Clinical status, biochemical profile and management of a single cohort of patients with arginase deficiency

Arginase deficiency is a rare autosomal recessive urea cycle disorder (UCD) caused by mutations in the ARG1 gene encoding arginase that catalyses the hydrolysis of arginine to ornithine and urea. Patients have hyperargininaemia and progressive neurological impairment but generally suffer fewer metabolic decompensations compared to other UCDs. The objective is to describe the clinical features, biochemical profile, neuroradiological findings and experience of managing children with arginase deficiency. Twenty‐year retrospective review of patient medical records at a single metabolic centre was performed. Six patients from three unrelated families were identified. Mean age at first symptom was 3.3 (1.5–9.0) years, while mean age at diagnosis was 8.8 (0.16–15.92) years. Four patients developed spastic diplegia and two of six with spastic quadriplegia with classical features including hyperreflexia, clonus and toe walking. This resulted in gait abnormalities that have been monitored using the GAITRite system and required Achilles tendon release in five children. Generalised tonic‐clonic seizures and/or absences were present in three of six children and were controlled with anticonvulsants. All patients had moderate learning difficulties. Neuroimaging showed cerebral/cerebellar atrophy in four patients and basal ganglia abnormalities in two. Arginine levels were universally elevated throughout follow‐up despite protein restriction, essential amino acid supplementation and ammonia scavengers, and neurological outcome was generally poor. Two patients died following severe metabolic decompensation in adolescence. Children with arginase deficiency continue to present a management challenge of what appears to be an inexorable course of neurocognitive impairment. Further insight into disease mechanisms may provide insight into novel treatment strategies.

100,000 Genomes Pilot on Rare-Disease Diagnosis in Health Care - Preliminary Report

BACKGROUND

The U.K. 100,000 Genomes Project is in the process of investigating the role of genome sequencing in patients with undiagnosed rare diseases after usual care and the alignment of this research with health care implementation in the U.K. National Health Service. Other parts of this project focus on patients with cancer and infection.

METHODS

We conducted a pilot study involving 4660 participants from 2183 families, among whom 161 disorders covering a broad spectrum of rare diseases were present. We collected data on clinical features with the use of Human Phenotype Ontology terms, undertook genome sequencing, applied automated variant prioritization on the basis of applied virtual gene panels and phenotypes, and identified novel pathogenic variants through research analysis.

RESULTS

Diagnostic yields varied among family structures and were highest in family trios (both parents and a proband) and families with larger pedigrees. Diagnostic yields were much higher for disorders likely to have a monogenic cause (35%) than for disorders likely to have a complex cause (11%). Diagnostic yields for intellectual disability, hearing disorders, and vision disorders ranged from 40 to 55%. We made genetic diagnoses in 25% of the probands. A total of 14% of the diagnoses were made by means of the combination of research and automated approaches, which was critical for cases in which we found etiologic noncoding, structural, and mitochondrial genome variants and coding variants poorly covered by exome sequencing. Cohortwide burden testing across 57,000 genomes enabled the discovery of three new disease genes and 19 new associations. Of the genetic diagnoses that we made, 25% had immediate ramifications for clinical decision making for the patients or their relatives.

CONCLUSIONS

Our pilot study of genome sequencing in a national health care system showed an increase in diagnostic yield across a range of rare diseases. (Funded by the National Institute for Health Research and others.).

Ten years of enzyme replacement therapy in paediatric onset mucopolysaccharidosis II in England

The outcome of 110 patients with paediatric onset mucopolysaccharidosis II (MPS II) since the commercial introduction of enzyme replacement therapy (ERT) in England in 2007 is reported. Median length of follow up was 10 years 3 months (range = 1 y 2 m to 18 years 6 month). 78 patients were treated with ERT, 18 had no ERT or disease modifying treatment 7 had haematopoietic stem cell transplant, 4 experimental intrathecal therapy and 3 were lost to follow up. There is clear evidence of improved survival (median age of death of ERT treated (n = 16) = 15.13 years (range = 9.53 to 20.58 y), and untreated (n = 17) = 11.43 y (0.5 to 19.13 y) p = .0005). Early introduction of ERT improved respiratory outcome at 16 years, the median FVC (% predicted) of those in whom ERT initiated <8 years = 69% (range = 34-86%) and 48% (25-108) (p = .045) in those started >8 years. However, ERT appears to have minimal impact on hearing, carpal tunnel syndrome or progression of cardiac valvular disease. Cardiac valvular disease occurred in 18/46 (40%), with progression occurring most frequently in the aortic valve 13/46 (28%). The lack of requirement for neurosurgical intervention in the first 8 years of life suggests that targeted imaging based on clinical symptomology would be safe in this age group after baseline assessments. There is also emerging evidence that the neurological phenotype is more nuanced than the previously recognized dichotomy of severe and attenuated phenotypes in patients presenting in early childhood.

International working group identifies need for newborn screening for mucopolysaccharidosis type I but states that existing hurdles must be overcome

Aim

Mucopolysaccharidosis type I is a lysosomal storage disorder that can result in significant disease burden, disability and premature death, if left untreated. The aim of this review was to elaborate on the diagnosis of mucopolysaccharidosis type I and the pros and cons of newborn screening.

Methods

An international working group was established to discuss ways to improve the early diagnosis of mucopolysaccharidosis type I. It consisted of 13 experts in paediatrics, rare diseases and inherited metabolic diseases from Europe and the Middle East.

