Allele‐specific loss of heterozygosity at the DAL‐1/4.1B (EPB41L3) tumor‐suppressor gene locus in the absence of mutation

Abstract

DAL-1/4.1B (EPB41L3)is a member of the protein 4.1 superfamily, which encompasses structural proteins that play important roles in membrane processes via interactions with actin, spectrin, and the cytoplasmic domains of integral membrane proteins. DAL-1/4.1B localizes within chromosomal region 18p11.3, which is affected by loss of heterozygosity (LOH) in various adult tumors. Reintroduction of this protein into DAL-1/4.1B-null lung and breast tumor cell lines significantly reduced the number of cells, providing functional evidence that this protein possesses a growth suppressor function not confined to a single cell type. For characterization of the mutational mechanisms responsible for loss of DAL-1/4.1B function in tumors, the exon-intron structure of DAL-1/4.1B was examined for mutations in 15 normal/tumor pairs of non-small cell lung carcinoma by single-strand conformation polymorphism analysis. These studies revealed that small intragenic mutations are uncommon in DAL-1/4.1B. Furthermore, LOH analysis on 129 informative early-stage breast tumors utilizing a new intragenic C/T single-nucleotide polymorphism in exon 14 revealed that LOH resulted in preferential retention of the C-containing allele, suggesting that allele-specific loss is occurring. These studies indicate that mechanisms such as imprinting or monoallelic expression in combination with loss of heterozygosity may be responsible for loss of the DAL-1/4.1B protein in early breast disease.

Promoter Methylation of DAL-1/4.1B Predicts Poor Prognosis in Non–Small Cell Lung Cancer

“…Although LOH on 18p11.3 was reported in f40% of NSCLCs (18) as well as 60% to 70% of meningiomas (17) or breast cancers (19), a subsequent study showed that LOH in this region was not correlated with loss of DAL-1 expression (17). Moreover, mutational screening failed to identify inactivating mutations of the DAL-1 gene (17,20). Because many tumor suppressor genes carrying the CpG islands in their upstream regions are often silenced by epigenetic mechanisms, we examined the status of the promoter methylation of the DAL-1 gene in the present study.…”

Loss of the Putative Tumor Suppressor Band 4.1B/Dal1 Gene Is Dispensable for Normal Development and Does Not Predispose to Cancer

“…However, this (4,5,15,23,24). Alas, studies examining the remaining allele of 4.1B have failed to detect any mutations (4,7,12). It is possible that other mechanisms may account for the lack of mutations, such as epigenetic modifications of the 4.1B allele.…”

Protein 4.1B expression is induced in mammary epithelial cells during pregnancy and regulates their proliferation

“…Similar results were seen with other mammary cell lines (data not shown). 4.1B contains a putative nuclear localization signal (NLS) (KRRK), as well as an upstream casein kinase II site (SAAE), a common feature in proteins containing an NLS (Kittiniyom et al, 2004). Furthermore, 4.1B has been shown to translocate from the plasma membrane into the nucleus in PC12 cells following differentiation (Gascard et al, 2004), and the 4.1 family members merlin, ezrin, and 4.1R localize within the nucleus under some conditions (Mattagajasingh et al, 1999;Batchelor et al, 2004;Muranen et al, 2005).…”

“…4.1B has also been implicated as a tumor suppressor protein in mammary carcinomas, which also frequently develop LOH at band 18p11.3 (Tran et al, 1999;Kittiniyom et al, 2004). Because loss of this region occurs with relatively high frequency in ductal carcinomas in situ (56%), it has been proposed to be an early event in tumor progression in the breast (Kittiniyom et al, 2001).…”