QUT Digital Repository: Biochemical markers associated with executive function in adolescents with early and continuously treated Phenylketonuria (ECT-PKU): a preliminary study. Rachael

QUT Digital Repository: http://eprints.qut.edu.au/

Sharman, Rachael R. and Sullivan, Karen A. and Young, Ross McD. and McGill, Jim (2009) Biochemical markers associated with executive function in adolescents with early and continuously treated Phenylketonuria (ECT-PKU). Clinical Genetics, 75(2).

© Copyright 2009 Blackwell Publishing Ltd.

Executive function in ECT-PKU 1

Running Head: Executive function in adolescents with PKU

Biochemical markers associated with executive function in adolescents with early and

continuously treated Phenylketonuria (ECT-PKU): a preliminary study.

Rachael Sharman1,2, Karen Sullivan1,2, Ross Young1 & Jim McGill3

1 = Institute of Health and Biomedical Innovation,

Queensland University of Technology, Queensland, Australia

2 = School of Psychology and Counselling,

Queensland University of Technology, Queensland, Australia

3 = Department of Metabolic Medicine, Royal Children’s Hospital,

Queensland, Australia.

Correspondence concerning this article should be addressed to Dr Karen Sullivan at

the School of Psychology and Counselling, Queensland University of Technology,

Carseldine Campus, Carseldine Q 4034, AUSTRALIA. telephone 617 3138 4609 |

fax 617 3138 4660 email [email protected]


Abstract

Debate continues as to whether executive function (EF), develop normally in children

with PKU. Using a mixed model, we measured EF in ten adolescent children with

PKU and six sibling controls, and examined associations between cognitive function

and phenylalanine (phe) as well as the phenylalanine:tyrosine ratio (phe:tyr).

Measurements were taken on two occasions anticipated to yield variation in

concurrent phe resulting from changes in dietary compliance (i.e. Christmas holiday

versus non-holiday period). A repeated measures ANOVA using Behaviour Rating

Inventory of Executive Function (BRIEF) scores yielded the following results: no

significant interactions; two significant group effects of substantially impaired

working memory and initiation skills in children with PKU than controls irrespective

of occasion; and, two significant time effects, suggestive of slightly poorer “non-

holiday” planning and organisation scores in both PKU and control groups. Further

analyses revealed that phe levels were not significantly different at the time occasions

we sampled, suggesting that holiday dietary compliance may be better than expected.

Correlations between EF and biochemistry in children with PKU showed that the

participant’s phe:tyr ratio across their lifetime and particularly prior to age 12 years,

demonstrated the greatest number of significant and strong positive correlations

across most scales of EF impairment.

KEYWORDS: Phenylketonuria; Executive Function; Phenylalanine:Tyrosine Ratio;

Adolescents; Dietary Compliance; BRIEF


Biochemical markers associated with executive function in children with early and

continuously treated Phenylketonuria (ECT-PKU): a preliminary study

Approximately 1 in 14,500 live births in Australia will be diagnosed with

phenylketonuria (PKU; 1) an inborn error of metabolism detected in Australia by

newborn screening. Elsewhere the incidence of PKU in Caucasian populations of

European descent has been reported as approximately 1 in 10,000 (2). PKU is of

particular interest because of its potential to contribute to our understanding of the

development of frontal lobe function, and continuing debate regarding the potential

for “normal” cognitive development in affected individuals.

PKU is a deficiency in the liver enzyme phenylalanine hydroxlase; which

results in very high levels of the amino acid phenylanine (phe) and low levels of

tyrosine. Phe is toxic to the developing brain, resulting in major neurological damage,

particularly to white matter tracts that develop post-natally. Low tyrosine is

hypothesised to lead to low dopamine production (as tyrosine is a precursor to

dopamine). The introduction of newborn screening for PKU has enabled earlier

detection and treatment (phe restricted diet) which has lead to dramatic improvements

in the neuropsychological function of people with PKU. Specifically, the severe and

inevitable intellectual impairment that developed as a consequence of untreated PKU

(3) no longer occurs in early and continuously treated (ECT) PKU. Research since

has focused on whether, and to what extent, cognitive function remains impaired in

individuals with this disorder (4, 5). Current evidence suggests that children with

ECT-PKU can achieve at least average IQs, although these scores may be slightly

lower than those of their siblings and parents (5,6). However, it has been suggested

that higher level cognitive skills, particularly executive functions, might be impaired

in children with ECT-PKU (5,7) with deficits persisting into adulthood (8).


Executive functioning (EF) describes abilities in goal directed behaviour,

inhibition of automatic responses, self-regulation, task-switching, integration of

information across space and time and working memory. Deficits in executive

function in children with ECT-PKU are more likely than in controls (5,7), in

particular deficits in working memory (9; 10) and processing speed (10,11). These

residual EF deficits lead to a prevalence rate of attention deficit disorder five times the

norm in children with ECT-PKU (12).

The mechanism hypothesised to underlie executive function deficits in

children with ECT-PKU remains largely biochemical, in that greater phe exposure

leads to compromised executive function (9,10,13). However, two studies have

identified that the ratio of blood phe to tyrosine (phe:tyr ratio) has been shown to be a

potentially more influential determinant of EF outcomes rather than phe-only

measures (9, 14).

