Correlation between LTR point mutations and proviral load levels among human T cell lymphotropic virus type 1 (HTLV-1) asymptomatic carriers

Abstract

Background: In vitro studies have demonstrated that deletions and point mutations introduced into each 21 bp imperfect repeat of Tax-responsive element (TRE) of the genuine human T-cell leukemia virus type I (HTLV-1) viral promoter abolishes Tax induction. Given these data, we hypothesized that similar mutations may affect the proliferation of HTLV-1-infected cells and alter the proviral load (PvL). To test this hypothesis, we conducted a cross-sectional genetic analysis to compare the near-complete LTR nucleotide sequences that cover the TRE1 region in a sample of HTLV-1 asymptomatic carriers with different PvL burden.

Methods: A total of 94 asymptomatic HTLV-1 carriers with both sequence from the 5' long terminal repeat (LTR) and a PvL for Tax DNA measured using a sensitive SYBR Green real-time PCR were studied. The 94 subjects were divided into three groups based on PvL measurement: 31 low, 29 intermediate, and 34 high. In addition, each group was compared based on sex, age, and viral genotypes. In another analysis, the median PvLs between individuals infected with mutant and wild-type viruses were compared.

Results: Using a categorical analysis, a G232A substitution, located in domain A of the TRE-1 motif, was detected in 38.7% (12/31), 27.5% (8/29), and 61.8% (21/34) of subjects with low, intermediate, or high PvLs, respectively. A significant difference in the detection of this mutation was found between subjects with a high or low PvL and between those with a high or intermediate PvL (both p < 0.05), but not between subjects with a low or intermediate PvL (p > 0.05). This result was confirmed by a non-parametric analysis that showed strong evidence for higher PvLs among HTLV-1 positive individuals with the G232A mutation than those without this mutation (p < 0.03). No significant difference was found between the groups in relation to age, sex or viral subtypes (p > 0. 05).

Conclusions: The data described here show that changes in domain A of the HTLV-1 TRE-1 motif resulting in the G232A mutation may increase HTLV-1 replication in a majority of infected subjects.

Figure 1

Alignment of HTLV-1 LTR sequence elements determined from DNA amplified from 94 individuals with different concentration of proviral loads: (31 low (< 50 copies/1000 PBMCs), 29 intermediate (50-100 copies/1000 PBMCs), and 34 high (> 100 copies/1000 PBMCs). Domains A, B and C are indicated by a gray background, and their position in the U3 region is presented relative to ATK prototype (accession number J02029). Dots indicate nucleotide identity to the ATK prototype.

Figure 2

Number of HTLV-1 subjects harboring wild-type or G232A and A184G mutant: viruses correlated with proviral loads. Black lines represent the median proviral load for each group of subjects shown.