Association of the 1513C polymorphism in the P2X7 gene with familial forms of chronic lymphocytic leukaemia (2004)

B-cell chronic lymphocytic leukaemia (CLL) is unique in showing a threefold increased incidence in closely related family members compared with other lymphoproliferative diseases (Houlston et al, 2003). A candidate gene for this familial incidence is the P2X7 gene, which codes for a cytolytic receptor, activated by extracellular ATP. We have identified a loss-of-function single nucleotide polymorphism (1513 A→C) in the P2X7 gene and reported an increase in frequency of this polymorphic allele in 36 patients with CLL (Wiley et al, 2002). In contrast, other recent reports did not find any difference in the frequency of this polymorphic allele between normal and CLL subjects (Starczynski et al, 2003; Zhang et al, 2003). To further explore the involvement of the P2X7 as a susceptibility gene in CLL, we expanded our previous study to include 42 cases of familial and 74 cases of sporadic CLL. A total of three intergenerational pairs and six sibling pairs and eight single familial cases (affected relatives unavailable for study) were recruited from Eastern Australian centres. DNA samples from an additional five parent-child and three sibling pairs were obtained from the UK, courtesy of Dr D. Catovsky. Normal subjects of Western European descent (n = 411) with no history of haematological disease were recruited from the partners of patients or staff from our institutions. As expected, the median age at diagnosis was lower in familial CLL than for the sporadic CLL cohort (Table I). However, there were no significant differences in the IgVH mutational status or stage at diagnosis between familial and sporadic CLL (Table I). The P2X7 1513C genotype was determined by sequencing the polymerase chain reaction (PCR) products of exon 13 (Wiley et al, 2002) and results are shown in Table II. The 1513C frequency was significantly increased in familial CLL patients (0·286) compared with normal subjects (0·157) with an odds ratio (OR) of 2·1 (P = 0·008). In contrast, the 1513C frequency in patients with sporadic CLL was very similar to that observed in the normal subjects, with an OR close to one. To account for the difference in the background allelic frequency between the Australian and UK normal populations, we also conducted two independent sub-analyses and found that the ORs for carrying a 1513C allele were 1·8 [95% confidence interval (CI): 0·9–3·5, P = 0·08] and 2·2 (95% CI: 1·0–5·0, P = 0·03) for the Australian and UK restricted analyses respectively. The UK normal subjects (n = 443) were pooled from the Zhang et al (2003) and Starczynski et al (2003) studies. While the OR remained similar to the combined analysis of 2·1, only the UK sub-analysis reached significance because of reduction in sample size. This increase in the 1513C frequency in familial CLL suggests an association between inheritance of this polymorphism and familial CLL and explained the high 1513C frequency reported in our previous study (Wiley et al, 2002) that included a number of familial cases. The IgVH mutational status was also determined as previously described (Thunberg et al, 2002) but the difference in the 1513C frequency between 60 mutated (0·167) and 22 unmutated patients (0·273) did not reach statistical significance (P = 0·13), consistent with other studies (Thunberg et al, 2002; Starczynski et al, 2003; Zhang et al, 2003). Therefore, this loss-of-function polymorphism may have a limited role in disease progression. The frequency of 1513C in Australian normal subjects of 0·157 was lower than the values of 0·216 and 0·182 in recent UK studies (Starczynski et al, 2003; Zhang et al, 2003). A possible explanation for this would be the inclusion of subjects of Indian background in these UK studies, as we have observed an increase in 1513C frequency of 0·36 (n = 25) in this ethnic group. However, our present data agrees with others that there is no difference in the 1513C frequency between the sporadic CLL and normal controls. In conclusion, our data suggests a possible role for the P2X7 receptor in susceptibility to familial CLL or, alternately, the 1513C allele may be in linkage disequilibrium with a nearby susceptibility gene. This possibility is supported by a recent study of 18 families with CLL (Goldin et al, 2003), which gave a non-parametric linkage score of 2.81 between CLL and a microsatellite marker, 2000 kb centromeric to the P2X7 gene. Further study of genetic loci close to 12q24 is warranted in familial forms of CLL. This work was supported by grants from the National Health and Medical Research Council, the Cure Cancer Australia Foundation, the Leukaemia Foundation of Australia, and the Swedish Cancer Society.