The phe:tyr ratio effect on EF is due to a hypothesised link to dopamine levels,

such that a higher phe:tyr ratio may lead to a lower dopamine environment for the

developing brain, compared to high phe alone. Phenylalanine and tyrosine are both

large neutral amino acids and share the same transporter mechanisms to cross the

blood brain barrier. Very high phe inhibits tyrosine transport into the brain so less is

available for dopamine formation. Increased plasma levels of tyrosine may not

improve this competition for transport if phenylalanine is also increased. A low

phe:tyr ratio occurs when the plasma concentration of tyrosine is high relative to

phenylalanine. Despite findings that the phe:tyr ratio may be an important

determinant of function, the impact of increasing dietary tyrosine to improve the ratio,

and therefore EF, has not been widely investigated and findings so far have been

inconsistent (15,16).


There remains a need for further studies of EF deficits among children with

ECT-PKU (4), particularly those utilising longitudinal rather than cross-sectional

designs (8) and with ecologically valid executive function tests (17 – 19). There are

also differences in the way studies operationalise “phe exposure”. The majority of

studies have used phe recorded at the time of neuropsychological testing variously

referred to as “concurrent” phe (20) or “current” phe (8). However, it may be also

important to study “recent” phe exposure (i.e. an accumulation over the previous

weeks or months) since short term phe fluctuations have been shown to affect

cognition (21). Studies of the effects of lifetime phe exposure, also referred to as

“historical” phe, have also been reported (8,20) and these studies also show strong

relationships with cognitive variables.

The possibility that the PKU cognitive profile might be partly underpinned by

structural defects such as hypomyelination has also been suggested (20, 22).

Evidence for abnormalities of this type comes from animal studies (23), as well as

human imaging (8, 20, 24) and histomorphological dissection studies (25). Review

studies suggest that the neuroanatomical structures susceptible to developmental

defects in PKU include those white matter tracts that develop post-natally, such as

those of the corpus callosum and subcortical white matter (22). More recent imaging

studies have implicated areas such as the cerebrum and hippocampus (26). Consistent

associations between neuroanomatical anomalies and metabolic parameters have

been difficult to demonstrate (20, 26, 27), and there is debate as to their possible

reversibility (28). Interestingly, where neuroanatomical abnormalities in people with

PKU have been identified, the structures affected typically include those implicated in

mediating deficits seen on neuropsychological examination and may provide another


explanation for poor performance that is independent of, or acts in conjunction with,

effects due to neurochemical dysfunction.

Given the ongoing debate regarding the exact nature and severity of EF

deficits in ECT-PKU children, and a continuing need to better understand the

biochemical factors that might contribute to such deficits, the aim of this study was to

examine executive function in ECT-PKU children across two different time periods

(holiday versus non-holiday) intended to capture fluctuations in concurrent phe. Our

hypothesis predicted that the stringent phe-restricted diet would be more difficult to

maintain during the Christmas holiday period; which would in turn lead to higher

concurrent phe and subsequent deterioration in EF. A further purpose of this study

was to determine which metabolic indicators including phe and tyrosine based indices

were associated with EF in children with ECT-PKU.

Method

Participants

The families of twenty-nine children with a diagnosis of ECT-PKU between

the ages of eight and 17 years were mailed a study kit, comprising study details and

questionnaires, via the Royal Children’s Hospital, Brisbane, Queensland, Australia.

PKU diagnosis was based on plasma phenylalanine > 800 mmol/L at newborn

screening. Participants were also found to have: a) commenced treatment

immediately after diagnosis; b) continued treatment up until the time of data

collection (i.e. no discontinuation of diet/supplementation) and; c) undertaken regular

(approximately monthly) blood checks. Participants were also required by the

administration of another psychological test instrument (results not reported in this

paper) to have a minimum reading age of eight years. Therefore, this sample is likely


to be positively skewed in terms of functional IQ as no severely affected participants

were included.

Parents were asked to select a sibling between eight and 17 years of age and

closest in age to the child with PKU as a control subject. The purpose of including a

sibling group in this study was primarily to provide a control for score variation that

might occur over time as a result of family and/or environmental effects associated

with Christmas holiday versus non-holiday behaviours.

Thirteen families with children with ECT-PKU responded, and eight of those

families provided data for a non ECT-PKU child (sibling control); however, data from

one sibling control was excluded from analysis due to failure to comply with study

instructions; and data from one ECT-PKU participant was rejected because this

participant was suspected of having an exceptional form of PKU1. With these

exclusions, the “Christmas holiday” sample comprised 12 children with ECT-PKU

and 7 sibling controls. Children in the ECT-PKU group were slightly older (age M =

14.4 years, SD = 2.08) than sibling controls (age M = 13.1 years; SD =2.57, although

this difference was not significant, t(17) = 1.18, p = .253. The ratio of males to

females was higher in the ECT-PKU group (3:1) than sibling controls (approximately

1:1).