Results

It is becoming increasingly clearer that the delay between symptom onset and clinical diagnosis is considerable for mucopolysaccharidosis type I and other rare lysosomal storage disorders, despite numerous awareness campaigns since therapies became available. Diagnosis currently depends on recognising the signs and symptoms of the disease. The practice of newborn screening, which is being explored by pilot programmes around the world, enables early diagnosis and consequently early treatment. However, these studies have highlighted numerous new problems and pitfalls that must be faced before newborn screening becomes generally available.

Conclusion

Newborn screening for mucopolysaccharidosis type I offers the potential for early diagnosis and early pre‐symptomatic treatment, but existing hurdles need to be overcome.

Expression of CD105 on expanded mesenchymal stem cells does not predict their chondrogenic potential

OBJECTIVE

Total bone marrow-derived mesenchymal stem cell (BMSC) populations differ in their potential to undergo chondrogenesis, with individual BMSCs differing in their chondrogenic capacity. The aim of this study was to explore the use of CD105 as a marker to isolate a chondrogenic subpopulation of BMSCs from the total, heterogeneous population.

DESIGN

BMSCs were isolated from patients undergoing total hip replacement and following expansion (Passage 1-Passage 5), CD105 expression was investigated by FACS analysis. FACS was also used to sort BMSCs based on the presence of CD105 (CD105(+)/CD105(-)) or their amount of CD105 expression (CD105(Bright)/CD105(Dim)). After 3 or 5 weeks of differentiation, chondrogenic potential was determined by thionine staining for glycosaminoglycan (GAG) content and by detection of collagen type II using immunohistochemistry.

RESULTS

Expanded total BMSC populations were composed almost exclusively of CD105(+) cells, the percentage of which did not correlate to subsequent chondrogenic potential; chondrogenic potential was observed to diminish with culture although CD105 expression remained stable. Similarly, differences in chondrogenic potential were observed between donors despite similar levels of CD105(+) BMSCs. Comparison of CD105(Bright) and CD105(Dim) BMSCs did not reveal a subpopulation with superior chondrogenic potential.

CONCLUSIONS

Chondrogenic potential of BMSCs is often linked to CD105 expression. This study demonstrates that CD105 expression on culture expanded BMSC populations does not associate with a chondroprogenitor phenotype and CD105 should not be pursued as a marker to obtain a chondroprogenitor population from BMSCs.

Response of 33 UK patients with infantile-onset Pompe disease to enzyme replacement therapy

BACKGROUND

Enzyme replacement therapy (ERT) for infantile-onset Pompe disease has been commercially available for almost 10 years. We report the experience of its use in a cohort treated at three specialist lysosomal treatment centres in the UK.

METHODS

A retrospective case-note review was performed, with additional data being gathered from two national audits on all such patients treated with ERT. The impact on the outcome of various characteristics, measured just prior to the initiation of ERT (baseline), was evaluated using logistic regression.

RESULTS

Thirty-three patients were identified; 13/29 (45%) were cross-reactive immunological material (CRIM) negative, and nine were immunomodulated. At baseline assessment, 79% were in heart failure, 66% had failure to thrive and 70% had radiological signs of focal pulmonary collapse. The overall survival rate was 60%, ventilation-free survival was 40% and 30% of patients were ambulatory. Median follow-up of survivors was 4 years, 1.5 months (range 6 months to 13.5 years). As with previous studies, the CRIM status impacted on all outcome measures. However, in this cohort, baseline failure to thrive was related to death and lack of ambulation, and left ventricular dilatation was a risk factor for non-ventilator-free survival.

CONCLUSION

The outcome of treated patients remains heterogeneous despite attempts at immunomodulation. Failure to thrive at baseline and left ventricular dilation appear to be associated with poorer outcomes.

Seizures Due to a KCNQ2 Mutation: Treatment with Vitamin B6

There is increasing evidence that vitamin B6, given either as pyridoxine or pyridoxal 5'-phosphate, can sometimes result in improved seizure control in idiopathic epilepsy. Whole-exome sequencing was used to identify a de novo mutation (c.629G>A; p.Arg210His) in KCNQ2 in a 7-year-old patient whose neonatal seizures showed a response to pyridoxine and who had a high plasma to CSF pyridoxal 5'-phosphate ratio, usually indicative of an inborn error of vitamin B6 metabolism. This mutation has been described in three other patients with neonatal epileptic encephalopathy. A review of the literature was performed to assess the effectiveness of vitamin B6 treatment in patients with a KCNQ2 channelopathy. Twenty-three patients have been reported to have been trialled with B6; in three of which B6 treatment was used alone or in combination with other antiepileptic drugs to control seizures. The anticonvulsant effect of B6 vitamers may be propagated by multiple mechanisms including direct antagonist action on ion channels, antioxidant action on excess reactive oxygen species generated by increased neuronal firing and replenishing the pool of pyridoxal 5'-phosphate needed for the synthesis of some inhibitory neurotransmitters. Vitamin B6 may be a promising adjunctive treatment for patients with channelopathies and the wider epileptic population. This report also demonstrates that an abnormal plasma to CSF pyridoxal 5'-phosphate ratio may not be exclusive to inborn errors of vitamin B6 metabolism.

