Scene 1: Michelle is saved by the starving bacteria

1
Scene 1 Michelle is saved by the starving
bacteria

• After this lesson, the student will be able to
• explain recessive inheritance using
  phenylketonuria (PKU) as an example
• describe the Guthrie test for screening newborn
  babies for PKU
• explain how PKU can be treated by a special diet
  and how this could postpone the effects of PKU
  until the next generation
• discuss some of the economic issues associated
  with screening for the treatment of genetic
  conditions
• explain why females with PKU need to be followed
  very closely in their reproductive years to avoid
  the problems associated with maternal PKU.

2
Michelles Situation

• Stacy and Paul Chance are referred to the
  genetics center because their newborn daughter,
  Michelle, had elevated phenylalanine levels as
  determined by newborn screening. The diagnosis of
  phenylketonuria (PKU) is made.

3
Mandatory PKU Testing

• The couple wonders why their child was tested.
  They have two other children, Alan (age 6) and
  Frank (12) who are apparently healthy, active,
  intelligent boys. The couple couldn't remember
  any special testing that either boy had at birth.
  State mandated newborn screening is explained to
  them. A description of PKU and its inheritance is
  provided, and dietary management is discussed.

4
PKU Testing Guthrie Test

• Mandatory statewide testing around day 2
  (5/child)
• Bacteria grows on blood sample if excess
  phenylalanine is present
• Treatment begins immediately
• Why is 5/child more cost effective for the state
  then no testing at all?

5
What is PKU?

• PHENYLKETONURIA (PKU)
• PAH gene on long arm of chromosome 12 is
  absent/reduced
• Excess phenylalanine interferes with brain
  development
• Autosomal recessive disorder
• 110,000-15,000 births
• Effects retardation, seizures, destructive,
  restlessness

6
Prognosis

• With treatment Normal development
• With minimal treatment Intellectual defects
• Without treatment Retardation

7
Treatments

• Low phenylalanine diet
• Baby fed special formula (10,000/year)
• Vegetables and low phenylalanine foods added
  later in development
• Regular blood samples taken and phenylalanine
  diet adjusted as needed
• Eventually, a normal diet can be obtained

8
Chemistry of PKU

• Normally, phenylalanine is converted into
  tyrosine and then into a protein
• Enzyme phenylalanine hydroxylase is
  absent/reduced
• Phenylalanine builds up in blood

9
Chances reaction

• Paul denies his contribution to his daughter's
  condition. He is a successful businessman, proud
  of his healthful lifestyle. He doesn't smoke, he
  jogs six miles a day and he enjoys a healthy
  diet. He attributes Michelle's condition to a
  "weakness on Stacy's side of the family" and
  refuses to believe that he has any "bad genes."
  The genetic counselor mentions that everyone
  carries an estimated 6-10 "unusual" recessive
  genes which cause no clinical problems in the
  carriers, but if two carriers of the same unusual
  recessive gene were to have children together,
  each child would have a 1 in 4 chance of
  inheriting the recessive condition for which the
  genes were responsible.

10
GeneticCounseling

• Paul is relieved to know that being a carrier
  neither reflects negatively on his manhood nor
  does it isolate him from his peers. He quickly
  turns the situation to his advantage and states,
  "Well, if we're all carriers of some bad genes, I
  guess I'm better off than most of you. At least I
  know what one of my unusual genes is." Stacy does
  not try to defend herself against Paul's earlier
  accusations but is relieved at his acceptance of
  the cause of Michelle's condition.
• The further complication of maternal PKU is
  mentioned. Paul and Stacy are scheduled to meet
  with a nutritionist, and follow-up appointments
  for monitoring blood phenylalanine levels are
  made.
• Upon leaving, Stacy thanks the genetics staff and
  says, "Well I guess we won't be seeing you again
  for a long time."
• Little does she know...

11
Essay Topic

• Assume that you are Stacy or Paul Chance.
  Describe in writing your feelings after your
  visit to the genetics center.