Given comorbid psychological diagnosis in children with ECT-PKU is not

uncommon, we also collected a brief medical history. No control sibling was

identified by their parent as having a historical or current diagnosis of ADHD, autism

1 Data on this participant’s phe levels prior to age six years were missing; average phe level between six to 12 years of age was over double recommended Australian maximum at 735μmol/L; average phe after age 12 years over double recommended maximum at 1013 μmol/L, yet parents and treating metabolic physician reported normal cognitive function. That is, this case had no known cognitive or neurological deficiencies in the context of relatively high phe exposure. Koch et al., (2000) have recommended that such cases be regarded as “exceptional” (p. 1093), suggesting that they may represent an atypical form of PKU. Given the focus of this study on classical PKU, this case was excluded from data analysis.


spectrum disorder (ASD), learning difficulty, depression or anxiety. Compared to

four (33%) of the children with ECT-PKU, who were identified by their parents as

having a historical or current diagnosis of psychological/learning difficulties. One of

those four children was also identified as having ADHD and ASD and was on current

medication (Ritalin).

Three months later participants were approached for a second time and asked

to complete the same measures. Ten of the twelve eligible children with ECT-PKU

and their siblings participated in this phase of the study, yielding one group of ten

follow-up participants with ECT-PKU (8 males; age M = 14.4 years, SD = 2.16) and

six sibling controls (4 male; age M = 14.0 years, SD = 2.21) respectively. The

treatment characteristics of ECT-PKU participants included in the final sample are

shown in Table 1. When compared to Australian treatment guidelines (29), the phe

levels recorded for this group, reveal them to be a relatively adherent group, with the

mean phe level for “under 12s” just over the recommended Australian maximum for

this age group (i.e., <360 umol), and the mean phe level for “over 12’s” just under the

upper limit for this age group (i.e., <500 umol).

Insert Table 1 about here

Materials

Executive function

Executive function was assessed by parent questionnaire – the Behaviour

Rating Inventory of Executive Function (BRIEF; 30). The BRIEF is a standardized

clinical measure of children’s executive function designed to reflect the

multidimensionality of this construct. Since its publication in 2000, the BRIEF has


been used widely in research applications to assess executive function in children

with: PKU (11), traumatic brain injury (31,32) ADHD and Tourette syndrome (33)

and other acquired developmental disorders (34, 35).

The BRIEF parent questionnaire contains 86 statements regarding the child’s

behaviour. Parents were instructed to rank each statement against instances of their

child’s behaviour during the preceding “two weeks”, given this study’s focus on

concurrent phe and recent instances of behaviour. Statements are ranked on a “never,

sometimes, often” basis and assess a wide range of executive functions reflected in

eight “scales”: working memory, initiation, shifting, inhibition, planning and

organisation, organisation of materials, emotional control and monitoring. These

scales contribute to three index scores: the Global Executive Composite (GEC), which

is composite of each and every BRIEF scale score; the Behavioral Regulation Index

(BRI), comprising emotional control, inhibit and shift scale scores; and the

Metacognition Index (MI), which is a composite of initiate, working memory,

planning/organisation, organisation of materials, and monitoring. BRIEF scores have

good internal consistency and test reliability (33) and the BRIEF provides an

ecologically valid indication of EF of how the child actually functions in their

environment on a day to day basis.

T-scores were calculated to provide a clinically relevant measure of function,

corrected for age and sex. A T-score of 50 is considered average, with higher scores

representing worse executive functioning; a T-score above 65 indicates clinical

impairment.


Biochemistry

Measures of concurrent phe and tyrosine as well as the phe:tyr ratio were

collected for participants with ECT-PKU via guthrie card. These scores were

regarded as “concurrent” because participants were instructed to complete

questionnaires on the same day they provided their blood sample for analysis.

Lifetime phe exposure, lifetime tyrosine exposure, and the lifetime phe:tyr

ratio were calculated by averaging the blood test results for each participant obtained

from the regular blood tests from birth (excluding the original blood result used for

diagnosis) to the end of 20052. Finally, the average level of phe recorded for the year

preceding data collection (i.e., 2005) was calculated for each ECT-PKU participant

using archived laboratory records.

Procedure

Data were collected on two occasions chosen for their potential to yield

different concurrent phe levels. Participants were asked to complete the first set

questionnaires in the first week of January 2006; a period selected because of its

proximity to Christmas and New Year, thought to be associated with poorer dietary

compliance and thus higher phe levels. Participants were asked to complete the

second set of questionnaires in March 2006, hypothesised to be a relatively benign

period in terms of dietary compliance.

2 One participant had significant missing data (first six years of their life), having moved to Queensland from interstate, so was excluded from analyses regarding effects of “lifetime” levels. This is because phe levels during infancy and early childhood are usually significantly lower than later years so any “lifetime average” levels calculated missing the first six years are likely to be substantially inflated.


Results

The performance of ECT-PKU and control participants who completed

executive function questionnaires on both occasions is shown in Table 2. This table

shows descriptive statistics for the eight BRIEF scale scores and BRIEF index scores.