Inborn Error of Metabolism (IEM) screening in Singapore by electrospray ionization-tandem mass spectrometry (ESI/MS/MS): An 8 year journey from pilot to current program

IEM screening by ESI/MS/MS was introduced in Singapore in 2006. There were two phases; a pilot study followed by implementation of the current program. The pilot study was over a 4 year period. During the pilot study, a total of 61,313 newborns were screened, and 20 cases of IEM were diagnosed (detection rate of 1:3065; positive predictive value (PPV) of 11%). Regular self-review, participation in external quality assessment and the Region 4 Genetic collaborative programs (http://www.region4genetics.org/) had led to the robust development of our current NBS MS/MS program. Overall, from July 2006 to April 2014, we screened a total of 177,267 newborns. The mean age at the time of sampling was 47.9h. Transportation of samples to the testing laboratory averaged 0.92 day. Upon receipt of sample, the NBS result was available within 1.64 days and within 3.8 days if a second tier test was required. Using absolute cut-off values in place of the initial 99th percentile reference range for the analyte markers and the introduction of two 2nd tier tests (MMA and Succinylacetone) had significantly reduced the high recall rate from an initial 1.5% during the period 2006-07 to 0.12% in 2013. The NBS MS/MS program was supported by a centralized confirmatory/diagnostic testing laboratory and a rapid response team of metabolic specialists. The detection rate was 1: 3165 (1:2727 if maternal conditions were also included). There were 23 newborns affected with organic acidemias (incidence: 1:6565), 23 with fatty acid oxidation disorders (incidence: 1:6565), and 10 with amino acidopathies (incidence 1:17,726). The performance metrics for the screening test were acceptable (sensitivity: 95.59%, specificity: 99.85%, PPV: 20%, FPR: 0.15). Participation in the NBS MS/MS program by hospitals was voluntary, and in 2013, the uptake rate was 71% of the annual births. We hope that newborn screening by MS/MS will become a standard of care for all babies in Singapore.

A distinct mitochondrial myopathy, lactic acidosis and sideroblastic anemia (MLASA) phenotype associates with YARS2 mutations

Nuclear-encoded disorders of mitochondrial translation are clinically and genetically heterogeneous. Genetic causes include defects of mitochondrial aminoacyl-tRNA synthetases, and factors required for initiation, elongation and termination of protein synthesis as well as ribosome recycling. We report on a new case of myopathy, lactic acidosis and sideroblastic anemia (MLASA) syndrome caused by defective mitochondrial tyrosyl aminoacylation. The patient presented at 1 year with anemia initially attributed to iron deficiency. Bone marrow aspirate at 5 years revealed ringed sideroblasts but transfusion dependency did not occur until 11 years. Other clinical features included lactic acidosis, poor weight gain, hypertrophic cardiomyopathy and severe myopathy leading to respiratory failure necessitating ventilatory support. Long-range PCR excluded mitochondrial DNA rearrangements. Clinical diagnosis of MLASA prompted direct sequence analysis of the YARS2 gene encoding the mitochondrial tyrosyl-tRNA synthetase, which revealed homozygosity for a known pathogenic mutation, c.156C>G;p.F52L. Comparison with four previously reported cases demonstrated remarkable clinical homogeneity. First line investigation of MLASA should include direct sequence analysis of YARS2 and PUS1 (encoding a tRNA modification factor) rather than muscle biopsy. Early genetic diagnosis is essential for counseling and to facilitate appropriate supportive therapy. Reasons for segregation of specific clinical phenotypes with particular mitochondrial aminoacyl tRNA-synthetase defects remain unknown. © 2013 Wiley Periodicals, Inc.

Mutations in the mitochondrial complex I assembly factor NDUFAF1 cause fatal infantile hypertrophic cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is frequently fatal in infancy. Mitochondrial disease causing infantile HCM is characterised by extreme biochemical and genetic heterogeneity, but deficiency of respiratory chain complex I is observed relatively frequently. Identification of the precise genetic basis has prognostic implications for the likelihood of neurological involvement.

OBJECTIVE

The authors' objective is to report two heterozygous missense mutations in the NDUFAF1 gene as a cause of fatal infantile HCM in a patient with isolated complex I deficiency.

METHODS

The authors investigated a cohort of 30 paediatric patients with complex I deficiency using biochemical and genetic approaches. The patients were clinically heterogeneous; phenotypes included HCM, Leigh syndrome, other encephalomyopathies and multisystem disease. Complex I assembly was evaluated using Blue Native polyacrylamide gel electrophoresis.

RESULTS

Sequence analysis of NDUFAF1 revealed compound heterozygous missense mutations (c.631C>T;p.Arg211Cys and c.733G>A;p.Gly245Arg) in one patient with fatal infantile HCM. These changes were absent in 240 ethnically matched control alleles. No NDUFAF1 mutations were observed in the remaining patients. Functional studies demonstrated a severe reduction in NDUFAF1 protein in Western blots of patient fibroblasts and accumulation of abnormal complex I assembly intermediates on Blue Native polyacrylamide gel electrophoresis.

CONCLUSIONS

The authors report a case of fatal infantile HCM caused by missense mutations in NDUFAF1 associated with complex I misassembly. Establishing a genetic diagnosis in mitochondrial cardiomyopathy is challenging and achieved in only a minority of cases because of complex genetics. A precise genetic diagnosis is important to provide accurate prognostic and genetic counselling advice regarding recurrence risks and to guide future reproductive options.

