Preliminary Study Of The Relationship Between Axin Gene, A Tumor Suppressor Gene, And The Development Of Rectal Carcinoma And Astrocytomic Neopalsms

Background Axin, which was initially identified as a regulator of embryonic development in 1997, plays a critical role in controlling axis formation during embryonic development in which it is a negative regulator of Wnt signaling pathway. Later it was also considered as a tumor suppressor gene. Alterations of Wnt signaling pathway appear to contribute to pathogenesis of several human adenocarcinomas including cerebellar medulloblastomas, hepatocellular carcinomas, hepatoblastomas and primative neuroectodermal tumors. Mutations of Axin have been identified previously in cerebellar medulloblastomas, hepatocellular carcinomas, brest carcinoma and hepatoblastomas. In addition, Axin also exerts functions in JNK signal pathway and cell apoptosis.Rectal carcinoma and astrocytomic neopalsms are common cancers in human. Whether Axin functions as a tumor suppressor in these two tumors has not been reported yet. Since tumorgenesis is a very complicated multi-steped course and a lot of tumor suppressor gene alterations involved in it, deep study of the tumorsuppressor genes is of great significance to clarify the cause of rectal carcinoma and astrocytomic neopalsms and is helpful to the prevention and treatment of the diseases.Objectives: (1) To detect point mutations of the Axin gene in rectal carcinoma and astrocytomic neopalsms to determine the role of Axin gene in the development of the above tumors on the DNA level and provide valuble information for further study; (2) To detect the presence of Axin in astrocytomic neopalsms tissue and evaluate whether the presence of the Axin protein is related to astrocytomic neopalsms formation and whether gene point mutations have an influence on protein production; to detect the expression of p-catenin in astrocytomic neopalsms samples. P-catenin is one of the target proteins of Axin and is a key molecule in the Wnt signal pathway. From the results, determine in what way Axin functions as a tumor suppressor and whether P-catenin and the Wnt signal pathway are invoved in astrocytomic neopalsms formation.Methods: (1) To detect point mutations of Axin gene in 51 cases of rectal carcinoma and 28 cases of astrocytomic neopalsms, PCR-SSCP analysis, subcloning and sequencing analysis were used. Five blood samples from unaffected people were included as normal control; (2) Immunohistochemistry was used to exam the expression of Axin and P-catenin in astrocytomic neopalsms tissues and the results were statictcally analyzed.Results: Aberrantly migrating bands appeared in 26(26/51) different rectal carcinoma cases and 1 blood sample. After subcloning and sequencing analysis of PCR products of single strand DNA eluted from these bands, 18 out of the 26 aberrantbands contained sequence alterations in the intron 4, all of which were at the 17th nucleotide. The remaining 8 DNA samples contained nucleotide changes distributed among exons 1,2,4,6 and 10, with 3 having silent mutations and 4 missense mutations and 1 nonsense mutation. The missense mutations included K203M, N307K, H394N and E852G, and one nonsense mutation was Y305-stop. K203M means a lycine at residue 203 was changed into a methionine. The 5 (10%)tissue samples with missense mutations and nonsense mutation included 4 cases of well-differentiated rectal carcinoma and 1 case of mucinous adenocarcinoma; (2) Three missense mutations in exon 10 of Axin gene were detected in 6 out of 28 cases of astrocytomic neopalsms, including 3 cases of V836M, 2 cases of R841Q and 1 case of D862N. In addition, another three silent mutations were detected in other 3 cases; (3) In normal brain tissue Axin was expressed in cytoplasm of neurons but not in gliacytes, while in astrocytomic neopalsms Axin was present in tumor cells, at a positive rate of nearly 30%; (4) P-catenin expressed in the tumor cytoplasms of 2 caese of astrocytomic neopalsms, and one of which expressed both Axin and P-catenin.Conclusions: (1) From 51 rectal carcinoma samples, we detecdted 5 single alterations that might result in amino acid changes, with a mutation rate o...