Biomedical subjects

Yongzhao Shao

Publications and source records attributed to Yongzhao Shao.

6 recordsLinked to original sources

PubMed2006

Mixture cure model with an application to interval mapping of quantitative trait loci.

When censored time-to-event data are used to map quantitative trait loci (QTL), the existence of nonsusceptible subjects entails extra challenges. If the heterogeneous susceptibility is ignored or inappropriately handled, we may either fail to detect the responsible genetic factors or find spuriously significant locations. In this article, an interval mapping method based on parametric mixture cure models is proposed, which takes into consideration of nonsusceptible subjects. The proposed model can be used to detect the QTL that are responsible for differential susceptibility and/or time-to-event trait distribution. In particular, we propose a likelihood-based testing procedure with genome-wide significance levels calculated using a resampling method. The performance of the proposed method and the importance of considering the heterogeneous susceptibility are demonstrated by simulation studies and an application to survival data from an experiment on mice infected with Listeria monocytogenes.

Algorithms↗

PubMed2006

The combination of ionizing radiation and peripheral vaccination produces long-term survival of mice bearing established invasive GL261 gliomas.

PURPOSE: High-grade glioma treatment includes ionizing radiation therapy. The high invasiveness of glioma cells precludes their eradication and is responsible for the dismal prognosis. Recently, we reported the down-regulation of MHC class I (MHC-I) products in invading tumor cells in human and mouse GL261 gliomas. Here, we tested the hypothesis that whole-brain radiotherapy (WBRT) up-regulates MHC-I expression on GL261 tumors and enhances the effectiveness of immunotherapy. EXPERIMENTAL DESIGN: MHC-I molecule expression on GL261 cells was analyzed in vitro and in vivo by flow cytometry and immunohistochemistry, respectively. To test the response of established GL261 gliomas to treatment, mice with measurable (at CT imaging) brain tumors were randomly assigned to four groups receiving (a) no treatment, (b) WBRT in two fractions of 4 Gy, (c) vaccination with irradiated GL261 cells secreting granulocyte-macrophage colony-stimulating factor, or (d) WBRT and vaccination. Endpoints were tumor response and survival. RESULTS: An ionizing radiation dose of 4 Gy maximally up-regulated MHC-I molecules on GL261 cells in vitro. In vivo, WBRT induced the expression of the beta2-microglobulin light chain subunit of the MHC class I complex on glioma cells invading normal brain and increased CD4+ and CD8+ T cell infiltration. However, the survival advantage obtained with WBRT or vaccination alone was minimal. In contrast, WBRT in combination with vaccination increased long-term survival to 40% to 80%, compared with 0% to 10% in the other groups (P < 0.002). Surviving animals showed antitumor immunity by rejecting challenge tumors. CONCLUSION: Ionizing radiation can be successfully combined with peripheral vaccination for the treatment of established high-grade gliomas.

Animals↗

PubMed2006

Radiation sensitivity of GL261 murine glioma model and enhanced radiation response by flavopiridol.

Response of a solid tumor to radiation treatment depends, in part, on the intrinsic radiosensitivity of tumor cells, the proliferation rate of tumor cells between radiation treatments and the hypoxic state of the tumor cells. A successful radiosensitizing agent would target S-phase cells and hypoxia. Recently, we demonstrated the anti-tumor effects of flavopiridol in the GL261 murine glioma model might involve 1) recruitment of tumor cells to S-phase (Newcomb et al Cell Cycle 2004; 3:230-234) and 2) an anti-angiogenic effect on the tumor vasculature by downregulation of hypoxia-inducible factor -1alpha (HIF-1alpha) (Newcomb et al Neuro-Oncology 2005; 7:225-235). Given that flavopiridol has demonstrated radiosensitizing activity in several murine tumor models, we tested whether it would enhance the response of GL261 tumors to radiation. In the present study, we evaluated the intrinsic radiation sensitivity of the GL261 glioma model using the tumor control/cure dose of radiation assay (TCD(50)). We found that a single dose of 65 Gy (CI 57.1-73.1) was required to cure 50% of the tumors locally. Using the tumor growth delay assay, fractionated radiation (5 fractions of 5 Gy over 10 days) combined with flavopiridol (5 mg/kg) given three times weekly for 3 cycles produced a significant growth delay. Our results indicate that the GL261 murine glioma model mimics the radioresistance encountered in human gliomas, and thus should prove useful in identifying promising new investigational radiosensitizers for use in the treatment of glioma patients.

