A 3-year-old boy is being evaluated for recurrent respiratory infections. The patient's family immigrated to the United States 5 months after his birth. Since then, the boy has experienced multiple episodes of pneumonia and bronchitis, and has developed a persistent cough and failure to thrive. His older brother has no medical issues. A genetic test is performed and reveals a mutation in an exon of a gene that codes for a transmembrane chloride channel. The abnormal mRNA is isolated from cultured epithelial cells, and its complementary DNA is synthesized. Amplified cDNA samples from both the patient and his healthy sibling are analyzed using gel electrophoresis and compared to DNA fragments of known size to determine base pair length. The results are shown below. Which of the following is most likely responsible for this patient's condition?

Question

A 3-year-old boy is being evaluated for recurrent respiratory infections.  The patient's family immigrated to the United States 5 months after his birth.  Since then, the boy has experienced multiple episodes of pneumonia and bronchitis, and has developed a persistent cough and failure to thrive.  His older brother has no medical issues.  A genetic test is performed and reveals a mutation in an exon of a gene that codes for a transmembrane chloride channel.  The abnormal mRNA is isolated from cultured epithelial cells, and its complementary DNA is synthesized.  Amplified cDNA samples from both the patient and his healthy sibling are analyzed using gel electrophoresis and compared to DNA fragments of known size to determine base pair length.  The results are shown below.  Which of the following is most likely responsible for this patient's condition?

Quizplus · Accepted Answer

11ec160b_376e_9c95_9249_4bd4aa0a0d4b_MD0001_00 This patient has cystic fibrosis, which occurs due to mutations affecting the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The vignette states that the patient's mutation affects an exon, meaning that the mutation will be detectable in the mRNA sequence. Complementary DNA (cDNA) is double-stranded DNA that is synthesized from an mRNA template. In this case, cDNA was synthesized using CFTR mRNA from the patient and his brother. Subsequent gel electrophoresis shows that the patient's cDNA is 28 base pairs shorter than that of his brother, indicating the patient has a 28-base pair deletion affecting the coding region of CFTR. Deletion or insertion of a number of bases that is not divisible by 3 results in frameshift mutations. As the name implies, frameshift mutations alter the reading frame of the genetic code, resulting in the formation of nonfunctional proteins due to the incorporation of many incorrect amino acids. (Choice B) One or more complete codons (genetic triplet codes) are removed in an in-frame deletion, which does not affect the reading frame of subsequent codons. This patient's 28-base pair deletion is not divisible by 3, so it will result in a reading frame shift. (Choices C and D) Missense and nonsense mutations are due to single base substitutions that result in the placement of an incorrect amino acid or introduction of a premature stop codon, respectively. These mutations occur in exons and affect protein translation. However, in this scenario, gel electrophoresis is being performed on cDNA (not protein) and the results indicate the length of the mature RNA transcript. Termination of RNA transcription occurs by specific sequences in the 3' untranslated region that cause RNA hairpin loop formation and/or recruitment of termination factors. As a result, point mutations in the exons of a gene (such as those responsible for causing nonsense mutations) are unlikely to affect total mRNA (cDNA) size. (Choice E) Silent mutations can occur in both coding and noncoding regions, but they do not alter protein quantity or function and do not affect phenotype. This patient's mutation caused him to develop cystic fibrosis, so it is not a silent mutation. (Choice F) Trinucleotide expansions increase the number of trinucleotide repeats in the coding region of a gene, often resulting in large proteins with abnormal function. This patient's abnormal CFTR gene is shorter (not longer) than the normal gene from his sibling, which indicates a deletion has occurred. Educational objective: Deletion or addition of a number of bases that is not divisible by 3 in the coding region of a gene will cause a frameshift mutation. Frameshift mutations alter the reading frame of the genetic code, resulting in the formation of nonfunctional proteins. __________ References: The CFTR frameshift mutation 3905insT and its effect at transcript and protein level. (http://www.ncbi.nlm.nih.gov/pubmed/19724303)

A 3-Year-Old Boy Is Being Evaluated for Recurrent Respiratory Infections