Myc overexpression brings out unexpected antiapoptotic effects of miR-34a

Downregulation of microRNA-34a by Myc is known to be essential for tumorigenesis and improve tumor-cell survival. Conversely, upregulation of miR-34a by p53 is thought to enhance its acetylation and activity and contribute to the pro-apoptotic effects of this tumor suppressor. We sought to determine whether restoration of miR-34a levels in B-lymphoid cells with Myc overexpression would aid therapeutic apoptosis. Unexpectedly, delivery of miR-34a, which doesn't target p53 directly, severely compromised steady-state p53 levels. This effect was preceded and mediated by direct targeting of Myc, which sustained p53 protein levels via the Arf-Hdm2 pathway. As a result, in the presence of Myc, miR-34a inhibited p53-dependent bortezomib-induced apoptosis as efficiently as anti-p53 small interfering RNA. Conversely, inhibition of miR-34a using antisense RNA sensitized lymphoma cells to therapeutic apoptosis. Thus, in tumors with deregulated Myc expression, miR-34a confers drug resistance and could be considered a therapeutic target.

Use of a long-chain triglyceride-restricted/medium-chain triglyceride-supplemented diet in a case of malonyl-CoA decarboxylase deficiency with cardiomyopathy

Malonyl coenzyme A (CoA) decarboxylase (EC 4.1.1.9, MCD) deficiency, or malonic aciduria, is a rare inborn error of metabolism characterised by a variable phenotype of developmental delay, seizures, cardiomyopathy and acidosis. There is no consensus for dietary treatment in this condition. This case describes the effect of a long-chain triglyceride (LCT)-restricted/medium-chain triglyceride (MCT)-supplemented diet upon the progress of an affected child. A full-term Asian girl of birth weight 3590 g was screened for malonic aciduria after birth due to a positive family history. She had elevated urine malonic and methylmalonic acids and was presumably homozygous for a deleterious mutation in the MLYCD gene. Her echocardiography showed mild cardiomyopathy at 0.5 months of age, but heart function was good. She was treated with carnitine 100 mg/kg per day and continued a high-energy formula feed, as her growth was slow. At 3 months of age, echocardiography showed deteriorating cardiac function with a fractional shortening of 18%. She started an angiotensin-converting enzyme (ACE) inhibitor (Captopril). Over the next few months, her diet was altered to comprise 1.9% energy from LCT, 25% from MCT and the remainder carbohydrate. Cardiac function improved and was optimal at 23 months of age, with a fractional shortening of 28% and good systolic function. During a period of low MCT intake, her cardiac function was noted to deteriorate. This reversed and stabilised following reinstatement of the diet. This case of malonic aciduria with cardiomyopathy demonstrates improvement in cardiac function attributable to LCT-restricted/MCT-supplemented diet.

RNAi-mediated germline knockdown of FABP4 increases body weight but does not improve the deranged nutrient metabolism of diet-induced obese mice

Objective:

To investigate the impact of reduced adipocyte fatty acid-binding protein 4 (FABP4) in control of body weight, glucose and lipid homeostasis in diet-induced obese (DIO) mice.

Methods:

We applied RNA interference (RNAi) technology to generate FABP4 germline knockdown mice to investigate their metabolic phenotype.

Results:

RNAi-mediated knockdown reduced FABP4 mRNA expression and protein levels by almost 90% in adipocytes of standard chow-fed mice. In adipocytes of DIO mice, RNAi reduced FABP4 expression and protein levels by 70 and 80%, respectively. There was no increase in adipocyte FABP5 expression in FABP4 knockdown mice. The knockdown of FABP4 significantly increased body weight and fat mass in DIO mice. However, FABP4 knockdown did not affect plasma glucose and lipid homeostasis in DIO mice; nor did it improve their insulin sensitivity.

Conclusion:

Our data indicate that robust knockdown of FABP4 increases body weight and fat mass without improving glucose and lipid homeostasis in DIO mice.

Changes in gait pattern as assessed by the GAITRite™ walkway system in MPS II patients undergoing enzyme replacement therapy

Patients with MPS II often present with limitations to functional mobility. With the advent of enzyme replacement therapy (ERT), robust assessment tools are important to assess response to treatment. The aim of this study was to see if the GAITRite™ system (electronic pressure sensitive walkway) could identify any changes to gait pattern following commencement of ERT. Six boys with MPS type II were assessed at baseline and at intervals post commencing ERT. Four individual characteristics of gait were studied - velocity, cadence, step length and base of support. Changes in parameters for each individual could be analysed and be compared with age matched controls. The data generated from the GAITRite™ indicated all six boys had changes to their gait pattern. The most notable changes were in velocity, step length and base of support. The GAITRite™ was found to identify changes in gait parameters in this group of patients. It is an accessible way of providing both quantitative and qualitative analysis of gait in the clinical environment, and could potentially be used to monitor response to treatment. Larger studies are needed to corroborate our findings, as well as to establish the GAITRite™ as a monitoring tool.