To test for statistically significant differences across time (holiday vs non-

holiday), between groups (control vs ECT-PKU), and as a product of time by group

interactions, a series of repeated measures ANOVAs were run. For each analysis, the

dependent measure was derived from the BRIEF (separate ANOVAs were run for

each scale score and index). There were two independent variables for each ANOVA.

These were (a) time, a within groups factor with two levels (Christmas holiday and

non-holiday) and (b) group, a between subjects factor with two levels (control versus

ECT-PKU). With alpha set at 0.05, four significant effects were observed (see Table

2 for full ANOVA results, including group means and standard deviations). Two of

the four significant effects were time main effects suggesting deterioration over time

in parents’ ratings of their children’s planning and organisational behaviour

respectively, but this effect was clinically weak (in the order of three or four scale

points, on average). The third and fourth effects were a significant group effect on the

BRIEF initiate and working memory scales (see Table 2). On closer inspection, these

group effects were due to worse reported performance by ECT-PKU participants than

controls, irrespective of occasion. On average, ECT-PKU participants were separated

from controls by more than one standard deviation on these scales (T score difference

of 10 = 1 SD). Finally, no significant group by time interactions were found,

suggesting that the performance of ECT-PKU children was not differentially affected

over time, compared to controls.



To further explore these findings, a series of exploratory analyses focused on

the biochemical status of ECT-PKU children was conducted. Given that we expected

ECT-PKU children’s performance to be worse in January compared to March

(because we assumed dietary compliance would be more difficult during holidays),

we undertook a comparison of concurrent biochemical markers (phe, tyr, and phe:tyr

ratio) over time. Using a within subjects t-test to test for differences in concurrent phe

between holiday and non-holiday periods, no significant differences were found, t(9)

= 1.029, p = .331. Further, when compared with historical measures, no significant

differences between phe in 2005 and our two testing occasions were found. Even

when adjusted for age/different dietary restrictions (i.e. using historical phe levels

after age 12 when a less restrictive diet is recommended in Australia), no significant

differences were observed between historical phe (post 12 years of age only) and

concurrent phe in January, t (7) = 1.886, p = .101, or March, t (7) = .819, p = .440.

Suffice to say these analyses demonstrated that the concurrent phe levels collected for

this study at both holiday and non-holiday periods appeared relatively consistent with

historical phe levels during the preceding year and across the children’s lifetime.

Concurrent biochemistry and executive function

To explore the relationship between biochemical markers and performance on

measures of executive function, a series of correlations were performed. The results

of correlational analyses revealed only two significant associations between

concurrent biochemical markers and performance on the BRIEF scale and index

scores. Specifically, we found a significant correlation between concurrent phe at


time one3 and the BRIEF scales working memory, r (12) = .64, and planning, r (12) =

.73 suggesting that higher concurrent phe levels were associated with worsening

performance on these scales

Correlational analyses using historical biochemical markers were also

undertaken and significant results are shown in Table 3. Notably, we found no

significant correlations between our “recent” (averaged across 2005) measures of

phe:tyr or tyrosine and executive function.


Table 3 indicates the extent of significant positive correlations between

executive function and the biochemical markers lifetime phe:tyr, phe:tyr prior to 12

years, and to a lesser extent, lifetime phe, suggesting that longer exposure to

biochemical markers underpin worsening executive function. The overall pattern of

results was similar regardless of whether time one or time two executive function

scores were used. The only significant association with lifetime tyr was a negative

association with the EF scale of ‘organise’. Importantly, working memory and initiate

scores, the only scores identified as significantly different between ECT-PKU

participants and controls in this study, were significantly and positively correlated

with historical phe:tyr indices.

Discussion

Taking the significant time effects first, we found a modest increase in parent

ratings of poorer non-holiday versus holiday EF in the scales of plan and organise

across both groups (PKU and controls). The result was clinically weak but achieved

statistical significance due the power of the design (repeated measures) and because

3 The correlation between concurrent phe and these scales of the BRIEF at time two were not significant.


each and every parent rated their children’s EF on these scales as slightly worse

during the school term. A number of questions in the BRIEF relate to school

performance, so it may be that parents unanimously rated their children’s performance

as slightly worse during the school term due to recent instances of such behaviour,

that were not evident during the Christmas holiday period.

The primary purpose of this study was to explore differences in the executive

function in children with ECT-PKU (a) across two different time periods (i.e., a

comparison of performance measured at a period expected to be associated with poor

dietary compliance and higher concurrent phe, with a period of usual dietary

compliance/ lower concurrent phe), and; (b) compared to sibling controls.

The predicted changes of elevated phe levels in the ECT- PKU group as a

consequence of Christmas holiday occasion was not found. This expectation was

based on reports that adherence with PKU dietary guidelines can be more difficult for

patients and their families during holiday/celebratory periods (36). The similarity of

holiday and non-holiday phe levels, and their similarity to age-adjusted historical

measures may suggest that improvements in dietary supplements (formulas) and in the

availability of low phe foods and recipes or advances in patient education and

counselling to support to dietary control (37) has made it easier for families with

children with ECT-PKU to manage dietary restrictions at such times of year.

