Pauci- and Multibacillary Leprosy: Two Distinct, Genetically Neglected Diseases

<div><p>After sustained exposure to <i>Mycobacterium leprae</i>, only a subset of exposed individuals develops clinical leprosy. Moreover, leprosy patients show a wide spectrum of clinical manifestations that extend from the paucibacillary (PB) to the multibacillary (MB) form of the disease. This “polarization” of leprosy has long been a major focus of investigation for immunologists because of the different immune response in these two forms. But while leprosy per se has been shown to be under tight human genetic control, few epidemiological or genetic studies have focused on leprosy subtypes.</p><p>Using PubMed, we collected available data in English on the epidemiology of leprosy polarization and the possible role of human genetics in its pathophysiology until September 2015. At the genetic level, we assembled a list of 28 genes from the literature that are associated with leprosy subtypes or implicated in the polarization process. Our bibliographical search revealed that improved study designs are needed to identify genes associated with leprosy polarization. Future investigations should not be restricted to a subanalysis of leprosy per se studies but should instead contrast MB to PB individuals. We show the latter approach to be the most powerful design for the identification of genetic polarization determinants. Finally, we bring to light the important resource represented by the nine-banded armadillo model, a unique animal model for leprosy.</p></div

Family based sample and study design.

<p>Two sets of families were employed: those with T1R-affected offspring and those with leprosy but T1R-free offspring. The T1R-affected subset comprised 229 offspring belonging to 221 families while the T1R-free subset comprised 229 offspring in 209 families. Offspring were matched by clinical leprosy subtype in the two family sets. In a first analysis stage, the transmission disequilibrium test (TDT) was used to estimate significance of association of <i>LRRK2</i> variants with disease in each subset. In a second stage, a formal heterogeneity test was performed to identify <i>LRRK2</i> variants preferentially associated with T1R.</p

LRRK2 variants preferentially associated with T1R.

<p>LRRK2 variants preferentially associated with T1R.</p

Host versus pathogen control of <i>LRRK2</i> expression levels.

<p><i>LRRK2</i> expression levels for 53 unrelated subjects are indicated on the y-axis and stratified according to rs2404580 genotypes on the x-axis. The left panel represents baseline expression while the right panel indicates gene expression levels following stimulation with <i>M</i>. <i>leprae</i> antigen.</p

Studies identifying 28 genes associated with clinical forms of leprosy or its polarization.

<p>Studies identifying 28 genes associated with clinical forms of leprosy or its polarization.</p

Multivariate analysis of <i>LRRK2</i> variants with T1R

<p>Multivariate analysis of <i>LRRK2</i> variants with T1R</p

Proposed mechanism for LRRK2 in T1R.

<p>The LRRK2 M2397T amino acid substitution affects protein turnover. The methionine variant of LRRK2 displays a half-life of approximately 8 hours while the half-life of the threonine variant is 18 hours [<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0004412#pntd.0004412.ref034" target="_blank">34</a>]. LRRK2 arrests the NFAT transcription factor in the cytoplasm through a complex mechanism mediated by Ca²⁺ [<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0004412#pntd.0004412.ref036" target="_blank">36</a>]. This prevents NFAT to migrate to the nucleus and trigger the expression of pro-inflammatory cytokines [<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0004412#pntd.0004412.ref035" target="_blank">35</a>]. The M2397 allele is in tight linkage disequilibrium with alleles of SNPs that promote an increase in <i>LRRK2</i> expression creating a compensatory mechanism to counterbalance the shorter LRRK2-M2397 half-life. This compensatory mechanism is abrogated in the presence of <i>M</i>. <i>leprae</i> antigen. Hence, the effect of the M2397T amino acid substitution is most pronounced in the presence of <i>M</i>. <i>leprae</i> antigen.</p

Haplotype analysis of independent signal of association with T1R.

<p>Haplotype analysis of independent signal of association with T1R.</p

Statistical impact of the hypothesis used to test for an association between a genetic variant and leprosy polarization.

<p>In the common case-control study design, a standard test of association between a genetic variant and the phenotype under study is to compare allelic frequencies (f) between the group of cases and the group of controls. Here, we consider a sample including an even number of controls and leprosy cases, equally distributed between PB and MB. In addition, we fixed the frequency of the allele of interest to 0.5 among the controls (f_C = 0.5). In each panel, f_PB and f_MB (x and y axis) are the allele frequencies among PB and MB individuals, respectively. The <b>first strategy</b> directly compares MB to PB cases (Panel A). The <b>second strategy</b> first tests PB versus controls (Panel B) and MB versus controls (Panel C) before performing a heterogeneity test MB versus PB, possibly after testing leprosy per se versus controls (panel D). The red SNP is a variant for which f_MB ≠ f_PB and for which the association is significant with the first strategy but not with the second (the red SNP is in the gray area where the null hypothesis H₀ cannot be rejected). This second strategy is indeed hampered by correcting for multiple testing (testing MB versus PB after testing PB versus controls and MB versus controls) as well as the usual low power of heterogeneity tests.</p

Concordance of leprosy polarization phenotypes according to WHO-82, WHO-96, or physician definitions in a Vietnamese sample.

<p>Venn diagram for the number of patients with PB leprosy (left) and MB leprosy (right) according to three definitions: WHO-82, WHO-96, and physician in a Vietnamese sample. The surfaces of overlap are approximately proportional to the number of individuals identically classified by one, two, or three definitions. There was no individual classified PB by a physician and MB by WHO-82 + WHO-96 as well as no individual classified MB by a physician and PB by WHO-82 + WHO-96.</p