Expression of ezrin in glial tubes in the adult subventricular zone and rostral migratory stream

Ezrin is a member of the ERM (ezrin-radixin-moesin) family of membrane-cytoskeletal linking proteins. ERM proteins are involved in a wide variety of cellular functions including cell motility, signal transduction, cell-cell interaction and cell-matrix recognition. A recent in situ hybridization study showed that the mRNA encoding ezrin is expressed in neurogenic regions of the mature brain including the subventricular zone (SVZ) and rostral migratory stream (RMS); however, the specific cell types expressing ezrin and their relationship to migrating and proliferating cells in these regions have not been characterized previously. In this study, we used immunocytochemistry to perform double labeling with a variety of cell-type specific markers to characterize the expression of ezrin in the SVZ and RMS of adult mice. Ezrin was expressed at high levels in both the SVZ and RMS where ezrin-immunopositive processes formed a trabecular network surrounding the proliferating and migrating cells. Ezrin-positive cells co-labeled with the glial makers S100beta and GFAP (glial fibrillary acidic protein), but only minimally with the early neuronal markers beta III tubulin and polysialylated form of neural cell adhesion molecule 1 (PSA-NCAM), indicating that ezrin was expressed primarily in the glial tube cells. Ezrin positive cells also expressed beta-catenin, a membrane-complex protein previously implicated in the regulation of stem-cell proliferation and neuronal migration. Glial tube cells act as both precursors of, and a physical channel for, migrating neuroblasts. Bi-directional signals between glial tube cells and migrating neuroblasts have been shown to regulate the rates of both proliferation of the precursor cells and migration of the newly generated neuroblasts. Our finding that ezrin and beta-catenin are both present at the cell membrane of the glial tube cells suggests that these proteins may be involved in those signaling processes.

Randomised controlled trial of essential fatty acid supplementation in phenylketonuria

OBJECTIVE

The long-chain polyunsaturated fatty acids (LC-PUFA) status of children with PKU is often compromised. LC-PUFA, which are important fatty acids in the development of the CNS, can be synthesised endogenously from the parent essential fatty acids (EFA) provided dietary intakes are adequate. This study was designed to assess the biochemical effect over a 20-week period of a phe-free protein substitute that has been supplemented with a balanced blend of n-3 and n-6 EFAs on LC-PUFA status of children with PKU.

DESIGN, SETTING AND SUBJECTS

Fifty three community-living children aged 1-10 years diagnosed with PKU in the newborn period were recruited from seven tertiary centres in the UK and France and randomised to a fat-free control formula or the EFA-supplemented test-treatment formula in an open, prospective study. Forty four children completed the study (20 controls, 24 test-treatments). Fatty acid status was assessed at entry and 20-weeks follow-up. Three day dietary diaries were recorded at 20 weeks' follow-up. The safety, efficacy and palatability of the test-treatment formula were also assessed.

RESULTS

The test-treatment group had significantly higher intakes of fat and EFA than the control group. There was a significant between group difference (P=0.04) in increases in median docosahexaenoic acid (DHA) concentrations in erythrocyte phospholipids, which increased by 19% in the test-treatment group and by 0.5% in the control group over the study period. Growth and phe control were satisfactory in all subjects.

CONCLUSIONS

Supplementing the diets of children with PKU with a balanced blend of n-6 and n-3 EFA improves DHA status without compromising AA status.

Temporomandibular joint destruction in mucolipidosis type III necessitating gastrostomy insertion

Mucolipidosis III is a genetically heterogeneous lysosomal disorder characterised by progressive symptoms and signs, the commonest being skeletal pain due to bony destruction. We describe a patient who developed severe destruction of the temporomandibular joints leading to difficulties with speech and feeding, necessitating gastrostomy insertion.

CONCLUSION

Temporomandibular joint involvement has not been previously reported in mucolipidosis III.

The significance of elevated CSF lactate

The final diagnosis of 158 patients who had a cerebrospinal fluid (CSF) lactate concentration greater than 2 mmol/l was ascertained. The conditions included seizures, inflammatory changes, and proven metabolic disorders. For the diagnosis of congenital lactic acidoses, CSF lactate should ideally be measured in a seizure free patient after any acute illness.

Developmental delay: when to suspect and how to investigate for an inborn error of metabolism

The purpose of this review is to provide a practical guideline on the suspicion and investigation of inborn errors of metabolism (IEMs) as cause of developmental delay. Developmental delay is a common paediatric problem. Inborn errors of metabolism are a rare cause of developmental delay. However, it is important to detect IEMs for several reasons: accurate counselling may be given regarding recurrence risk; metabolic decompensation may be avoided; and specific treatments may be available. Certain clinical situations are more likely to point to an IEM as the cause of developmental delay. This review highlights the risk factors in the history, the important examination findings, and the appropriate biochemical investigation of the child with developmental delay. Following these guidelines makes "missing" an IEM unlikely.

Ornithine aminotransferase deficiency: diagnostic difficulties in neonatal presentation

We describe two unrelated cases of ornithine aminotransferase (OAT) deficiency with rare neonatal presentation of hyperammonaemia. The diagnosis in the neonatal presentation of OAT deficiency is hampered as hyperornithinaemia is absent. Enzyme and mutation studies confirmed the diagnosis. OAT deficiency should be included in differential diagnosis of neonatal hyperammonaemia.