The results of group comparisons of executive function in children with ECT-

PKU and sibling controls revealed that EF scales of initiate and working memory

differed significantly between groups, with substantially worse function (more than

one standard deviation apart on average) among children with ECT-PKU compared to

controls on both occasions. However, the group results are limited by a small sample

size and underpowered analyses on all EF scales bar working memory. It is possible


that impairment on other measures of EF would be demonstrated with a larger sample

size (e.g. note the large mean differences observed between controls and ECT-PKU

on Plan and Metacognition Index). Nonetheless, our results support the proposition

that working memory in particular, continues to show the strongest and most robust

impairment in children with ECT-PKU, even within such a small sample.

We explored the relationship between executive function and a broader range

of biochemical markers than has been used previously. The majority of past studies

have focused on associations between behaviour/executive function and concurrent

and/or historical phe only, and our inclusion of measurements of tyrosine, both

concurrently and historically, extends previous research paradigms that have not

explored the impact of tyrosine specifically.

Overall our results provide relatively little support for the idea that concurrent

biochemistry is strongly and consistently related to EF performance. This finding is

consistent with several previous reports (8, 20, 38, 39) but not others (9, 40-42). This

inconsistency may be a result of differences in EF measurement (ecologically valid

tasks versus laboratory based measures), phe (mean or median phe levels or summary

phe scores based on variable blood collection regimes), participant age and treatment

history (compliant versus non-compliant samples assessed on or before possible

critical periods for phe exposure, or a combination thereof - see (43-45) for a fuller

discussion of some of these issues). Our analysis of concurrent versus recent versus

lifetime biochemistry in this sample of adolescents demonstrated that phe levels were

highly consistent across the lifespan. This may explain why some studies that focus

on concurrent phe only continue to show significant correlations, as in our sample

concurrent phe was shown to be a good measure of lifetime phe, and this has been

demonstrated by others (46, 47).


The most consistent pattern of associations we found was between EF and

participant’s lifetime and < 12 years of age phe:tyr ratio. The EF measures that

proved most impaired between controls and children with ECT-PKU (Working

Memory and Initiate) showed strong and significant correlations with historical

phe:tyr measures. Interestingly, recent (2005) and concurrent phe:tyr showed no

relationship whatsoever with measures of EF, and given the strongest correlations

were observed between phe:tyr below age 12 years and EF, this may indicate a

potential sensitive period in brain development that needs further follow up.

We identified one significant negative correlation between lifetime tyrosine

and the Organise scale, suggesting that tyrosine on its own is not usually associated

with performance on executive function tasks in children with ECT-PKU.

Nonetheless our one significant negative correlation was in the direction predicted by

the dopamine theory i.e. lower tyrosine = higher EF impairment. The reason why

tyrosine itself is neither strongly nor consistently related to executive function, but the

phe:tyr ratio is, remains unclear. Increased plasma levels of tyrosine may not improve

the competition for transport if phenylalanine is also increased. A low phe:tyr ratio

occurs when the plasma concentration of tyr is high relative to phenylalanine and this

should allow more tyrosine to cross into the brain (48).

Overall the results of this study add to the body of literature that suggests that

specific aspects of executive function may be impaired relative to controls in children

and adolescents with ECT-PKU, especially working memory. In terms of

contributing factors to EF impairment in children with ECT-PKU, the strongest

association we found was between selected historical biochemical parameters

(especially phe:tyr ratio) and performance, though future studies are needed to

identify if and how these factors may be linked causally. Although our sample was


small in terms of absolute numbers of participants, the effect sizes observed and

strength of association with biochemical markers were generally robust. In addition,

the results of this study, by accounting for a broader range of historical biochemical

parameters than has been studied previously, has strongly suggested that phe:tyr data

should be taken into consideration in the design of future studies.


Acknowledgements

The study reported was approved by the Queensland University of Technology’s Human

Research Ethics Committee (ref no. 4294H) and Royal Children’s Hospital and Queensland

Health Service District Ethics Committee. All participants signed written consent forms

informing them of our intention to publish results. Financial assistance for this project was

provided by the School of Psychology and Counselling QUT (AUD $2300) and the Metabolic

Dietary Disorders Association (AUD $500). The authors would like to thank metabolic nurse

Ms Anita Inwood for assistance during the data collection phase of this project, Dr Toni Jones

who contributed to early discussions regarding the measurement of executive function in

children with PKU, and QUT School of Psychology and Counselling biostatistician Dr

Patricia Obst.

Parts of this project have previously been presented at:

Sharman R, Sullivan KA, Young R, McGill J. Biochemical markers associated

with executive function in children with early and continuously treated phenylketonuria.

Paper presented at Australian Psychological Society conference, Brisbane, October 2007.

Sharman R, Sullivan KA, Young R, McGill J. Biochemical markers associated

with executive function in children with early and continuously treated phenylketonuria.

Paper presented at Australian College of Clinical Neuropsychologists conference,

Mooloolaba, September 2007.

Sharman R, Sullivan KA, Jones T, Inwood A, McGill, J. Attention Deficit

Hyperactivity Disorder, executive function and depression in children with early and

continuously treated phenylketonuria. Poster presented at the 10th International Congress

of Inborn Errors of Metabolism, Tokyo, Japan, September 2006.

