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Since sweat testing was introduced there have been individuals who are atypical. They have features of cystic fibrosis but a normal or borderline sweat test. This includes mild lung disease (Gan et al, 1995), pancreatitits (Cohn et al, 1998) and bilateral congenital absence of the vas deferens (CAVBD) (Dumur et al, 1990).
This has lead to the recognition that the spectrum of mutations in the CFTR gene gives rise to a very variable clinical phenotype that may not be predictable from the genotype.
Some mutations may act concurrently with other mutations on the same allele. An example of this is exon 9 splicing which is influenced by the polythymidine sequence of intron 8 which precedes the splicing receptor site (Massie et al, 2001). This polythymidine tract is polymorphic with sequences of 5, 7, 9 thymidines. Because CFTR missing exon 9 splicing is non functional and exon-9 splicing is inversely proportional to the length of the thymidine sequences, the 9T variant allows normal reading of the gene while the 5T variant is associated with the highest level of non-functional CFTR protein (Massie et al, 2001).The commonest polymorphism is the seven thymidine (7T) variant and the DF508 mutation occurs exclusively on the 9T variant. This variation may be important in some CFTR mutations eg R117H . The presence of R117H/508 on a background of 5T is associated with an elevated sweat test and clinical CF. R117H in association with 7T is associated with a normal, borderline or elevated sweat test and variable clinical presentation. (Massie et al, 2001; Rave-Harel et al, 1997; Kiesewetter et al, 1993).
Environment factors and the influence of modulator genes are also likely to impact on phemotypic expression..