Phenotypes of California CF Newborn Screen-Positive Children with CFTR 5T Allele by TG Repeat Length

Background: At the cystic fibrosis transmembrane conductance regulator (CFTR) gene (IVS8)-(TG)m(T)n locus, a lower number of thymidines (legacy names 9T vs. 7T vs. 5T) and a higher number of (TG) repeats (TG-11 vs. 12 vs. 13) are associated with decreasing translation of functional CFTR protein in vitro. Methods: Retrospective cohort study comparing phenotypes of California CF newborn screen-positive children (followed 2-8 years) who had two CF-causing mutations (diagnosed as CF) with those who had one mutation from a panel of 40 CF-causing mutations (CF40(mut)) and one (IVS8)-(TG) 11, 12, or 13-5T mutation detected by sequencing (diagnosed as CFTR-related metabolic syndrome [cRMS]). Results: The study included 428 children, of which 234 had two CF-causing mutations, and were used to compare with the other 194 children with one CF-causing mutation and one isolated 5T allele [CF40(mut)/(TG)13-5T = 21, CF40(mut)/(TG) 12-5T = 85, and CF40(mut)/(TG) 11-5T = 88]. Among children with CF40(mut)/(TG) 13-5T, 38% were diagnosed with CF by 8 years, based on sweat chloride results and clinical presentation. Six percent of those with CF40(mut)/(TG) 12-5T, and none with CF40(mut)/(TG) 11-5T, reached diagnostic criteria. Conclusions: CFTR (IVS8)-(TG) m-5T allele (TG) tract length determination provides valuable information in predicting the risk of developing a CF phenotype. Of the three types of 5T alleles evaluated, screen-positive children with genotype CF40(mut)/(TG) 13-5T progressed from CRMS to CF at a high rate, while there was little evidence of clinical disease in those with CF40(mut)/(TG) 11-5T. Additional data from longer follow-up intervals are needed to fully understand the natural history of individuals with a CF40(mut)/(TG) m-5T genotype.