Sharman R, Sullivan KA, Jones T, McGill J. ADHD, executive function and

depression in children with early and continuously treated phenylketonuria. Paper

presentation of wave one data at the Inaugural Developmental Cognitive Neuropsychology

Conference, Westmead, Sydney, March, 2006.


References

1. Wiley V, Greed L, Francis I, Ranieri, E, Thomas A, Pitt J, et al. 5 year audit of newborn screening in Australia. J Inherit Metab Dis 2006; 29 Suppl 1: 86.

2. Eisensmith RC, Woo SL. Population genetics of phenylketonuria. Acta Paediatr 1994; 407 Suppl: 19-26.

3. Pitt D, Danks, D. The natural history of untreated phenylketonuria over 20 years. J Paediatr Child Health 1991; 27:189-190.

4. Griffiths P. Neuropsychological approaches to treatment policy issues in phenylketonuria. Eur J Pediatr 2000; 159 Suppl 2: 82-6.

5. Welsh MC, Pennington BF, Ozonoff S, Rouse B, McCabe ERB Neuropsychology of early treated phenylketonuria: specific executive function deficits. Child Dev 1990; 61:1697- 1713.

6. Weglage J, Pietesch M, Funders B, Koch HG, Ullrich K. Deficits in selective attention processes in early treated children with phenylketonuria – result of impaired frontal lobe function? Eur J Pediatr 1996; 155: 200-4.

7. Smith ML, Klim P, Hanley WB. Executive function in school aged children with phenylketonuria. J Dev Phys Disabil 2000;12: 317-332.

8. Brumm VL, Azen C, Moats RA, Stern AM, Broomand C, Nelson M, et al. Neuropsychological outcome of subjects participating in the adult PKU collaborative study: A preliminary review. J Inherit Metab Dis 2004; 27: 549-66. 9. Diamond A, Prevor MB, Callender G, Druin DP. Prefrontal cortex cognitive deficits in children treated early and continuously for PKU. Monogr Soc Res Child Dev 1997; 62(4): i-iv. 10. Huijbregts SC, de Sonneville LM, van Spronsen FJ, Licht R, Sergeant JA. The neuropsychological profile of early and continuously treated phenylketonuria: orienting, vigilance, and maintenance versus manipulation-functions of working memory. Neurosci Biobehav Rev 2002; 26(6): 697-712. 11. Antshel KM, Waisbren SE. Timing is everything: executive functions in children exposed to elevated levels of phenylalanine. Neuropsychology 2003; 17(3): 458-68. 12. Arnold GL, Vladutiu CJ, Orlowski CC, Blakely EM, DeLuca J. Prevalence of stimulant use for attentional dysfunction in children with phenylketonuria. J Inherit Metab Dis 2004; 27(2):137-43. 13. Leuizzi V, Pansini M, Sechi E, Chiarotti F, Carducci C, Levy G et al. Executive function impairment in early-treated PKU subjects with normal mental development. J Inherit Metab Dis 2004; 27:115-25. 14. Luciana M, Sullivan J, Nelson CA. Associations between phenylalanine-to-tyrosine ratios and performance on tests of neuropsychological function in adolescents treated early and continuously for phenylketonuria. Child Dev 2001; 72(6): 1637– 53.


15. Lykkelund C, Nielsen JB, Lou HC, Rasmussen V, Gerdes AM, Christensen E et al. Increased neurotransmitter biosynthesis in phenylketonuria induced by phenylalanine restriction or by supplementation of unrestricted diet with large amounts of tyrosine. Eur J Pediatr 1988; 148: 238–45. 16. Smith ML, Hanley WB, Clarke JT, Klim P, Schooneheyt W, Austin V et al. Randomised controlled trial of tyrosine supplementation on neuropsychological performance in phenylketonuria. Arch Dis Child 1998; 78:116–21. 17. Anderson P. Assessment and development of executive function during childhood, Child Neuropsychol 2002; 8(2): 71 – 82. 18. Gioia GA, Isquith PK. Ecological Assessment of Executive Function in Traumatic Brain Injury. Dev Neuropsychol 2004; 25:135-58. 19. Sbordone RJ. Ecological validity issues in neuropsychological testing. Brain Injury Source 2000; 4: 10 -12. 20. Anderson PJ, Wood SJ, Francis DE, Coleman L, Warwick L, Casanelia S. et al. Neuropsychological functioning in children with early-treated phenylketonuria: impact of white matter abnormalities. Dev Med Child Neurol 2004; 46: 230-8. 21. Huijbregts SC, de Sonneville LM, Licht R, van Spronsen FJ, Verkerk PH, Sergeant JA. Sustained attention and inhibition of cognitive interference in treated phenylketonuria: associations with concurrent and lifetime phenylalanine concentrations. Neuropsychologia 2002; 40(1): 7-15. 22. Dyer CA. Pathophysiology of phenylketonuria. Ment Retard Dev Disabil Res Rev 1999; 5: 104-13. 23. Reynolds R, Burri R, Mahal S, Herschkowitz N. Disturbed myelinogenesis and recovery in hyperphenylalaninemia in rats: an immunohistochemical study. Exp Neurol 1992; 115: 347-67. 24. Moller HE, Weglage J, Bick U, Wiedermann D, Feldmann R, Ullrich K. Brain imaging and proton magnetic resonance spectroscopy in patients with phenylketonuria. Pediatrics 2003; 112:1580–1583. 25. Bauman ML, Kemper TL. Morphologic and histoanatomic observations of the brain in untreated human phenylketonuria. Acta Neuropathol 1982; 58(1): 55-63. 26. Pfaendner NH, Reuner G, Pietz J, Jost G, Rating D, Magnotta VA et al. MR imaging-based volumetry in patients with early-treated phenylketonuria. Am J Neuroradiol 2005; 26: 1681-5. 27. Koch R, Burton B, Hoganson G, Peterson R, Rhead W, Rouse B et al. Phenylketonuria in adulthood: a collaborative study. J Inherit Metab Dis 2002; 25(5) 333-46.


