New classification of HLA-DRB1 alleles supports the shared epitope hypothesis of rheumatoid arthritis susceptibility

OBJECTIVE

The shared epitope hypothesis was formulated to explain the involvement of HLA-DRB1 in rheumatoid arthritis (RA). However, several studies, which considered only the HLA-DRB1 alleles shown to be associated with RA risk, rejected this hypothesis. In this report, we propose that a different classification of HLA-DRB1 alleles be considered, based on the amino acid sequence at position 70-74.

METHODS

The fit of both HLA-DRB1 classifications was tested in 2 groups of RA patients. All subjects were recruited through the European Consortium on Rheumatoid Arthritis Families, and included 100 patients with isolated RA and 132 patients with at least 1 affected sibling.

RESULTS

The new classification produced risk estimates that fit all of the observed data, i.e., the distribution of the HLA-DRB1 genotype in the 2 patient groups, and the distribution of parental alleles shared by affected sibpairs. The risk of developing RA under this new classification depends on whether the RAA sequence occupies position 72-74 but is modulated by the amino acid at position 71 (K confers the highest risk, R an intermediate risk, A and E a lower risk) and by the amino acid at position 70 (Q or R confers a higher risk than D).

CONCLUSION

A new classification based on amino acid sequence allows us to show that the shared epitope RAA sequence at position 72-74 explains the data, with the risk of developing RA modulated by the amino acids at positions 70 and 71.

Dense genome-wide linkage analysis of rheumatoid arthritis, including covariates

OBJECTIVE

Rheumatoid arthritis (RA) is a heterogeneous disease that exhibits a complex genetic component. Previous RA genome scans confirmed the involvement of the HLA region and generated data on suggestive signals at non-HLA regions, albeit with few overlaps in findings between studies. The present study was undertaken to detect potential RA gene regions and to estimate the number of true RA gene regions, taking into account the heterogeneity of RA, through performance of a dense genome scan.

METHODS

In a study of 88 French Caucasian families (105 RA sibpairs), 1,088 microsatellite markers were genotyped (3.3-cM genome scan), and a multipoint model-free linkage analysis was performed. The statistical assessment of the results relied on 10,000 computer simulations. A covariate-based multipoint model-free linkage analysis was performed on the locations of regions with suggestive evidence for linkage.

RESULTS

Involvement of the HLA region was strongly confirmed (P = 6 x 10(-5)), and 19 non-HLA regions showed suggestive evidence for linkage (P < 0.05); 9 of these overlapped with regions suggested in other published RA genome scans. A routine 12-cM genome scan with the same families would have detected only 7 of the 19 regions, including only 4 of the 9 overlapping regions. From the 10,000 computer simulations, we estimated that 8 +/- 4 regions (mean +/- SD) were true-positives. RA covariate-based analysis provided additional linkage evidence for 3 regions, with age at disease onset, erosions, and HLA-DRB1 shared epitope as covariates.

CONCLUSION

The results of this study provide evidence of 19 non-HLA RA gene regions, with an estimate of 8 +/- 4 as true-positives, and provide additional evidence for 3 regions from covariate-based analysis.

Validation of the reshaped shared epitope HLA-DRB1 classification in rheumatoid arthritis

Recently, we proposed a classification of HLA-DRB1 alleles that reshapes the shared epitope hypothesis in rheumatoid arthritis (RA); according to this model, RA is associated with the RAA shared epitope sequence (72–74 positions) and the association is modulated by the amino acids at positions 70 and 71, resulting in six genotypes with different RA risks. This was the first model to take into account the association between the HLA-DRB1 gene and RA, and linkage data for that gene. In the present study we tested this classification for validity in an independent sample. A new sample of the same size and population (100 RA French Caucasian families) was genotyped for the HLA-DRB1 gene. The alleles were grouped as proposed in the new classification: S₁ alleles for the sequences A-RAA or E-RAA; S₂ for Q or D-K-RAA; S_3D for D-R-RAA; S_3P for Q or R-R-RAA; and X alleles for no RAA sequence. Transmission of the alleles was investigated. Genotype odds ratio (OR) calculations were performed through conditional logistic regression, and we tested the homogeneity of these ORs with those of the 100 first trio families (one case and both parents) previously reported. As previously observed, the S₂ and S_3P alleles were significantly over-transmitted and the S₁, S_3D and X alleles were under-transmitted. The latter were grouped as L alleles, resulting in the same three-allele classification. The risk hierarchy of the six derived genotypes was the same: (by decreasing OR and with L/L being the reference genotype) S₂/S_3P, S₂/S₂, S_3P/S_3P, S₂/L and S_3P/L. The homogeneity test between the ORs of the initial and the replication samples revealed no significant differences. The new classification was therefore considered validated, and both samples were pooled to provide improved estimates of RA risk genotypes from the highest (S₂/S_3P [OR 22.2, 95% confidence interval 9.9–49.7]) to the lowest (S_3P/L [OR 4.4, 95% confidence interval 2.3–8.4]).

