Genetic isolates represent exceptional resources for the mapping of complex traits but not all isolates are similar. We have selected a genetic and cultural isolate, the village of Talana from an isolated area of Sardinia, and propose that this population is suitable for the mapping of complex traits. A wealth of historical and archive data allowed the reconstruction of the demographic and genealogical history of the village. Key features of the population, which has grown slowly with no significant immigration, were defined by using a combination of historical, demographic and genetic studies. The genealogy of each Talana inhabitant was reconstructed and the main maternal and paternal lineages of the village were defined. Haplotype and phylogenetic analyses of the Y chromosome and characterisation of mitochondrial DNA haplogroups were used to determine the number of ancestral village founders. The extent of linkage disequilibrium (LD) was evaluated by the analysis of several microsatellites in chromosomal region Xq13.3, which was previously used to asses the extension of LD. Genealogical reconstructions were confirmed and reinforced by the genetic analyses, since some lineages were found to have merged prior to the beginning of the archival records, suggesting an even smaller number of founders than initially predicted. About 80% of the present-day population appears to derive from eight paternal and eleven maternal ancestral lineages. LD was found to span, on average, a 5-Mb region in Xq13.3. This suggests the possibility of identifying identical-by-descent regions associated with complex traits in a genome-wide search by using a low-density marker map. The present study emphasises the importance of combining genetic studies with genealogical and historical information.

Essential hypertension (EH) is a complex disorder that results from the interaction of a number of susceptibility genes and environmental factors. We studied an isolated Sardinian village (Talana) in which the prevalence of hypertension is comparable to that in most Western populations. Talana exhibits features, such as slow demographic growth, high inbreeding, a low number of founders, stable lifestyle and culture, and accurate genealogical records, that make it suitable for the study of complex disorders. Clinical assessment of the entire adult population (N= approximately 1,000) identified approximately 100 hypertensive subjects. For our study, we selected the individuals with the most-severe EH (i.e., diastolic blood pressure >100 mm Hg), belonging to a single deep-rooted pedigree (12 generations), whose common ancestors lived in the 17th century. We performed a three-stage genomewide search using 36 affected individuals, by means of parametric linkage and allele-sharing approaches. LOD scores >1 were observed on chromosomes 1, 2, 13, 15, 17, and 19 (stage I). The most striking result was found in a 7.57-cM region on chromosome 2p24-p25. All five nonparametric linkage statistics estimated by the SimWalk2 program lie above the significance threshold of P<.008 for the whole region. Similar significance was obtained for 2p24-25 when parametric linkage (LOD score 1.99) and linkage disequilibrium mapping (P=.00006) were used, suggesting that a hypertension-susceptibility locus is located between D2S2278 and D2S168. This finding is strengthened by a recent report of linkage with marker D2S168 in a hypertensive sib-pair sample from China.

Uric acid nephrolithiasis (UAN) is a common disease with an established genetic component that presents a complex mode of inheritance. While studying an ancient founder population in Talana, a village in Sardinia, we recently identified a susceptibility locus of approximately 2.5 cM for UAN on 10q21-q22 in a relatively small sample that was carefully selected through genealogical information. To refine the critical region and to identify the susceptibility gene, we extended our analysis to severely affected subjects from the same village. We confirm the involvement of this region in UAN through identical-by-descent sharing and autozygosity mapping, and we refine the critical region to an interval of approximately 67 kb associated with UAN by linkage-disequilibrium mapping. After inspecting the genomic sequences available in public databases, we determined that a novel gene overlaps this interval. This gene is divided into 15 exons, spanning a region of approximately 300 kb and generating at least four different proteins (407, 333, 462, and 216 amino acids). Interestingly, the last isoform was completely included in the 67-kb associated interval. Computer-assisted analysis of this isoform revealed at least one membrane-spanning domain and several N- and O-glycosylation consensus sites at N-termini, suggesting that it could be an integral membrane protein. Mutational analysis shows that a coding nucleotide variant (Ala62Thr), causing a missense in exon 12, is in strong association with UAN (P=.0051). Moreover, Ala62Thr modifies predicted protein secondary structure, suggesting that it may have a role in UAN etiology. The present study underscores the value of our small, genealogically well-characterized, isolated population as a model for the identification of susceptibility genes underlying complex diseases. Indeed, using a relatively small sample of affected and unaffected subjects, we identified a candidate gene for multifactorial UAN.