Animals↗

PubMed2006

Interval mapping of quantitative trait loci for time-to-event data with the proportional hazards mixture cure model.

Interval mapping using normal mixture models has been an important tool for analyzing quantitative traits in experimental organisms. When the primary phenotype is time-to-event, it is natural to use survival models such as Cox's proportional hazards model instead of normal mixtures to model the phenotype distribution. An extra challenge for modeling time-to-event data is that the underlying population may consist of susceptible and nonsusceptible subjects. In this article, we propose a semiparametric proportional hazards mixture cure model which allows missing covariates. We discuss applications to quantitative trait loci (QTL) mapping when the primary trait is time-to-event from a population of mixed susceptibility. This model can be used to characterize QTL effects on both susceptibility and time-to-event distribution, and to estimate QTL location. The model can naturally incorporate covariate effects of other risk factors. Maximum likelihood estimates for the parameters in the model as well as their corresponding variance estimates can be obtained numerically using an EM-type algorithm. The proposed methods are assessed by simulations under practical settings and illustrated using a real data set containing survival times of mice after infection with Listeria monocytogenes. An extension to multiple intervals is also discussed.

Algorithms↗

PubMed2005

Possible association of p53 codon 72 polymorphism with susceptibility to adult and pediatric high-grade astrocytomas.

Polymorphisms in codon 72 of the p53 tumor suppressor gene have been associated with susceptibility to human cancer. We wished to evaluate whether variant allelic forms of the p53 protein were associated with brain tumors. In this study, we scored 135 brain tumor samples (92 adult and 43 pediatric cases consisting of 64 high-grade astrocytomas and 71 non-astrocytomas) for the P53 Arg72Pro polymorphisms. Our data show that the genotype frequencies of P53 Arg72Pro vary not only between patients with brain tumors and controls, but also between different histological subtypes of brain tumors. Specifically, we found (i) that the genotype distributions of the P53 Arg72Pro between all brain tumors and controls were statistically significant (P < 0.001) as well as their variant allele frequencies between cases and controls (P < 0.001); (ii) that there was a significant increase in the Arg/Pro heterozygous genotype among high-grade astrocytomas compared with non-astrocytomas (P = 0.002); and (iii) that there was a significant increase in the Arg/Pro heterozygous genotype among high-grade astrocytomas containing transdominant as well as recessive p53 mutations compared with controls (P = 0.002). Our results suggest a possible association between P53 Arg72Pro polymorphisms and susceptibility to brain tumors, particularly high-grade astrocytomas.

Adolescent↗

PubMed2003

A marginal likelihood model for family-based data.

This paper presents a marginal likelihood model for family-based data based upon the transmission of marker alleles from each heterozygous parent to his/her affected children. The proposed model, extending the maximum-likelihood-binomial (MLB) method and the disequilibrium maximum-likelihood-binomial (DMLB) method (Abel et al. 1998; Abel & Müller-Myhsok, 1998; Huang & Jiang, 1999), is adaptive to linkage disequilibrium (LD) and linkage heterogeneity. Compared with other procedures, the likelihood ratio test (LRT) derived from the proposed model enjoys superior qualities. First, simulations indicate that the power of the LRT is greater than that of the TDT or DMLB in all of our studied scenarios. Second, when we applied the LRT and other tests to a Tourette Syndrome data, the result was data favorable to the use of the LRT. Therefore, we recommend the use of the LRT as an additional linkage test wherever applicable, especially when the amount of LD is uncertain.

Alleles↗