Introduction
Pediatric acute lymphoblastic leukemia and lymphoma (ALL) is the most
common childhood malignancy, accounting for approximately 25% of all
pediatric cancer diagnosis. Chemotherapy inhibiting folate metabolism,
particularly methotrexate (MTX), has remained a core component of ALL
therapy since the first published remissions as a monotherapy in 1948.
Currently within Children’s Oncology Group (COG)-based protocols, MTX is
administered in by multiple modes of delivery including intravenous,
intrathecal, and oral administration during all phases of ALL treatment.
One of the major contributions of MTX is to combat leukemia in sanctuary
sites, namely the CNS and testicles, thereby preventing local relapse.
The antileukemic activity of MTX is attributed to its ability to disrupt
folate metabolism. MTX enters the cell through the reduced folate
carrier and then polyglutamated which helps retain the drug
intracellularly. It is also believed that the polyglutamated form is
more bioactive, inhibiting MTX’s main targets, Dihydrofolate Reductase
and Thymidylate Synthase, which are important for DNA synthesis. Through
the disruption of folate metabolism, there is also reduction in the
conversion of homocysteine to methionine resulting in decreased protein
synthesis and methylation. Methylenetetrahydrofolate reductase (MTHFR)
is an enzyme that also exists in this pathway and serves as the
irreversible, rate limiting step of the conversion of
5,10-methylentetrahydrofolate (5,10-CH2-THF) to
5-methyltetrahydrofolate (5-CH-THF). Two important single nucleotide
polymorphisms (SNPs), C677T and A1298C, are frequently identified in the
general population and thought to be clinically significant based upon
relative enzymatic function. The C677T SNP is thought to reduce MTHFR
activity to 60-70% and 30% in heterozygous and homozygous individuals
respectively. This SNP has also been associated with elevated
homocysteine, which may lead to a prothrombotic state. The A1298C SNP is
more active with 60 and 80% in its homozygous and heterozygous form,
respectively. However, for those with one copy of the A1298C SNP,
activity is estimated to be slightly decreased at 50-60% when
coexisting with a single C677T SNP, otherwise known as compound
heterozygosity.
Patients treated with MTX can experience a variety of multisystem
toxicities including, but not limited to, myelosuppression,
transaminitis, subacute leukoencephalopathy, mucositis, and renal
toxicity. The influence of MTHFR SNPs on toxicity has been examined
previously and results have remained controversial. It should be noted
that most studies to date have either focused on high dose MTX (HDMTX)
with typical doses of 5 g/m2/dose given intravenously,
examined ethnically homogenous populations, or have not differentiated
between differing modalities of MTX treatment. In a retrospective
analysis of CCG-1891 after analyzing for common polymorphic genes
involved in drug metabolism, the MTHFR C677T genotype was determined to
have no significant effect on cumulative oral MTX received during
maintenance, although a 25-30% reduction of cumulative dose was
observed across the population. This method of analysis, however, can be
misleading as it fails to account for several confounders, including
medication holds, average dose received, and compliance.
The aim of our study was to directly examine the influence of MTHFR SNPs
on methotrexate tolerance at various phases of treatment, with
particular attention given to oral methotrexate which is given during
the maintenance phase of treatment, the last and longest phase of
treatment on COG-based ALL treatment protocols. Dosing of MTX in
maintenance along with oral 6-mercaptopurine (6-MP), is regulated by
both degree of myelosuppression (goal absolute neutrophil count
500-1500/ul and Platelet count > 50,000 cell/ul) and to a
lesser extent liver toxicity. There is extreme variability between
patients likely from a multitude of factors including individual
pharmacogenomics. As part of the folate metabolic pathway, MTHFR has the
potential to influence MTX efficacy, as well as toxicity, and those with
certain MTHFR genotypes may be at increased risk for toxicity.
Our overarching goal was to utilize our local ALL patient population to
identify clinically significant genotypes that would identify
individuals who need reduced MTX dose adjustment upfront and therefore
provide guidance to providers taking care of these patients.
Furthermore, we also examined the influence of MTHFR genotypes on other
phases of treatment as well as risk for developing additional toxicities
such as subacute leukoencephalopathy and thrombosis.