Recent studies indicate that, whereas the Sardinian population as a whole is comparable to outbred populations for linkage disequilibrium (LD) mapping of common variants, LD in Sardinian sub-isolates is more extended, making these populations particularly suitable for this approach. To evaluate the extent of LD between microsatellite markers, we compared different sub-populations within Sardinia selected on the basis of their geographical position and isolation: two small isolated villages (Talana, Urzulei), two larger but remote areas (Ogliastra, Nuoro province) and a cohort of samples representing the wider Sardinian population. LD analysis was carried out by using six microsatellite markers that are located on Xq13.3 and that have been extensively studied in different populations. We found different extents and patterns of LD in the sub-population samples depending on their degree of isolation and demographic history. All LD measurements and haplotype analyses indicate that there is a decreasing trend from Talana (the most inbred population, LD up to 9.5-11.5 Mb) to the more outbred Sardinian population (LD only for intervals <2 Mb). In one village (Talana), five haplotype classes accounting for 80% of the entire sample perfectly matched five Ogliastra clusters, supporting the origin of the village from the Ogliastra genetic pool. In contrast, the other village (Urzulei) showed a different pattern of haplotypes with a closer relationship to the Nuoro region sub-population. LD analyses therefore show that even neighbouring isolate villages may differ in their genetic background. Here, we highlight the importance of selecting appropriate populations and/or sub-populations for the analysis of complex traits. Isolated sub-populations showing different extents of LD can provide a powerful method for mapping complex traits by LD scanning at relatively low marker density.

Breast cancer is the most common malignancy in women, with an incidence that varies between 40 and 90 per 100 000 (standardized rate) worldwide. Breast cancer is the most frequent female tumour in Italy, representing about 25% of all female tumours as reported in Italian registries (Zanetti et al, 1997).A positive family history is known to be a high risk factor for developing the disease: 5-10% of all breast cancers arise in individuals carrying a germline mutation and are usually considered hereditary forms (Claus et al, 1991). Two major breast cancersusceptibility genes, BRCA1 and BRCA2, have been cloned (Miki et al, 1994;Wooster et al, 1995) and both are thought to account for 30-60% of hereditary breast cancer (Serova et al, 1997;Szabo et al, 1997;Vehmanen et al, 1997aVehmanen et al, , 1997b. However, large-scale mutation analyses conducted in several populations suggest the existence of additional breast cancer-susceptibility gene(s). BRCA1 mutations are responsible for the majority of familial breast cancer associated with ovarian carcinoma, for about 50% of cases with breast cancer alone and for very few male breast cancer cases (Easton et al, 1993;Stratton et al, 1994;Narod et al, 1995). It has been estimated that women carrying a germline mutation in BRCA1 have a risk ranging from 80 to 90% for developing breast cancer and from 44 to 63% for developing ovarian cancer (Easton et al, 1993(Easton et al, , 1995Ford et al, 1994; Miki et al, 1994;Wooster et al, 1994). BRCA2 mutations account for a similar proportion of inherited breast cancer and are frequently associated with male breast cancer (Wooster et al, 1995). Breast cancer risk in females carrying BRCA2 mutations is calculated to be similar to that conferred by BRCA1 mutations (Easton et al, 1993Ford et al, 1994; Miki et al, 1994;Wooster et al, 1994). BRCA1 and particularly BRCA2 families are often affected by other tumours such as prostate, liver, pancreas, lung, stomach and colorectum (Wooster et al, 1995;Gudmundsson et al, 1996;Phelan et al, 1996;Thorlacius et al, 1996;Vehmanen et al, 1997b;Tonin et al, 1998). Except for higher incidences of ovarian cancer in families with mutations in a 3.3-kb region of exon 11 of BRCA2 (the so-called ovarian cancer cluster region [OCCR]; Gayther et al, 1997), no other significant association between genotype and phenotype was described. BRCA1 and BRCA2 mutations are for the most part frame-shifts due to small deletions leading to premature translation termination (Wooster et al, 1995;Phelan et al, 1996;Tavtigian et al, 1996;Gayther et al, 1997).Some of these mutations are prevalent in genetically homogeneous populations as a consequence of a founder effect. A single BRCA2 mutation accounts for the majority of hereditary breast cancer in Iceland Thorlacius et al, 1996) and for 40% of male breast cancer cases , whereas three different founder mutations (185delAG and 5382insC in BRCA1, and 6174delT in BRCA2) have a high frequency in Ashkenazi Jews (Roa et al, 1996). Although at different rates, BRCA1 and BRCA2 f...