28. Cleary MA, Walter JH, Wraith JE, White F, Tyler K, Jenkins JP. Magnetic resonance imaging in phenylketonuria: reversal of cerebral white matter change. J Pediatr 1995; 127: 251-25. 29. Australian Society for Inborn Errors of Metabolism. The PKU Handbook. Alexandra, Victoria: Human Genetic Society of Australia; 2005. 30. Gioia GA, Isquith PK, Guy SC, Kenworthy L. Behaviour Rating Inventory of Executive Function, Professional Manual. Psychological Assessment Resources Inc: Lutz, FL; 2000. 31. Mangeot A, Armstrong K, Colvin AN, Yeates KO, Taylor HG. Long-term executive function deficits in children with traumatic brain injuries: assessment ising the Behavior Rating Inventory of Executive Function (BRIEF). Child Neuropsychol 2002; 8: 271–84. 32. Vriezen ER, Pigott SE. The relationship between parental report on the BRIEF and performance-based measures of executive function in children with moderate to severe traumatic brain injury. Child Neuropsychol 2002; 8: 296–303. 33. Mahone EM, Cirino PT, Cutting LE, Cerrone PM, Hagelthorn KM, Hiemenz JR et al. M.B. Validity of the behavior rating inventory of executive function in children with ADHD and/or Tourette syndrome. Arch Clin Neuropsychol 2002; 17: 643-62. 34. Baron SI. Test Review: Behaviour Rating Inventory of Executive Function. Child Neuropsychology 2000; 6: 235-238. 35. Gioia GA, Isquith PK, Kenworthy L, Barton RM. Profiles of everyday executive function in acquired and developmental disorders. Child Neuropsychol 2002; 8: 121 – 37. 36. Bilginsoy C, Waiztman N, Leornard C, Ernst S. Living with phenylketonuria, perspectives of patients and their families. J Inherit Metab Dis 2005; 28(5): 639–45. 37. MacDonald, A. Diet and compliance in phenylketonuria. Eur J Pediatr 2000; 159 Suppl 2: 136–41. 38. Mazzocco MMM, Nord AM, van Doorninck WJ, Greene CL, Kovar CG, Pennington BF. Cognitive development in children with early-treated phenylketonuria. Dev Neuropsychol 1994; 10: 133-51. 39. White DA, Norton MJ, Mandernach T, Huntington K, Steiner RD. Age-related working memory impairments in children with prefrontal dysfunction associated with phenylketonuria. J Int Neuropsychol Soc 2002; 8: 1 -11. 40. Brunner RJ, Jordan MK, Berry HK. Early-treated phenylketonuria: neuropschologic consequences. J Pediatr 1984; 102: 831-5. 41. Koch R, Azen C, Friedman EG, Williamson ML. Paired comparison between early treated phenylketonuria children and their matched sibling controls on intellectual and school achievement test results at eight years. J Inherit Metab Dis1984; 7: 86-90. 42. Weglage J, Funders B, Wilken B, Schubert B, Ullrich K. School performance and intellectual outcome in adolescents with phenylketonuria. Acta Paediatrica 1993; 82: 582-6.


43. Schweitzer-Krantz S, Burgard P. Survey of National guidelines for the treatment of phenylketonuria. Eur J Pediatr 2000; 159 Suppl 2: 70-3. 44. Walter J, White F, Hall S, MacDonald A, Rylance G, Boneh A et al. How practical are recommendations for dietary control in phenylketonuria? The Lancet 2002; 360: 55-7. 45. Wappner R, Cho S, Kronmal RA, Schuett V, Seashore, M. Management of phenylketonuria for optimal outcome: a review of guidelines for phenylketonuria management and a report of surveys of parents, patients, and clinic directors. Pediatrics 1999, 104: 68. 46. Griffiths P, Campbell R, Robinson. Executive function in treated phenylketonuria as measured by the one-back and two-back versions of the continuous performance test. J Inherit Metab Dis 1998; 21: 125-135. 47. VanZutphen KH, Packman W, Sporri L, Neeham MC, Morgan C, Weisiger K, Packman S. Executive functioning in children and adolescents with phenylketonuria. Clinical Genetics 2007; 72: 13-18. 48. Koch R, Moseley KD, Yano S, Nelson M, Moats R. Large neutral amino acid therapy and phenylketonuria: a promising approach to treatment. Mol Genet Metab 2003; 79:110-3.