The ITGAV rs3738919-C allele is associated with rheumatoid arthritis in the European Caucasian population: a family-based study

The integrin αvβ3, whose αv subunit is encoded by the ITGAV gene, plays a key role in angiogenesis. Hyperangiogenesis is involved in rheumatoid arthritis (RA) and the ITGAV gene is located in 2q31, one of the suggested RA susceptibility loci. Our aim was to test the ITGAV gene for association and linkage to RA in a family-based study from the European Caucasian population.

Two single nucleotide polymorphisms were genotyped by PCR-restriction fragment length polymorphism in 100 French Caucasian RA trio families (one RA patient and both parents), 100 other French families and 265 European families available for replication. The genetic analyses for association and linkage were performed using the comparison of allelic frequencies (affected family-based controls), the transmission disequilibrium test, and the genotype relative risk.

We observed a significant RA association for the C allele of rs3738919 in the first sample (affected family-based controls, RA index cases 66.5% versus controls 56.7%; P = 0.04). The second sample showed the same trend, and the third sample again showed a significant RA association. When all sets were combined, the association was confirmed (affected family-based controls, RA index cases 64.6% versus controls 58.1%; P = 0.005). The rs3738919-C allele was also linked to RA (transmission disequilibrium test, 56.5% versus50% of transmission; P = 0.009) and the C-allele-containing genotype was more frequent in RA index cases than in controls (RA index cases 372 versus controls 339; P = 0.002, odds ratio = 1.94, 95% confidence interval = 1.3–2.9).

The rs3738919-C allele of the ITGAV gene is associated with RA in the European Caucasian population, suggesting ITGAV as a new minor RA susceptibility gene.

Comparative study of fluoride bioavailability following the administration of sodium fluoride alone and in combination with different calcium salts

To assess whether an interval of a few hours would be advisable between an intake of sodium fluoride (NaF) and that of calcium salts when treating osteoporotic patients with vertebral collapse, we carried out three pharmacokinetic studies in 12 healthy fasting volunteer subjects to compare the fluoride bioavailability provided by NaF alone and NaF combined with two calcium salts. The results were as follows: (1) When NaF is accompanied by calcium, the fluoride peak level is lower and delayed. (2) Fluoride absorption varied greatly among individuals in both experiments, but none of the 6 subjects proved to be nonabsorbers. (3) The areas under the curves obtained with each of the three preparations were not significantly different, but 24-h urinary fluoride was significantly lower in volunteers receiving simultaneously NaF and calcium salts than in volunteers receiving only NaF.

HSPD1 is not a major susceptibility gene for rheumatoid arthritis in the French Caucasian population

The heat shock 60-kDa protein 1 (HSP60) is involved in immune and inflammatory reactions, which are hallmarks of rheumatoid arthritis (RA). HSP60 is encoded by the HSPD1 gene located on 2q33, one of the suggested RA susceptibility loci in the French Caucasian population. Our aim was to test whether HSPD1 is a major susceptibility gene by studing families from the French Caucasian population. Three single nucleotide polymorphisms (SNPs) were studied in 100 RA trio families, and 100 other families were used for replication. Genetic analyses were performed by comparing allelic frequencies, by applying the transmission disequilibrium test, and by assessing the genotype relative risk. We observed a significant RA association for the C/C genotype of rs2340690 in the first sample. However, this association was not confirmed when the second sample was added. The two other SNPs and the haplotype analysis did not give any significant results. We conclude that HSPD1 is not a major RA susceptibility gene in the French Caucasian population.

[Acquired toxoplasmic chorioretinitis with a late onset]

Maternal toxoplasma infection was responsible for congenital toxoplasmosis with ocular and cerebral manifestations in the infant. The acquired character of the toxoplasmosis in the 5th month of pregnancy was confirmed by serologic data and isolation of the parasite from the placenta. Five years later, an evolutive chorioretinitis was found in the mother. The toxoplasma origin of the ocular injury was proven by the study of aqueous humor. This case report is compared with the rare similar cases in the literature. The late discovery of ocular impairment is discussed.

[Evaluation of reticulocyte count on Cell-Dyn 3500 automate: comparison with a flow cytometry technique]

Cell-Dyn 3500 automate (Abbott) provides reticulocyte count using a recent software, after ribosomal RNA colouring with methylene blue reagent. The aim of this study was to evaluate results from Cell-Dyn 3500, and to compare them with the routine laboratory technique by flow cytometry (FacScan, Becton-Dickinson). Cell-Dyn 3500 analytical parameters (inter- and intra-assay precision, sample-to-sample contamination) were good and in accord to manufacturer's specifications. Reticulocyte values from blood samples kept at 4 degrees C during 3 days were unchanged. Usual values in 145 normal patients were found between 71.3 and 78.3 G/l with Cell-Dyn 3500, between 119.3 and 130.2 G/l with FacScan. Both techniques were compared in 87 patients with various anemias: Cell-Dyn 3500 showed more frequently high reticulocyte values in regenerative anemias (hemorrhagic), but was less performant for classification of aregenerative anemias. Reticulocyte count with Cell-Dyn 3500 is a semi automatic, easy to perform and swift technique, using no toxic reagent but unitary test tubes kept at room temperature. The test cost is however high, and Cell-Dyn 3500 cannot be used during the analysis.