Chromosomal loci influencing the susceptibility to the parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Abstract

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the dysfunction of the nigrostriatal dopaminergic pathway. Although its etiology is not yet fully understood, an interaction of genetic predisposition and environmental factors is frequently discussed. The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) can evoke PD-like symptoms and neuropathological changes in various species, including mice. It was found repeatedly that mouse strains differ in their susceptibility to MPTP, which might serve as a model for genetic predisposition to neurodegeneration of the nigrostriatal system. In the present study, F2 intercross mice, derived from parental strains with high (C57BL/6J) versus low (BALB/cJ) MPTP susceptibility, were treated with MPTP and phenotyped for dopamine (DA) loss in the neostriatum, a highly sensitive marker of nigrostriatal dysfunction. A subsequent quantitative trait loci analysis revealed a gender-dependent locus for DA loss on chromosome 15 and a putative locus on chromosome 13. A number of potential candidate genes, including the membrane dopamine transporter, are located in the respective areas. Several mechanisms that are possibly involved in the control of the action of MPTP on the nigrostriatal system are discussed.

Figure 1.

Distribution pattern of the phenotype “neostriatal dopamine depletion after MPTP administration.” (C57BL/6 × BALB/c)F₂ hybrids were treated with four doses of 15 mg MPTP-HCl/g body weight in 2 hr intervals, and neostriatal DA levels were determined postmortem by HPLC, with electrochemical detection 7 d later. Absolute numbers of animals are shown in 10% intervals of DA depletion separately for males (n = 179) and females (n = 132). “Depletion” is defined as dopamine loss in percentage compared with controls. DA depletion was significantly higher in males than in females (t test; t = 16.1; df = 309; p < 0.001). Saline-treated controls, which are C57BL/6 × BALB/c)F₂ hybrids, had neostriatal dopamine levels of 15.23 ± 0.41 and 15.28 ± 0.40 for males (n = 23) and females (n = 24), respectively (nanograms of DA per milligram of wet tissue weight; means ± SEM).

Figure 2.

Simulation: neostriatal DA depletion outcome if males severely affected by MPTP had died (in percentage loss of control values; means ± SEM). The black column indicates the actually observed DA depletion after MPTP treatment in the male (C57BL/6 × BALB/c)F₂ hybrid mice; the white column indicates the respective mean for the females. A total of 192 females was treated with MPTP, but only 132 of them survived treatment. The DA depletion of the deceased females (n = 61 or 31.6%) could not be determined. None of the males had died. The gray column reflects the simulated outcome if the 31.6% most affected males of our sample would have died as a consequence of MPTP treatment. Note that even after the simulated death of 57 male mice the gender difference is ∼30% points.

Figure 3.

The negative logarithm of the p value (LOD score) plotted against map location (cM) along the length of proximal to mid chromosome 15 (Chrom. 15) for both females and males. Numbers along the bottom of the plot are marker numbers, e.g., -226 denotes the map location of D15Mit226. Data are from step 1 involving the extreme scoring tails of the trait distribution because more markers were used than in step 2. A highly significant gender difference is apparent from the 3.7 orders of magnitude difference in p values observed between the two genders.

Figure 4.

The degree of dopamine depletion by genotype for the D9Mit4 marker on Chr. 9 as a function of genotype at another locus (D3Mit147) on Chr. 3. An interaction is demonstrated by the fact that the phenotypic effect of the Chr. 9 QTL reverses direction as a function of the Chr. 3 modifier locus genotype at D3Mit147. Neither locus had a significant main effect in the Map-Manager analysis, which does not assess interactions, but emerged as significant only in the PAIRSCAN analysis mainly because of the strength of the interaction shown above (p = 0.0008).

Figure 5.

Two-dimensional plot of PAIRSCAN results for the nine chromosomes of greatest interest using a 2.5 cM grid. These chromosomes were chosen for display either because they contained a significant or suggestive QTL or because they were involved in an interaction attaining p < 0.01. The bottom half shows the LOD scores for the full model (df = 8) for marker pairs, which includes the main effects of each marker plus their interaction, whereas the top half shows LOD scores for the interaction alone (df = 4). Areas in white failed to reach p < 0.05 (LOD 3.4, bottom plot; LOD 2.1, top plot), whereas those exceeding this relaxed criterion are shaded from gray to black as a function of the LOD score scale shown at right. For visual clarity, the LOD scores in the top half were increased by 1.64-fold. Although all interactions attaining p < 0.05 are shown, only one attained significance (Chr. 3 × 9) and another closely approached significance (Chr. 5 × 13). Both are denoted by arrows. The 5% genome-wide significance thresholds estimated by permutation tests were LOD 7.4 (p = 4 × 10^-5) in the bottom plot and 4.6 (p = 3 × 10^-4) in the top one.