CASE FILES: BIOCHEMISTRY
A N SW E R S TO C A SE 8: C Y ST IC FIB R O SIS
A 3-year-old Caucasian male has a history of chronic pulmonary
and gastrointestinal problems and has a positive chloride sweat test. A family
history of similar symptoms is also present.
Likely diagnosis: Cystic fibrosis, which is an autosomal recessive
disorder resulting in defective chloride ion channels of exocrine glands
and epithelial tissues of the pancreas, sweat glands, and mucous glands
in the respiratory, digestive, and reproductive tracts.
Gel electrophoresis: Separates deoxyribonucleic acid (DNA) chains of
varying length and can allow identification of a specific gene sequences.
C L IN IC A L C O R R E L A T IO N
Cystic fibrosis is an inherited condition affecting approximately 1 in 2500
white individuals. Affected patients usually have abnormal mucus secretion
and eccrine sweat glands leading to respiratory infections, gastrointestinal
obstruction, pancreatic enzyme dysfunction leading to malabsorption of nutri-
ents, and excessive electrolyte secretion. The protein cystic fibrosis trans-
membrane conductance regulator (CFTR) is defective, leading to abnormal
chloride transport. Approximately 70 percent of mutations are accounted for
by deletion of three specific base pairs at the F 508 position of the CFTR.
Oligonucleotide probes can be used to assay for this mutation, but other tests
are required for the less common mutations.
A PPR O A C H TO G E L E L E C T R O P H O R E SIS
A N D C L O N IN G
Understand the process of gel electrophoresis.
Know the difference between various types of blots.
Be familiar with DNA sequencing using the Sanger dideoxynucleotide
Understand the process and uses of cloning DNA.
Blotting: The transfer of proteins or nucleic acids from an electrophoresis
gel to a membrane support (such as nitrocellulose or nylon). The membrane
blot is then incubated with probes that bind the molecules of interest.
Complementary DNA (cDNA): A sequence of DNA copied from messen-
ger ribonucleic acid (mRNA), which does not contain introns that were
present in native DNA.