Heterozygote Screening of Cystic Fibrosis

Lab Report

Heterozygote Screening Of Cystic Fibrosis

Abstract

Cystic Fibrosis is one of the most common lethal hereditary autosomal recessive disorders. Cystic Fibrosis is regarded as a disease of Caucasians, because of its high incidence and carrier frequency in the Caucasian population. The gene responsible for cystic fibrosis is cystic fibrosis transmembrane conductance regulator, which encodes for a channel that control chloride movement out of the cell. The mutation of this gene that causes a loss of function or loss of expression results in the classical manifestation of the disease. In this practical experiment, we are assigned a sample from the class population and a sample from the Jone's family (Sample 7: Isobel's DNA sample). Screening for cystic fibrosis for the ΔF508 mutation was done in both sample, addition of 3 other mutations, G542X, G551D and R553X were done to the sample of Isobel's DNA. It was found the carrier frequency of the class population is lower than the expected of 1 in 25 and Isobel is a carrier with a heterozygous for G551D mutation.

Introduction

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) situated on 7q31 is the gene responsible for the hereditary disease cystic fibrosis (CF). It is one of the most common lethal autosomal recessive disorder causing mortality in Caucasians

(Voynow et al., 2008). In Australia, the incidence rate is 1 in 2500 and a carrier frequency of 1 in 25 (Massie et al., 2005). The disease is caused by either the loss of function or the loss of expression mutations in the CFTR gene. CFTR is a channel that has a critical role of active secretion of electrolytes and fluid in epithelial cells (Zheng et al., 2004). The defect in the CFTR protein upsets the functions of the channel, preventing the regulation of flow of electrolytes and fluid across cell membranes; Resulting to the classical manifestation of cystic fibrosis.

CFTR was identified through positional cloning located on 7q31.2, which spans 250 kb of DNA and contains 27 exons (Riordan et al., 1989; Rommens et al., 1989). Modifications to the sequence or in the synthesizing process of the CFTR gene lead to loss of functions of the product. Over a thousand different mutations in the CFTR gene have been found in patients, with the most common mutation ΔF508 that occurs in approximately 70% of all CF chromosomes (Joseph L. Bobadilla, 2002). This mutation is a 3 nucleotide base pair deletion in exon 10, leading to the loss of phenylalanine at the 508th amino acid residue (Gallati, 2003). This resulted in a severe reduction of function in CFTR protein.

In this practical, DNA based genetic testing was used to screen for mutations in the CFTR gene. 4 mutant alleles which are well characterized