The present results show that MUC1 homologs are restricted to mammalian species. MUC1 has no sequence similarity with the other membrane-bound mucins, except for the presence of a sea urchin sperm protein-enterokinase-agrin (SEA) domain. The results indicate that the MUC1 SEA domain originated from heparin sulfate proteoglycan of basement membrane (HSPG2; perlecan), an inducer of tumor cell growth. MUC4 has no SEA domain, but does have (i) a NIDO domain that evolved from an ancestor common to nidogen, and (ii) AMOP and VWD domains that originated from an ancestor common to the Sushi-domain containing protein. MUC16 contains multiple SEA domains that are found in a chicken gene and were subsequently repeated through duplication events. The SEA domains in MUC16 appear to have evolved from agrin before the divergence of birds and mammals. These findings indicate that MUC1, MUC4 and MUC16 evolved from distinct ancestors and that the membrane-bound mucins consist of different subgroups based on their genetic backgrounds.

Introduction

Mucins are highly glycosylated proteins that contain over 50% of their mass as O-linked oligosaccharides. A common feature of the mucins is variable numbers of tandem repeats with a high proportion of serines and threonines that are modified by O-glycosylation. Mucins can also be subject to N-glycosylation (Strous and Dekker, 1992). The 20 mucin-type glycoproteins identified to date are classified as a family based on the presence of an extensively O-glycosylated tandem repeat structure (Human Genome Organization Gene Nomenclature Committee; http://www.hugo-international.org/hugo/). The mucins have been subdivided into the secreted and membrane-bound forms. The secreted mucins contribute to the formation of a physical barrier that provides protection for epithelial cells lining the respiratory and gastrointestinal tracts and the ductal surfaces of organs such as the liver, pancreas and kidney. The secreted MUC2, MUC5AC, MUC5B and MUC6 mucins are encoded by a cluster of genes at the chromosomal locus 11p15 and exhibit amino acid homology in regions outside the tandem repeats (Desseyn et al., 1998). The finding that genetic inactivation of the MUC2 gene induces the formation of intestinal tumors has supported a tumor suppressor function (Velcich et al., 2002). Moreover, the secreted mucins are not known to contribute to tumor development or progression.

The membrane-bound mucins also function in formation of the protective mucous gel through ectodomains that extend from the apical cell surface. These mucins characteristically have a sea urchin sperm protein, enterokinase and agrin (SEA) domain located between the O-glycosylated tandem repeats and the transmembrane domain. The membrane-bound MUC1 has variable numbers of highly glycosylated 20 amino acid tandem repeats, a SEA domain, a transmembrane domain and a 72 amino acid cytoplasmic tail (Gendler et al., 1988, Siddiqui et al., 1988, Merlo et al., 1989) (Fig. 1). MUC3A, MUC3B, MUC17, MUC13 and MUC12 each contain tandem repeat regions, a SEA domain and a transmembrane domain, and in addition have epidermal growth factor (EGF)-like domains (Porchet et al., 1991, Fox et al., 1992, Gross et al., 1992, Nollet et al., 1998, Moniaux et al., 1999, Williams et al., 1999) (Fig. 1). MUC4 also has EGF-like domains, but lacks a SEA domain (Fig. 1). MUC16 has multiple SEA domains and a transmembrane region, but no EGF-like domains (O'Brien et al., 2001, Yin and Lloyd, 2001, Yin et al., 2002, Maeda et al., 2004) (Fig. 1).

The MUC1, MUC4 and MUC16 mucins are aberrantly overexpressed in diverse human carcinomas. With transformation and loss of polarity, carcinoma cells express MUC1 (CA15-3) at high levels over the entire cell surface and in the cytoplasm (Kufe et al., 1984). MUC1 is overexpressed in most carcinomas of the breast, prostate, lung, pancreas, ovary and other epithelia. Multiple myeloma cells, lymphomas and certain leukemias also express MUC1 at high levels (Takahashi et al., 1994, Brossart et al., 2001, Teruya-Feldstein et al., 2003). Notably, overexpression of MUC1 is sufficient to induce transformation (Li et al., 2003) and resistance to stress-induced apoptosis (Yin and Kufe, 2003, Ren et al., 2004, Yin et al., 2004). MUC4 is overexpressed predominantly in pancreatic cancers (Swartz et al., 2002) and silencing of MUC4 is associated with decreased pancreatic tumor cell growth and metastasis (Singh et al., 2004). In addition, MUC4 functions as a ligand for the ErbB2 receptor tyrosine kinase (Carraway et al., 2003). MUC16 (CA125) is overexpressed by ovarian carcinomas and was first identified as a serum marker for women with ovarian cancer (Bast et al., 1981). The overexpression of MUC1, MUC4 and MUC16 by human carcinomas and the functional studies linking MUC1 and MUC4 with transformation have supported the importance of these mucins in tumor development and progression. However, little is known about their relatedness and their evolution.

In the present work, we have used available sequence information to study the evolution and relationships among MUC1, MUC4, MUC16 and the other membrane-bound mucins. The results indicate that MUC1, MUC4 and MUC16 evolved from distinct ancestors. These findings support classification of the membrane-bound mucins into different subgroups based on their genetic backgrounds.