Histological profile of tumours from MYCN transgenic mice H C Moore, 1 K M Wood, 2 M S Jackson, 3 M A Lastowska, 3 D Hall, 3 H Imrie, 1 C P F Redfern, 1 P E Lovat, 1 F Ponthan, 1 K O’Toole, 1 J Lunec, 1 D A Tweddle 1 1 Northern Institute for Cancer Research, Newcastle University, Newcastle upon Tyne, UK; 2 Department of Cellular Pathology, Royal Victoria Infirmary, Newcastle upon Tyne, UK; 3 Institute of Human Genetics, Newcastle University, Newcastle upon Tyne, UK Correspondence to: Dr D A Tweddle, Northern Institute for Cancer Research, Paul O’Gorman Building, Newcastle University, Framlington Place, Newcastle upon Tyne NE2 4HH, UK; D.A. Tweddle@newcastle.ac.uk Accepted 18 July 2008 Published Online First 4 August 2008 ABSTRACT Background: MYCN is the most commonly amplified gene in human neuroblastomas. This proto-oncogene has been overexpressed in a mouse model of the disease in order to explore the role of MYCN in this tumour. Aims: To report the histopathological features of neuroblastomas from MYCN transgenic mice. Methods: 27 neuroblastomas from hemizygous trans- genic mice and four tumours from homozygous mice were examined histologically; Ki67 and MYCN immunocyto- chemistry was performed in 24 tumours. Results: Tumours obtained from MYCN transgenic mice resembled human neuroblastomas, displaying many of the features associated with stroma-poor neuroblastoma, including heterogeneity of differentiation (but no overt ganglionic differentiation was seen), low levels of Schwannian stroma and a high mitosis karyorrhexis index. The tumours had a median Ki67 labelling index of 70%; all tumours expressed MYCN with a median labelling index of 68%. The most striking difference between the murine and human neuroblastomas was the presence of tingible body macrophages in the transgenic mouse tumours reflecting high levels of apoptosis. This has not previously been described in human or other murine neuroblastoma models. Conclusions: These studies highlight the histological similarities between tumours from MYCN transgenic mice and human neuroblastomas, and reaffirm their role as a valuable model to study the biology of aggressive human neuroblastoma. A neuroblastic tumour, the most common extra- cranial solid tumour of childhood, 1 is an embryonal malignancy of neural crest origin with the primary tumour arising at a site within the sympathetic nervous system. 1 Heterogeneity is the hallmark of human neuroblastomas, with spontaneous regres- sion and maturation being commonly described. 2 The different biological and morphological char- acteristics of the tumours can be used to predict their clinical behaviour; histologically they can be placed into one of three categories: neuroblastoma, ganglioneuroblastoma (nodular or intermixed) or ganglioneuroma. 2 Amplification of the proto-oncogene MYCN,a member of the MYC family of oncogenes, is a well characterised genetic abnormality of some neuro- blastomas. 1 Amplification is found in approximately 25% of tumours and is associated with advanced stages of the disease and a poor patient prognosis. 3–8 Until recently, the most widely used animal models to study neuroblastoma have been ortho- topic or heterotopic rodent xenograft models, and they have been generally used to study drug efficacy. To test the hypothesis that amplification of MYCN contributes to tumourigenesis, and to explore the role of MYCN in neuroblastoma, a transgenic mouse model was created in which the MYCN oncogene was targeted to neuroectodermal cells of developing mice under the influence of a rat tyrosine hydroxylase promoter creating mice that overexpress MYCN in neural crest derived cells. 9 10 Mice develop neuroblastoma tumours resembling those seen in humans, and comparative genomic hybridisation (CGH) has identified consistent regions of chromosomal gain and loss syntenic to those that are found in human neuroblastoma, indicating that this murine model is representative of the human disease. 9 Furthermore, tumours from hemizygous mice have also been reported to undergo amplification of the MYCN transgene. 11 These transgenic mice tumours will hopefully be useful in the identification of additional genes involved in the tumourigenic process, and in the development of therapies to treat patients suffer- ing from neuroblastoma. The aim of this study was to report the histopathological features in 27 neuroblastomas from mice hemizygous for the MYCN transgene and four from mice homozygous for the transgene, to determine how closely they resemble the pathology of human neuroblastoma. METHODS MYCN transgenic mouse tumours Twenty-seven formalin-fixed, paraffin-embedded neuroblastomas from mice hemizygous for the MYCN transgene from strain 129X1/SvJ were studied. Ethical approval for this study was obtained from the local research ethics committee. Mice were examined every other day after weaning for palpable tumours or evidence of hindleg paralysis, and when present were sacrificed and the tumour removed for study. For comparison, four tumours from mice homozygous for the MYCN transgene were also studied (129X1/SvJ strain and CBA strain). Table 1 shows character- istics of the tumours in terms of latency and site of primary tumour. H&E staining of at least one section from each tumour was examined by light microscopy and the histopathological features in terms of differentiation, presence of Schwannian stroma, estimated mitosis karyorrhexis index (MKI) and presence of calcification were recorded using the International Neuroblastoma Pathology Classification (INPC). 2 The MKI is defined as the percentage of cells in mitosis and karyorrhexis (apoptosis)/5000 neuroblastic cells scored. It is classified as low if ,2%, intermediate if 2–4% and high if .4%. 2 Original article 1098 J Clin Pathol 2008;61:1098–1103. doi:10.1136/jcp.2007.054627