Screening for Retrovirus Genomes in Childhood Acute Lymphoblastic Leukemia

Acute lymphoblastic leukemia (ALL) is the most common type of childhood malignancy worldwide and Mexico has one of the highest reported incidence rates at 49.5 cases per million. Infections have been strongly suggested to be a causative factor for ALL; however, the identity of the agent involved is presently unknown. In many animal species, members of the Retroviridae family are responsible for leukemias. The murine mammary tumor virus (MMTV) is associated with leukemia and breast cancer in mice and has been suggested to be associated with human breast cancer. The T-cell lymphotropic virus 1 (HTLV1) is the causative factor of adult T cell leukemia. In this study, we assessed whether MMTV and HTLV1/2 are also involved in childhood ALL.

95 children from four Mexican states and Mexico City with untreated B cell ALL, aged 8 months to 16 years were included in the study. Bone marrow samples were screened using conventional PCR assays. Because the mutation rate is considerably high in retroviruses, false negatives due to inadequate primer recognition are likely. To avoid that, two sets of primers targeting different regions of the retroviral genomes were used and the PCR annealing temperatures were set at ≤ 55 °C. Also, the primers used in these assays had low similarity with human endogenous retroviral sequences to exclude false positives. The sensitivity of the MMTV PCR reactions was determined with plasmid DNA containing a region of the MMTV env gene and genomic DNA from CD1 mice spleens and for HTLV1/2 with DNA from the MJ cell line. Because ALL is defined by a frequency of at least 25% of leukemic blasts, the PCR sensitivities were set to detect in samples at least this frequency of infected cells. A nested PCR was also designed to confirm negative cases.

None of the samples were positive to any of the retroviruses. The study's statistical power to detect one or more MMTV or HTLV1/2 positive samples from our study population (N=95) for 20%, 15% or 10% hypothesized proportions of cases with genomic integration was quite high.

Our study does not support the involvement of MMTV or HTLV1/2 in the etiology of childhood acute lymphoblastic leukemia in samples from Mexico.

This work was partially funded by the Mexican Institute of Social Security through its program “Apoyo Financiero para el Desarrollo de Protocolos de Investigación en Salud en el IMSS” and by the Graduate Program of Doctor Degree in Biomedical Sciences, Medicine Faculty, National Autonomous University of Mexico, Mexico City, Mexico.

No relevant conflicts of interest to declare.