Relationship between Ethnicity, Methotrexate Dehydrofolate Reductase Polymorphisms, and Clinical Outcomes in Pediatric Leukemia Patients

Author: Angela M. Garcia

Primary Advisor: Hualou Liang, PhD

Committee Members:

Masters thesis, The University of Texas School of Health Information Sciences at Houston.

 
 

Acute Lymphoblastic Leukemia (ALL) is one of the first cancers reported to respond to chemotherapy. It primarily affects children and accounts for 75% of childhood leukemia and 25% of all childhood cancers. ALL is a lymphocytic leukemia characterized by a change in the white blood cells found in bone marrow cells. There are two types of white blood cells: B lymphocytes and T lymphocytes. The B lymphocytes regulate the production of antibodies. The T lymphocytes on the other hand are involved in cell mediated immunity and the stimulation of the B lymphocytes. The peak incidence of ALL is among children that are between two and five years of age with a predominance found in boys. This heterogeneous disease is diagnosed in 2500-3500 children per year in the United States with diagnosis in three to four white children per 100,000 cases. Studies have suggested that Hispanics and African American children with this disease have poorer outcomes than Caucasian children matched for severity of disease.( S Bahtia paper)

Most ALL patients exhibit diploidy or hyperdiploidy. Diploidy is when the chromosome number is doubled (92) while in hyperdipoloidy the chromosome number is more than doubled. The best prognosis has been found in children with a higher ploidy, specifically over 50 chromosomes. Other chromosomal findings (including specific translocations) are associated with specific high or low risk incidence of relapse.  The patients are divided into risk groups using a combination of statistically validated presentation features (i.e. age, WBC count, cytogenetics and others).  Clinical decisions regarding the intensity of therapy are guided by the risk grouping of each patient on protocols which are stratified into low, standard, and high-risk of relapse criteria.   The therapy for all levels of risk involves the heavy use of  methotrexate throughout the 2.5-3 years of treatment.

The alleles found on chromosomes are alternative forms of genes. Two polymorphisms that have been identified in the methotrexate dehydrofolate reductase (MTHFR) gene located on chromosome 1 at 1p36.3 are C677T and A1298C.  The former is a nucleotide change from a C to a T and results in a change from alanine to valine. The latter is a change from an A to C and results in a glutamine to alanine change. These polymorphisms have various effects, specifically on the folate metabolism.

The key reaction in the folate metabolism is the methylation of deoxyuridine monophosphate to generate thymidylate, which is needed for DNA synthesis. Disruption of this reaction leads to deficiency in folate levels. The folate metabolism involves several enzymes. Among them is the 5,10-Methylenetetrahydrofolate reductase (MTHFR) enzyme, which serves as the link between the folate and homocystein metabolisms. It converts 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate.

This regulating enzyme aids in the remethylation of methionine into homocystein. Disruption of the enzyme can lead to hyperhomocysteinemia, which has been linked with the C677T polymorphism.

Hypercysteinemia is caused by inhibiting the conversion of homocysteine to methionine. This reaction indirectly requires the formation of 5-methyltetrahydrofolate, which can be hindered by ALL treatments. Methotrexate (MTX) is one such chemotherapy. The inhibition of MTX can cause disruption in DNA synthesis and cellular replication. This folate antagonist temporarily reduces the production of blood cells by the bone marrow.  Leukemic lymphoblasts have been shown to have increased sensitivity (higher cellular death rates) when exposed to MTX compared with normal bone marrow cells. MTHFR polymorphisms exist in most populations tested.  The incidence of specific MTHFR mutations in Hispanic populations (both with and without leukemia) has not been thoroughly tested.  The objective of this pilot project is to perform MTHFR  mutation analysis for the two most common of the known MTHFR mutations and to attempt to correlate mutation status with various clinical parameters including presentation features, toxicity parameters, and overall clinical outcome.           

Genomic PCR was performed on Hispanic patient samples in order to ascertain the MTHFR isozyme frequency in this Hispanic population. Two MTHFR mutations were chosen for analysis.  The C677T mutation has been shown by multiple groups to result in loss of enzymatic activity (__% for homozygous, __% for heterozygous compared with the Wild Type configuration.  The A1298C mutation is not known to cause any change in enzymatic activity, but may be associated with specific ethnicities.  Isozyme genotypes, C677T and A1298C, where analyzed to find their correlation with Hispanic ALL patients treated with MTX