Cardiovascular changes in children with mucopolysaccharide disorders

The aims of this study were to evaluate cardiac involvement, assess risk factors and mortality, and define the outcome of cardiac abnormalities with age in the different types of mucopolysaccharidoses (MPS). The echocardiograms of 99 patients with MPS, aged 1-24 y (median age 10.3 y) were reviewed between 1978 and 2000. Mitral regurgitation (MR) was detected in 29 patients (29%). MR was more frequent in types IH [n = 11 (38%)], II [n = 10 (24%)] and III [n = 4 (20%)]. Sixteen patients (16%) developed aortic regurgitation (AR), seen mostly in types II [n = 9 (56%)] and IV [n = 4 (24%)]. AR and/or MR was detected in 37 patients and 8 had both abnormalities of borderline significance (odds ratio 2.95, 95% confidence interval 1.0-8.85, p = 0.05). Of 99 patients, 47 had a normal study on their first echocardiogram, whereas only 7 had a normal study on subsequent echocardiograms. Fifty-four (54%) had a single echocardiogram. Of these, 27 (50%) were abnormal and 27 normal. Forty-five patients had more than one echocardiogram, of which 25 (56%) were abnormal and 20 normal. In 13/20 (65%) a cardiac abnormality developed on a subsequent echocardiogram which was statistically significant (p = 0.002). Overall mitral and aortic valve abnormalities showed a positive association with age. Univariate analysis of risk factors showed that increasing age, MPS I and ejection fraction were significant risk factors for death. However, left ventricular hypertrophy, mitral valve abnormalities and type II MPS were not significant risk factors for death, with borderline significance for aortic valve abnormalities.

CONCLUSION

This study demonstrates the evaluation of ventricular function, which is a significant risk factor for death, along with increasing age and MPS I, and outlines the borderline significance of aortic valve abnormalities, which has not been mentioned in previous studies. It also shows that mitral valve lesions, commonly seen in MPS, were not a significant risk factor for death. The results emphasize the importance of performing serial echocardiograms in patients with MPS to assess ventricular function and the progression of cardiac abnormalities with age.

Diagnostic criteria for respiratory chain disorders in adults and children

BACKGROUND

Respiratory chain (RC) disorders are clinically, biochemically, and molecularly heterogeneous. The lack of standardized diagnostic criteria poses difficulties in evaluating diagnostic methodologies.

OBJECTIVE

To assess proposed adult RC diagnostic criteria that classify patients into "definite," "probable," or "possible" categories.

METHODS

The authors applied the adult RC diagnostic criteria retrospectively to 146 consecutive children referred for investigation of a suspected RC disorder. Data were collected from hospital, genetics, and laboratory records, and the diagnoses predicted by the adult criteria were compared with the previously assigned assessments.

RESULTS

The authors identified three major difficulties in applying the adult criteria:lack of pediatric-specific criteria; difficulty in segregating continuous data into circumscribed major and minor criteria; and lack of additivity of clinical features or enzyme tests. They therefore modified the adult criteria to allow for pediatric clinical and histologic features and for more sensitive coding of RC enzyme and functional studies. Reanalysis of the patients' data resulted in congruence between the diagnostic certainty previously assigned by the authors' center and that defined by the new general RC diagnostic criteria in 99% of patients.

CONCLUSIONS

These general diagnostic criteria appear to improve the sensitivity of the adult criteria. They need further assessment in prospective clinical and epidemiologic studies.

Facial appearance in glycogen storage disease type III

Amylo-1,6-glucosidase deficiency (glycogen storage disease type III) is associated with hypoglycaemia, hepatomegaly, raised transaminases and in most cases skeletal myopathy and cardiomyopathy. The disorder has not been considered to cause dysmorphism. We report consistent facial features in seven patients with GSD type III consisting of midface hypoplasia with a depressed nasal bridge and a broad upturned nasal tip, indistinct philtral pillars, and bow-shaped lips with a thin vermillon border. Younger patients had in addition deepset eyes. Several children had clinical problems such as persistent otitis media or recurrent sinusitis. The underlying aetiology of these features is unknown but the similarity in all our patient suggests that there is a facial phenotype for this disorder.

Disruption of an imprinted gene cluster by a targeted chromosomal translocation in mice

Genomic imprinting is an epigenetic process in which the activity of a gene is determined by its parent of origin. Mechanisms governing genomic imprinting are just beginning to be understood. However, the tendency of imprinted genes to exist in chromosomal clusters suggests a sharing of regulatory elements. To better understand imprinted gene clustering, we disrupted a cluster of imprinted genes on mouse distal chromosome 7 using the Cre/loxP recombination system. In mice carrying a site-specific translocation separating Cdkn1c and Kcnq1, imprinting of the genes retained on chromosome 7, including Kcnq1, Kcnq1ot1, Ascl2, H19 and Igf2, is unaffected, demonstrating that these genes are not regulated by elements near or telomeric to Cdkn1c. In contrast, expression and imprinting of the translocated Cdkn1c, Slc22a1l and Tssc3 on chromosome 11 are affected, consistent with the hypothesis that elements regulating both expression and imprinting of these genes lie within or proximal to Kcnq1. These data support the proposal that chromosomal abnormalities, including translocations, within KCNQ1 that are associated with the human disease Beckwith-Wiedemann syndrome (BWS) may disrupt CDKN1C expression. These results underscore the importance of gene clustering for the proper regulation of imprinted genes.

Increased risk of vitamin B12 deficiency in patients with phenylketonuria on an unrestricted or relaxed diet

OBJECTIVE

To investigate whether dietary relaxation or cessation in patients with phenylketonuria (PKU) predisposes to vitamin B12 deficiency.

STUDY DESIGN

Patients with PKU aged 11 to 38 years underwent a neurologic examination and dietetic assessment and were divided according to their diet into 1 of 3 groups: Strict - those on a strict low phenylalanine (phe) diet with amino acid, mineral, and vitamin supplements; Relaxed - those on a total protein intake of approximately 1 g/kg/d with 50% of this from natural protein and 50% from amino acid, mineral, and vitamin supplements; Unrestricted - those on no formal protein restriction and not taking amino acid supplements. Assays of blood samples were taken for vitamin B12 and folate levels by standard assays. Results were analyzed with Student t test.

