Regarding the APOB locus, neither the XbaI-RFLP nor the EcoRI-RFLP was able to discriminate between cases ¢ and controls (Table 2), while the 3 APOB-VNTR multial- lelic system revealed significant differences between the samples (Table 3): it is in fact clear that the frequency of alleles with fewer than 35 repeats is lower in cases than in controls (0.112 ± 0.019 vs 0.203 ± 0.023), as if the geno- types carrying these specific alleles were preferentially lost in the ageing population. It is worth noting that the modification of the gene pool with ageing seems to affect chiefly female cases, because the difference between male cases and controls does not reach statistical significance. The loss of specific alleles from the S gene pool (which is observed at the APOB locus but not at the TPO locus) does increase the intragroup homogeneity, as indicated by Pm values that were 0.09 in S vs 0.07 in C. This finding agrees with the hypothesis that unfavourable alleles, which could contribute to disease and death, have been eliminated from the S gene pool.

The difference between centenarian (S) and control (C) allele pools at the APOB locus is confirmed by D linkage studies. Surprisingly, while in S there is a strong linkage disequilibrium (D) between XbaI- and EcoR1-RFLPs (Table 4), in C the value of D is not significantly different from 0. Linkage disequilibrium between XbaI- and EcoRI-RFLPs has been reported for many sample popula- tions, although studies in samples of US Caucasians (Hegele et al. 1986), British Caucasians (Ferns et al. 1988) and southern Italians (De Benedictis et al. 1993) re- ported no linkage disequilibrium between these polymor- phisms. In the present case, however, the difference ob- served between the S and C samples is not due to ethnic or geographical differences, because S and C arise from one gene pool; rather, it could be supposed that recombi- nant chromosomes are preferentially lost in the ageing population. Interestingly, the XbaI- and EcoRI-RFLPs lie in DNA regions (inside exons 26 and 29, respectively) ad- jacent to those that code for the interaction domain of the apolipoprotein B with the LDL receptor (Blackart et al. 1986; Yang et al. 1986); furthermore, in exon 26, RNA editing of the mRNA or pre-mRNA changes codon 2153 (CAA, glutamine) to a stop codon (UAA) (Hodges and Scott 1992). It may be that recombination in this DNA re- gion (which is crucial for the correct physiology of apo- lipoprotein B) originates alleles unfavourable to longe- vity. On the other hand, in a study carried out on French families, it was found that recombination within the APOB gene appears to be relatively rare (Hallman et al. 1994). ¢ Also analyses of the 3 APOB-VNTR polymorphism with

Why are VNTR alleles with a low repeat number scarce ¢ in the centenarian gene pool? The 3 APOB-VNTR alleles exhibit significant linkage disequilibrium with APOB gene polymorphisms (Renges et al. 1992; De Benedictis et al. 1996). It is therefore possible that the loss of specific VNTR alleles is a consequence of selection against alleles of the APOB gene that predispose to disease and death (for instance alleles associated with coronary heart dis- ease). In such a case, the low incidence of specific VNTR alleles would be a consequence of their association with unfavourable APOB gene alleles. However, we should not ¢ discount the fact that the copy number of the 3 APOB- VNTR array could itself have some kind of function (Elis- worth et al. 1995). In this hypothesis long life expectancy

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