Table 1.

Treatment Variables and Phe Exposure (in µmols) of ECT-PKU Participants.

Phe levels N % of sample within recommended

guidelines

M SD min Max

Continuity of treatment / Frequency of blood test (weeks):

prior to age 12 after age 12

10

-

4.5

5.8

-

2.3

3.3

8.0

9.6

Concurrent phe (January, 2006) 10 40 601.30 204.95 270.00 970.00

Concurrent phe (March, 2006) 10 70 478.00 274.26 210.00 1100.00

Average phe aged < 12 years 10 70 383.00 96.91 267.00 580.00

Average phe aged >12 years 8 62.5 491.75 127.83 367.00 742.00

Lifetime phe 10 - 395.80 102.83 276.00 626.00

Note: Phe levels and continuity of treatment data prior to, and after, age 12 are shown separately because allowable limits of phe change at this age, according to current Australian PKU management guidelines (ASIEM, 2005). Continuity of treatment data was based on information current as of January 2006. Standard deviations for “blood test frequency” were not available for this study.


Table 2.

Descriptive Statistics and Results of Repeated Measures Analysis of Variance for ECT-PKU (N = 10) and Sibling Control Participants

(N = 6 using T-scores from the Behaviour Rating Inventory of Executive Function (BRIEF) at two time periods.

BRIEF Score Group January March F test Scales

Inhibit ECT-PKU 54.30 (14.38) 58.40 (17.59) Control 55.83 (8.54) 53.50 (8.36)

Shift ECT-PKU 53.60 (13.41) 58.10 (14.82) Control 45.33 (6.28) 48.67 (12.16)

Emotional ECT-PKU 56.40 (12.70) 57.3 (15.31) Control 58.17 (6.99) 57.67 (11.72)

Initiate ECT-PKU 63.40 (11.25) 62.50 (11.07) Control 49.33 (9.31) 53.67 (12.40) Sig. group effect: F = 5.099; p = .04; partial eta2 = .267; η2 = .557

Work Mem ECT-PKU 63.50 (10.42) 64.30 (12.22) Control 48.50 (8.90) 50.17 (12.73) Sig. group effect: F= 6.790; p = .021; partial eta2 = .327; η2 = .679

Planning ECT-PKU 60.10 (11.34) 64.00 (11.12) Control 48.67 (10.30) 53.17 (10.57) Sig. time effect: F = 6.517; p = .023; partial eta2 = .318; η2 = .661

Organise ECT-PKU 54.30 (9.75) 59.20 (8.15) Control 52.50 (13.01) 55.67 (12.92) Sig. time effect F = 5.116; p = .04; partial eta2 = .268; η2 = .558

Monitoring ECT-PKU 56.70 (8.74) 61.30 (9.53) Control 52.67 (6.68) 52.83 (9.62) Indexes

BRI ECT-PKU 55.80 (14.42) 58.30 (17.72) Control 54.17 (7.86) 54.33 (10.29)

MI ECT-PKU 61.20 (9.95) 63.90 (10.42) Control 50.17 (10.18) 53.00 (12.74)


GEC ECT-PKU 60.30 (11.28) 63.10 (13.24) Control 51.83 (12.59) 53.83 (12.59) Note: Higher T-scores on BRIEF scales indicate worse performance; GEC = Global Executive Composite (GEC) of each and every BRIEF scale; BRI (Behavioral Regulation Index) is a composite score comprising emotional control, inhibit and shift; Metacognition Index (MI) is a composite score of initiate, WM, Plan/org., org materials, and monitor. η2 is observed power.


Table 3

Significant Correlations Between Historical Biochemistry (Average Phe In 2005, Lifetime Phe, Lifetime Tyr, Lifetime Phe:Tyr and

Phe:Tyr prior to age 12) and Executive Function (Behavioual Rating Inventory of Executive Function Scale and IndexT- Scores) at Two Time

Periods in 10 adolescents with ECT-PKU.

Ave phe in 2005 Lifetime phe Lifetime tyr Lifetime phe:tyr Phe:tyr prior to 12 BRIEF January March January March January March January March January March Scales

Inhibit .671* .752* .737** .883** - - - - - .655* Shift .713** .80** - .793** - - .636* .804** .763** .820**

Emotion .691* .857* - .782** - - .673* .882** .796** .914** Initiate - - - - - - - .796** .668* .841**

WM - .693* - - - - .668* .841** .695* .813** Plan - - - - - - - .689* .682* .638*

Organise - - - - -.654* - - - - Monitor - .760* - .662* - - - .803** .604* .781**

Indexes BRI .743** .819** .651* .796** - - .671* .835** .763** .863** MI - - - - - .813** .709** .786**

GEC .647* .763* - .648* - - .675* .887** .786** .879** Note: * = p <.05; ** = p < 0.01.

Documents

QUT Digital Repository: Biochemical markers associated with executive function in adolescents with early and continuously treated Phenylketonuria (ECT-PKU): a preliminary study. Rachael