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Consistent cytogenetic aberrations and the consequent molecular alterations have considerably facilitated the classification of non-Hodgkin?s lymphoma (NHL). A cytogenetic abnormality involving chromosome region 1p36 is frequently detected in NHL cases and is associated with a poor prognosis and high risk of transformation from an indolent to a more aggressive NHL subtype. However, the molecular events mediating the biological consequences of this cytogenetic abnormality are poorly understood. TP73 is one of the most distally located candidate genes at the 1p36 locus, and is likely to be disrupted or deleted. Furthermore, we have observed frequent loss of heterozygosity in the P73 locus using fluorescence in situ hybridization (FISH) in NHL cases with the 1p36 abnormality. TP73 gene function is controlled by harmony between two isoforms with opposing functions, the full length TAp73 protein (pro-apoptotic) and the NH2-terminally truncated ΔNp73 protein (anti-apoptotic through antagonizing both TAp73 and p53). To determine whether 1p36 abnormalities alter p73 expression, we measured the mRNA expression of TAp73 and ΔNp73 by real-time PCR in diagnostic specimens of cytogenetically characterized NHL cases with and without 1p36 disruption. We observed a down-regulation of TAp73 in NHL cases with and without 1p36 disruption. However, there was a significant increase in the ΔNp73 expression (p<0.01) resulting in a significantly up-regulated (p<0.01) ΔNp73:TAp73 ratio among NHL cases harboring 1p36 abnormalities compared to NHL cases without 1p36 abnormalities. To evaluate the functional significance of this imbalance, we performed immunohistochemical analysis on diagnostic biopsy specimens from NHL cases with and without 1p36 abnormalities using TAp73, ΔNp73, cleaved caspase 3 (an apoptosis marker), and PCNA (a proliferation marker) antibodies on a tissue microarray. We observed a statistically-significant negative correlation between the ΔNp73:TAp73 ratio and the apoptosis marker cleaved caspase 3 (r=-0.614 ; p=0.015); and a significant positive correlation between ΔNp73 and the proliferation marker PCNA (r=0.532; p=0.041), indicating increased proliferation and decreased apoptosis with up-regulated ΔNp73. Additionally, we observed a significant positive correlation between TAp73 and cleaved caspase 3 (r=0.72; p=0.001), thus associating the TAp73 isoform with apoptosis. Together, our data demonstrates that 1p36 disruption leads to differential regulation of p73 gene expression, resulting in an imbalance in TP73 isoform expression (gain of ΔNp73 and loss of TAp73), with diminished apoptosis and increased proliferation thus explaining the prognostic effect of 1p36 disruption. These findings provide new insight into the differential regulation of p73 gene expression that is mediated by this cytogenetic abnormality.