RESULTS

Vitamin B12 levels were significantly lower in the PKU groups on relaxed or unrestricted diets compared with the normal population (P <.0001 [unrestricted] and.0034 [relaxed]). Folate levels were significantly elevated in all PKU groups (<.0001).

CONCLUSION

Patients with PKU who are no longer under strict dietary control may be at risk from vitamin B12 deficiency. We recommend that all patients should remain under medical and dietetic supervision and in particular have their vitamin B12 status monitored.

Oppositely imprinted genes p57(Kip2) and igf2 interact in a mouse model for Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is a clinically variable disorder characterized by somatic overgrowth, macroglossia, abdominal wall defects, visceromegaly, and an increased susceptibility to childhood tumors. The disease has been linked to a large cluster of imprinted genes at human chromosome 11p15.5. A subset of BWS patients has been identified with loss-of-function mutations in p57(KIP2), a maternally expressed gene encoding a G(1) cyclin-dependent kinase inhibitor. Some patients display loss of imprinting of IGF2, a fetal-specific growth factor that is paternally expressed. To understand how the same disease can result from misregulation of two linked, but unrelated, genes, we generated a mouse model for BWS that both harbors a null mutation in p57(Kip2) and displays loss of Igf2 imprinting. These mice display many of the characteristics of BWS, including placentomegaly and dysplasia, kidney dysplasia, macroglossia, cleft palate, omphalocele, and polydactyly. Some, but not all, of the phenotypes are shown to be Igf2 dependent. In two affected tissues, the two imprinted genes appear to act in an antagonistic manner, a finding that may help explain how BWS can arise from mutations in either gene.

Genetic counseling and prenatal diagnosis for the mitochondrial DNA mutations at nucleotide 8993

Mitochondrial genetics is complicated by heteroplasmy, or mutant load, which may be from 1%-99%, and thus may produce a gene dosage-type effect. Limited data are available for genotype/phenotype correlations in disorders caused by mtDNA mutations; therefore, prenatal diagnosis for mtDNA mutations has been hindered by an inability to predict accurately the clinical severity expected from a mutant load measured in fetal tissue. After reviewing 44 published and 12 unpublished pedigrees, we considered the possibility of prenatal diagnosis for two common mtDNA mutations at nucleotide 8993. We related the severity of symptoms to the mutant load and predicted the clinical outcome of a given mutant load. We also used the available data to generate empirical recurrence risks for genetic counseling, which may be used in conjunction with prenatal diagnosis.

Respiratory chain complex I deficiency: an underdiagnosed energy generation disorder

OBJECTIVE

To define the spectrum of clinical and biochemical features in 51 children with isolated complex I deficiency.

BACKGROUND

Mitochondrial respiratory chain defects are one of the most commonly diagnosed inborn errors of metabolism. Until recently there have been technical problems with the diagnosis of respiratory chain complex I defects, and there is a lack of information about this underreported cause of respiratory chain dysfunction.

METHODS

A retrospective review of clinical features and laboratory findings was undertaken in all diagnosed patients who had samples referred over a 22-year period.

RESULTS

Presentations were heterogeneous, ranging from severe multisystem disease with neonatal death to isolated myopathy. Classic indicators of respiratory chain disease were not present in 16 of 42 patients in whom blood lactate levels were normal on at least one occasion, and in 23 of 37 patients in whom muscle morphology was normal or nonspecific. Ragged red fibers were present in only five patients. Tissue specificity was observed in 19 of 41 patients in whom multiple tissues were examined, thus the diagnosis may be missed if the affected tissue is not analyzed. Nine patients had only skin fibroblasts available, the diagnosis being based on enzyme assay and functional tests. Modes of inheritance include autosomal recessive (suggested in five consanguineous families), maternal (mitochondrial DNA point mutations in eight patients), and possibly X-linked (slight male predominance of 30:21). Recurrence risk was estimated as 20 to 25%.

CONCLUSION

Heterogeneous clinical features, tissue specificity, and absence of lactic acidosis or abnormal mitochondrial morphology in many patients have resulted in underdiagnosis of respiratory chain complex I deficiency.

Generalised uridine diphosphate galactose-4-epimerase deficiency

The generalised form of epimerase deficiency galactosaemia has been described in only two children from unrelated families. Their progress is reported and three other affected children from these families are described. The initial presentation was similar to classic galactosaemia. Despite treatment all have shown poor growth and moderate learning difficulties. Three have sensorineural deafness and four have pronounced dysmorphic features. The two older female patients have normal pubertal development.

