Investigating HLXB9 as a biomarker in cancer

Abstract

A biomarker is a measurable biological characteristic that can be evaluated as an indicator of normal (physiological) or abnormal (pathogenic) processes. In cancer research, there remains a need for the identification of new biomarkers that can be used to close the gaps in the current understanding of cancer development, progression and treatment. HLXB9 is a homeobox gene located at 7q36. It encodes a transcription factor important in embryonic development. Its accurate regulation is significant in the organogenesis of the endodermal germ layer particularly in the development of the pancreas. After development, its expression is downregulated in the majority of adult tissues. Recently, aberrant expression of HLXB9 has been found in certain cancers such as hepatocellular carcinoma, testicular cancer, pancreatic cancer and leukaemia. The location of genes and chromosomes in the nuclei of healthy human cells has been shown to be non-random, therefore understanding the mechanisms that regulate nuclear genome organisation is important in understanding of cancer biology in cases where genes are relocated. The nuclear localisation of genes as a biomarker of tumour development in cancer is a relatively new but promising field in cancer research. Previous research by our group found overexpression of HLXB9 corresponded to an altered positioning of this gene in the nucleus of paediatric leukaemia patients harbouring the translocation t(7;12)(q36;p13). In this project, HLXB9 was evaluated as a biomarker in cancer development. In the first study, a new dual colour probe for the detection of the t(7;12)(q36; p13) was validated by fluorescence in situ hybridisation (FISH) in leukaemia patient samples previously described as harbouring the translocation. The expression of HLXB9 was then analysed by RT-PCR in 48 patients diagnosed with various haematological disorders. 25% of patients analysed expressed HLXB9. Additionally, HLXB9 expression in leukaemia was found in patients with normal copies of chromosome 7 suggesting HLXB9 expression can occur independently of chromosome 7 abnormalities. An attempt was made to evaluate the link between HLXB9 expression and its nuclear localisation in these patients. Four online databases were interrogated to identify cancer types and subtypes that exhibit differential HLXB9 expression. HLXB9 expression was not altered in the majority of cancer cases investigated. However, aberrant HLXB9 expression was found in cancer types not previously reported as showing differential HLXB9 expression such as kidney cancer, lung cancer and endometrial cancer. The identification of aberrant expression of HLXB9 in these cancer types provides a new avenue for research into understanding cancer development and progression in these tumour types. Finally, the expression of HLXB9 was analysed in four breast cancer cell lines by quantitative RT-PCR and immunofluorescence. Additionally, the prognostic significance of HLXB9 expression was evaluated in publicly available breast cancer survival databases (Kaplan Meier plotter and BreastMark). Altogether, the findings emerging from this thesis work show that, although the potential for HLXB9 to be used as biomarker is appealing, further work is required to confirm the value of this biological parameter in the diagnosis, prognosis and progression of cancer.