LOSS OF EXPRESSION OF A 55 KDA NUCLEAR-PROTEIN (NMT55) IN ESTROGEN RECEPTOR-NEGATIVE HUMAN BREAST-CANCER

We have identified and characterized a 55 kDa nuclear protein (referre d to as nmt55) from human breast tumors and MCF-7, human adenocarcinom a breast cell line, using site-directed monoclonal antibodies. Measure ments of estrogen receptors (ER) and progesterone receptors (PR), by l igand binding assays, in cytosols of 63 human breast tumors permitted classifications of these tumors into four phenotypes (ER+/PR+, ER+/PR- , ER-/PR-, ER-/PR+). Nuclear protein (nmt55) expression in these tumor s, as determined from Western blot analyses, showed a statistically si gnificant association (p = 0.001) with tumor hormonal phenotype. Revie w of the pathologic characteristics of tumors analyzed suggested that lack of nmt55 expression was significantly associated with mean tumor size (p < 0.03), mean ER (p = 0.001) and mean PR (p < 0.002), but was not associated with tumor stage, grade, or type. To further study this protein, we cloned and sequenced a 2.5 kb cDNA using a monoclonal ant ibody to nmt55. The complete predicted open reading frame encodes a pr otein with 471 amino acids and a calculated molecular mass of 54,169 D a. The deduced amino acid sequence exhibited unique regions rich in gl utamine, histidine, arginine, and glutamic acid. Northern blot analysi s of RNA from MCF-7 cells and ER+/PR+ human breast tumors showed a 2.6 kb mRNA. Southern blot analysis suggested the presence of a single co py of this gene. Chromosomal mapping, using fluorescent in situ hybrid ization (FISH), located nmt55 gene to the X chromosome, region q13. Th e extensive homology between nmt55 and RNA binding proteins suggested that nmt55 may be involved in hnRNA splicing. The strong association o bserved between expression of nmt55, tumor hormonal phenotype, mean tu mor size, mean ER, and mean PR content suggests that loss of nmt55 exp ression may be related to events involved in hormone insensitivity, tu mor differentiation, and unregulated tumor cell growth and metastases.