The distribution of the CFTR gene mutations on the 160 CF chromosomes is shown in Table 1. A total of 15 different mutations were detected.  The most frequent being delF508 (35.0%), followed  by 394delTT (3.13%), CFTRdele2,3(21kb) (1.25%), R334W (1.25%), G542X (0.62%), 2184insA (0.62%),  S1196X (0.62%), 3849+10kbT →C (1.25%), W1282X (0.62%), N1303K (1.25%), I488M (0.62%),1811+12A→C (1.25%), T663S (1.25%), I1226R ( 0.62%), 4005+9A→C  (0.62%).  Six novel mutations (I488M, 1811+12A→C, T663S, I1226R, 4005+9A→C) are described for the first time.

During this study two new polymorphisms were found - 2097A→C (A655A)  in 13 exon (0.62%), 3996G→C (V1288V) in 20 exon  (1.88 %).

So in this screening we could only characterize 50.0%  of CF chromosomes.  In 33.75% (27/80) of patients both mutant alleles were  characterized, 32.5% (26/80) were compound  heterozygotes with one unknown  allele and 33.75% (27/80) had unknown mutations on both chromosomes.

The delF508 mutation was presented on three  different M470V-TUB20 diallelic haplotypes, with the combination 1-2 accounting for 88.45% of all delF508 alleles (Table 4). One haplotype 1-1 could derive from the original by recombination and another one 2-1 derived from the 1-1 haplotype by a point mutation at the polymorphic  loci (M470V) or by recombination. Our results are in agreement with previous  data, which suggested  a single origin of the  delF508 mutation [27], because  delF508 arose from ancestral chromosome  with haplotype 1-2, which is uncommon in the actual background of normal chromosomes (30.2%) (c2=30.07, p<0.0005).

 Only three microsatellite IVS6aGATT-IVS8CA-IVS17CA haplotypes were found associated with delF508 and two of them 6-6-7 and 6-2-7 were common, 53.84% and 42.29%, respectively. Six different M470V-TUB20-IVS6aGATT-IVS8CA-IVS17CA haplotypes were discovered on delF508 chromosomes, two of them being most frequent. These were haplotypes 1-2-6-6-7 and 1-2-6-2-7 found with the frequencies of 50.00% and 35.00%, respectively. According to the hypothesis of Morral et al. [20], the association of two different haplotypes with one mutation is explained either by the replication error, like slipped-strand mispairing (slippage of the polymerase over the repeat sequence during the replication), or by crossingover error, which has arisen after the appearance of the delF508 mutation, resulting in the distribution of the 1-2-6-6-7 haplotype. It can be suggested that mutant chromosomes with the 1-2-6-6-7 and 1-2-6-2-7 haplotypes have already existed at that time, but their distribution was independent. Probably, because of this, in the regions, like Russia and Northern, and Central Europe, the 1-2-6-6-7 haplotype prevails, while in the Mediterranean countries the 1-2-6-2-7  haplotype is most frequent. Taking into consideration the prevalence of the 1-2-6-6-7  haplotype in CF patients from Russia, it can be suggested that delF508 chromosomes with this haplotype first appeared in Russia, and there were preferentially distributed as compared to the delF508 chromosomes with the 1-2-6-2-7  haplotype.

Our results suggest that the appearance of the delF508 mutation in the gene pool of Bashkortostan was associated with the migrations of the Slavic populations from the Eastern Europe.

            The other frequent mutation in our population  was 394delTT, and two  CF chromosomes with this mutation share the same haplotype 1-2-6-2-7, which  is also the same haplotype in which the delF508 arose (Table 2.).

Two chromosomes with  R334W were associated with  same haplotype 2-2-7-6-7, our results support  single origin of this  chromosomes in our CF population.

            Both chromosomes with mutation CFTRdele2,3(21kb) were  associated with same microsatellite haplotype 7-7-7. Two different diallelic haplotype associated with CFTRdele2,3(21kb) might be explained by recombination. Our results are in agreement with previous  data which suggested  a single Slavic origin of the CFTRdele2,3(21kb) mutation [29].  

The most common microsatellite haplotype on normal chromosomes 7-7-7 are associated with the largest number  of mutations (6).

One  CF chromosome with 3849+10kbT C were detected on haplotype 2-2-6-6-3, which did not present on normal chromosomes.

             Several mutations are associated with several haplotypes, as a result of slippage or  recombination.

            Table  5. lists the CFTR haplotypes for  52  CF chromosomes with unknown mutations.  Analysis of the M470V-TUB20-IVS6aGATT-IVS8CA-IVS17CA haplotypes on CF chromosomes with unknown mutations showed that the 2-2-7-7-7 haplotype was most frequent (26.9%). The differences between these and normal chromosomes were statistically significant (c2=7.24, p<0.008). It seems likely that CF patients from Bashkortostan, mostly belonging to the Turkic-speaking families, possess specific CFTR gene defect associated with the given haplotype. 

In general, on 52 mutant chromosomes with unknown mutations, a total of 30 different haplotypes were discovered with 38% of these not detected on normal chromosomes. The different haplotypes found suggest that most of this unknown CF alleles  carry different mutations, further confirming the high heterogeneity of Bashkortostan CF population.

Intragenic polymorphic markers seem most promising to be used as diagnostic markers.  To assess the possibility of utilization of the M470V, TUB20, IVS6aGATT, IVS8CA, and IVS17CA polymorphic systems for the DNA diagnostics of CF, the CF families were examined for the levels of heterozygosity relative these loci. These values constituted 48% for M470V, 46% for TUB20, 50% for IVS6aGATT, 91% for IVS8CA, and 24% IVS17CA. Analysis of the IVS6aGATT-IVS8CA-M470V-IVS17CA-TUB20 haplotypes makes it possible to increase the proportion of completely informative families up to 88%. Therefore, we conclude that the CFTR polymorphic markers examined are highly valuable for the family analysis and prenatal diagnostics of CF in Bashkortostan.

            The CFTR gene analysis confirmed the high heterogeneity of our CF population, which could be explained by the complex ethnic composition of the Bashkortostan population, resulting from the mixing of  different ethnical genetic pools ((European and Asian).