Genotypic hetereogeneity of the molecular basis of cystic fibrosis: the paradigm of lithuanian population genetic testing

Since cystic fibrosis (CF) is the most common lethal hereditary monogenic disease in Caucasian populations with an average prevalence of 1:2500 newborns, population screening by genetic testing for CF prenatal diagnosis and clinical phenotype determination seems worthwhile to implement in all countries as is the case for other common inherited disease screening protocols. Worldwide mutational analysis of the CF transmembrane regulator (CFTR) gene has revealed that p.F508del mutation accounts on the average for 70-80% of CF chromosomes from northern Europe and America. Yet, more than 1500 mutations of CFTR have been identified to cause CF in different populations. In addition, the phenotypic spectrum associated with mutations in the CFTR gene extends beyond the classically defined CF. Due to the high allelic and phenotypical heterogeneity of CF chromosomes, it is important to identify the CFTR mutation spectrum in specific populations to implement efficient molecular diagnostic protocols as well as to define a correct genotype-phenotype correlation. We have characterized the molecular basis of CF in Lithuania through the analysis of the CFTR gene in 98 unrelated chromosomes from 49 Lithuanian CF patients using both gene scanning and mutation screening methods. A CFTR mutation was characterized in 64.2% of CF chromosomes. The most frequent Lithuanian CF mutation is p.F508del (52.0%). Seven CFTR mutations, CFTRdele2,3(21 kb) (2.0%), p.N1303K (2.0%), p.R75Q (1.0%), p.G314R (1.0%), p.R553X (4.2%), p.W1282X (1.0%), and g.3944delGT (1.0%), accounted for 12.2% of Lithuanian CF chromosomes. Since standard PCR-based techniques did not allow us to characterize 35.8% of the CF Lithuanian chromosomes, we also analyzed three intragenic CFTR gene microsatellites, IVS8CA, IVS17bCA and IVS17bTA in the previously analyzed 49 Lithuanian patients as well as in their relatives for a total of 151 chromosomes (98 CF and 53 normal chromosomes). The most frequent CF haplotype 17-31-13 (21.4%) was shown to be the ancestral haplotype on which p.F508del mutation arose and, together with another 7 haplotypes, account for 67% of CF chromosomes. Haplotypes 16-30-13, 16-33-13, 16- 36-13, 17-31-11, 22-31-13 account for 10% of CF Lithuanian chromosomes with yet unidentified mutations and, with the exception of 16-30-13, were not found in normal chromosomes. Another 6 haplotypes (1% each) represent the remaining CF chromosomes characterized. Indirect linkage analysis to follow segregation of CF chromosomes using identified CFTR haplotypes revealed 82% informative families out of 22 families considered. Thus, screening of the eight CFTR disease-causing mutations, accounting for 64.2% of Lithuanian CF chromosomes, combined with haplotype linkage analysis in informative families, provide the basis for the implementation of clinical genetic testing of CF in Lithuania. The characterization of a comprehensive molecular epidemiology of CF in the Lithuanian population points to the importance of continuing to pursue small-scale genotyping research projects for reliable and efficient inherited disease genetic testing in clinical practice.