Interindividual variability in brain vulnerability to elevated blood phenylalanine concentrations in PKU patients - pilot study on amino acid transport in fibroblasts as a representative of the blood-brain barrier

At this moment, each single child with a positive heelstick for phenylketonuria
(PKU) and a confirmed diagnosis of PKU is put on the same strict dietary
treatment. However, this diet for life is socially demanding and quite
expensive, so treatment adherence usually declines with age. The question is
whether target phenylalanine levels should be the same for each patient, and
whether poor treatment adherence will have the same consequences for
neuropsychological outcome in all PKU patients.
In PKU, differences in brain vulnerability to elevated blood phenylalanine
concentrations are observed between patients. This can be best exemplified by
the fact that some late diagnosed PKU patients seem to have escaped from mental
retardation and have only been detected through affected siblings or children.
MRS examination in some of these patients revealed less increased brain
phenylalanine concentrations, despite markedly elevated blood phenylalanine
concentrations, possibly indicating a difference in the transport
characteristics of at least phenylalanine in these patients. However, in the
one gene investigated thus far that encodes for the LAT1 transporter, no DNA
polymorphisms have been identified that are associated with reduced brain
vulnerability to elevated blood phenylalanine concentrations.

To further characterize the transport of phenylalanine and other large neutral
amino acids across the blood-brain barrier in PKU patients with high and low
brain vulnerability to elevated blood phenylalanine concentrations, we will
investigate transport characteristics for phenylalanine, tyrosine, and
tryptophan as well as expression of different amino acid transporters in
fibroblasts of these patients. Based on these transport characteristics, we aim
to identify DNA polymorphisms in genes involved in amino acid transport, that
are associated with reduced brain vulnerability to elevated blood phenylalanine
concentrations in PKU patients.

Descriptive pilot study with an observational cross-sectional control group
design

Of each subject, fibroblasts, a single blood sample and urine will be
collected. Moreover, an IQ test will be performed in part of the subjects.
Regarding the group relatedness, this study will investigate the underlying
physiological mechanisms as well as the genetic basis for the difference in
brain vulnerability to elevated blood phenylalanine concentrations that can be
observed between individual PKU patients. To this purpose, as a first step, the
most extreme examples of patients with either a high or a low brain
vulnerability to elevated blood phenylalanine concentrations will be
investigated. The current neonatal screening programme and the direct
institution of a phenylalanine-restricted diet result in a near optimal outcome
for all PKU patients and thereby have made these differences in brain
vulnerability largely disappear. The only PKU patients who have clearly shown
to have either a high or low brain vulnerability to elevated blood Phe
concentrations are those who have been late-diagnosed and have either developed
severe mental retardation or have escaped from severe mental retardation. We
hypothesize, however, that PKU patients with a low brain vulnerability to
elevated blood Phe concentrations, who are early-diagnosed and have been
treated with a phenylalanine restricted diet from birth, may manifest with
behavioural problems due to brain deficiencies of essential amino acids other
than phenylalanine. As behavioural problems such as ADHD or autism spectrum
disorders are especially observed in some PKU children, this requires specific
PKU children to be included as well.