Analysis of HLA-DQA and HLA-DQB Frequencies in a Group of Sardinian Centenarians

Letizia Scola, Domenico Lio, Antonio Crivello, Giuseppina Candore, Giusi Irma Forte, Giuseppina Colonna-Romano, Mario Giovanni Pes, Ciriaco Carru, Luigi Ferrucci, Luca Deiana, Giovannella Baggio, Claudio Franceschi, and Calogero Caruso.

Human leukocyte antigen (HLA) alleles regulate the type and intensity of the immune response and might influence life expectancy. In previous case-control studies, the authors demonstrated that both HLA-DR and -DQ alleles are not associated with longevity in the Sardinian population. On the other hand, association studies are subjected to a number of possible confounding factors. Therefore, the authors typed the HLA-DQA1 and HLA-DQB1 alleles in 24 siblings (age range 85 to 97) of 17 centenarians by PCR-SSP. Sib-pair analysis showed non-significant differences between the observed and expected percentage of DQA* or DQB1* allele sharing. Therefore, these data strengthen the view that class II HLA genes have a marginal effect if any on the complex longevity trait.

The influence of genetic factors on variation in life expectancy might be estimated to be approximately 25%, and a number of studies have suggested that genetic determinants of longevity might at least partially reside in the polymorphisms for genes that regulate immune responses as human leukocyte antigen (HLA) genes. Some well-planned and -designed association studies performed in whites suggest that longevity is associated with the HLA-DR11 allele or HLA-B8, DR3 haplotype, although other studies do not confirm these observations.

More recently, the authors evaluated HLA-DRB1 frequency in Sardinian centenarians and controls and showed that HLA-DRB1*15 was increased in centenarians, although it was no longer significant after Bonferroni's correction. Indeed, association studies are subjected to a number of possible confounding factors, such as the homogeneity of the population concerning geographic origin. Selection of a very homogeneous population might allow one to overcome some confounding biases. In this regard, the Sardinian population is an ancient genetic isolate, the genetic pool of which is characterized by relatively little genetic flow from the different populations that have invaded the island during the last 3000 years.

On the other hand, the HLA region contains a remarkable number of genes forming some very stable haplotypes characterized by a strong linkage disequilibrium and probably conferring a selection advantage. Some of the highly conserved haplotypes are composed of the HLA-DR, DQ alleles, and, in some instances, HLA-DQ alleles seem to play a central role in association with diseases. This study evaluates the effect of HLA-DQ alleles in determining a longevity trait. The analysis of affected sib-pairs was applied to 24 Sardinian centenarian-sib (85- to 99-year-old) pairs. This strategy is an alternative powerful tool, allowing the authors to analyze the influence of genetic variants involved in complex phenotypes on ancestry and environmental influences independently.

Blood specimens were collected in tripotassium EDTA sterile tubes, and genomic DNA was extracted and stored at -20°C for different genetic analyses. DNA samples for HLA-DQ typing were available from 123 centenarians (83 women and 40 men), 92 controls (58 women and 34 men), and 24 siblings (14 women and 10 men) of 17 centenarians (7 women and 10 men). HLA-DQ alleles of all the subjects were genotyped by amplification refractory mutation system/polymerase chain reaction using primer sets and PCR conditions described by Bunce et al.

Pearson's test was applied to analyze differences among expected and observed HLA DQA1* and DQB1* identity, haploidentity, or non-identity frequencies in sib-pairs.

As previously reported, nonsignificant differences in HLA-DQA1* and -DQB1* allele frequencies were found between centenarians and controls (data not shown). On the other hand, for extreme longevity, moderate familial clustering has also been observed. Perls and coworkers found that the survival ratio for siblings of centenarians versus siblings of 73-year-olds was about four-fold that for people aged 80 to 94. A recent report on Danish centenarians suggests that siblings born after centenarians have a four-fold increased chance of becoming centenarians compared to their birth cohort. Table 1 describes the results of HLA-DQ allele typing of centenarian sib-pairs. The authors have not investigated the association ofSample. In addition, 24-85 year-old siblings of centenarians were enrolled following the reported protocol. Written informed consent for enrolling in the study and personal data management was obtained from all the subjects according to Italian law.

TABLE 1. ANALYSIS OF HLA-DQA1* AND DQB1* ALLELE SHARING IN 24 CENTENARIAN-SIB PAIRS FROM SARDINIA 
Nr of Centenarians Siblings alleles shared DQ DQ DQ DQ A1 B1 A1 B1 DQ DQ 
Sample Age Sex Typing Typing Sample Age Sex Typing Typing A1 B1 C1 
104 F 01 01 05 06 S1 92 F 01 03 03 06 
C2 105 F 01 05 02 05 S2 99 F 01 05 02 05 
C3 103 M 01 01 05 06 S3 86 F 01 01 05 06 
C4 101 M 01 02 02 05 S4 88 F 01 05 03 05 
C5 106 M 01 02 03 03 S5 97 M 03 03 03 03 
C6,104 M,01,05,02,03,S6,90,M,01,05,02,03
C7a,107,M,01,01,05,05,S7b,88,F,01,05,05,03
C8,104,M,01,05,02,06,S8,91,M,03,04,03,04
C9,102,M,01,01,05,05,S9,,81,M,,02,,03
C10,111,M,,01,,05,,06,S10,96,F,01,01,03,03
C11,103,F,03,05,02,03,S11,91,F,01,05,02,05
C12a,,103,F,01,05,02,05,S12b,,90,M,01,05,03,05
S12c,,88,F,01,05,02,05,S12d,,99,M,01,,05,,05
C13,,103,F,,01,,05,,06,S13,,96,M,,01,,05,,05
C14a,,101,F,01,05,03,05,S14b,,92,F,01,01,03,05
C15a,,,103,M,,,04,,,04,,,05,S15b,,,93,F,,,02,,,03
S15c,,,87,M,,,01,,,05,,,03
C16a,,,103,F,,,03,,,05,,,03,S16b,,,99,F,,,01,,,03,,,06
C17a,,,103,M,,,01,,,02,,,03,S17b,,,97,M,,,01,,,03,,,03