Peutz-Jeghers Syndrome STK11 19p13 Mutation

A 14-year-old boy presents with recurrent episodes of abdominal pain and iron-deficiency anemia. On examination, he has characteristic dark brown to blue-black macules on his lips and buccal mucosa. Genetic testing reveals a heterozygous loss-of-function mutation in the gene located at the locus marked **A** in the diagram. Which of the following best describes the molecular consequence of this mutation in Peutz-Jeghers syndrome?