Multiple mechanisms regulate imprinting of the mouse distal chromosome 7 gene cluster

Genomic imprinting is an epigenetic process that results in the preferential silencing of one of the two parental copies of a gene. Although the precise mechanisms by which genomic imprinting occurs are unknown, the tendency of imprinted genes to exist in chromosomal clusters suggests long-range regulation through shared regulatory elements. We characterize a 800-kb region on the distal end of mouse chromosome 7 that contains a cluster of four maternally expressed genes, H19, Mash2, Kvlqt1, and p57(Kip2), as well as two paternally expressed genes, Igf2 and Ins2, and assess the expression and imprinting of Mash2, Kvlqt1, and p57(Kip2) during development in embryonic and extraembryonic tissues. Unlike Igf2 and Ins2, which depend on H19 for their imprinting, Mash2, p57(Kip2), and Kvlqt1 are unaffected by a deletion of the H19 gene region, suggesting that these more telomeric genes are not regulated by the mechanism that controls H19, Igf2, and Ins2. Mutations in human p57(Kip2) have been implicated in Beckwith-Wiedemann syndrome, a disease that has also been associated with loss of imprinting of IGF2. We find, however, that a deletion of the gene has no effect on imprinting within the cluster. Surprisingly, the three maternally expressed genes are regulated very differently by DNA methylation; p57(Kip2) is activated, Kvlqt1 is silenced, and Mash2 is unaffected in mice lacking DNA methyltransferase. We conclude that H19 is not a global regulator of imprinting on distal chromosome 7 and that the telomeric genes are imprinted by a separate mechanism(s).

OCA-B is a functional analog of VP16 but targets a separate surface of the Oct-1 POU domain

OCA-B is a B-cell-specific coregulator of the broadly expressed POU domain transcription factor Oct-1. OCA-B associates with the Oct-1 POU domain, a bipartite DNA-binding structure containing a POU-specific (POU[S]) domain joined by a flexible linker to a POU homeodomain (POU[H]). Here, we show that OCA-B alters the activity of Oct-1 in two ways. It provides a transcriptional activation domain which, unlike Oct-1, activates an mRNA-type promoter effectively, and it stabilizes Oct-1 on the Oct-1-responsive octamer sequence ATGCAAAT. These properties of OCA-B parallel those displayed by the herpes simplex virus Oct-1 coregulator VP16. OCA-B, however, interacts with a different surface of the DNA-bound Oct-1 POU domain, interacting with both the POU(S) and POU(H) domains and the center of the ATGCAAAT octamer sequence. The OCA-B and VP16 interactions with the Oct-1 POU domain are sufficiently different to permit OCA-B and VP16 to bind the Oct-1 POU domain simultaneously. These results emphasize the structural versatility of the Oct-1 POU domain in its interaction with coregulators.

Oral health implications in children with inborn errors of intermediary metabolism: a review

Children with inborn errors of metabolism form an important group of children at risk of oral pathology. Their management includes dietary therapies which aim to promote normal growth and development but which are often highly cariogenic. It is important for paediatricians, dietitians and paediatric dentists to liaise closely in the management of these children. This paper reviews these disorders and the limited literature that exists on the oral health implications of those conditions that rely on dietary manipulation for their management, and presents suggestions for their dental care.

Structural flexibility in transcription complex formation revealed by protein-DNA photocrosslinking

The Oct-1 POU domain binds diverse DNA-sequence elements and forms a higher-order regulatory complex with the herpes simplex virus coregulator VP16. The POU domain contains two separate DNA-binding domains joined by a flexible linker. By protein-DNA photocrosslinking we show that the relative positioning of the two POU DNA-binding domains on DNA varies depending on the nature of the DNA target. On a single VP16-responsive element, the POU domain adopts multiple conformations. To determine the structure of the Oct-1 POU domain in a multiprotein complex with VP16, we allowed VP16 to interact with previously crosslinked POU-domain-DNA complexes and found that VP16 can associate with multiple POU-domain conformations. These results reveal the dynamic potential of a DNA-binding domain in directing transcriptional regulatory complex formation.

A maternally transmitted lethal neonatal progeroid syndrome with prominent genitourinary and gastrointestinal features

Twin brothers and their maternal uncle with a previously undescribed neonatal progeroid syndrome are presented. In addition to progeroid features, they had pseudo-obstruction of the urinary and gastrointestinal tracts, severe leucocytosis, liver dysfunction, and low complex III and IV in muscle but not in liver. Previously described neonatal progeroid syndromes and syndromes featuring pseudo-obstruction are discussed. The two most likely aetiological mechanisms are an X linked single gene disorder or a mitochondrial disorder. The evidence for these possibilities is presented.

The Oct-1 POU-specific domain can stimulate small nuclear RNA gene transcription by stabilizing the basal transcription complex SNAPc

The RNA polymerase II and III human small nuclear RNA promoters have a common basal element, the proximal sequence element, which binds the TATA box-binding protein-containing complex SNAPc. They also contain an enhancer characterized by a highly conserved octamer sequence, which constitutes a binding site for the broadly expressed POU domain transcription factor Oct-1. The POU domain is a bipartite DNA-binding domain consisting of a POU-homeo (POUH) domain and a POU-specific (POUs) domain joined by a flexible linker. Here, we show that the Oct-1 POU domain but not the related Pit-1 POU domain can facilitate the binding of SNAPc to the proximal sequence element, and activate transcription. The effect is probably mediated by protein-protein contacts, and 1 of 30 amino acid differences between the Oct-1 and Pit-1 POUs domains is the key determinant for the differential interaction with SNAPc and the ability to activate transcription. These results show that a function that is the hallmark of activation domains, namely, recruitment of a basal transcription complex resulting in activation of transcription, can be performed by a DNA-binding domain. In this case, subtle changes between activator DNA-binding domains, as subtle as a single amino acid difference, can profoundly affect interaction with the basal transcription machinery.

Histidinaemia: a benign metabolic disorder

Histidinaemia is a relatively common inherited metabolic disorder with an incidence similar to phenylketonuria. This paper reports the long term outcome of patients diagnosed by newborn screening in the north west of England. Between 1966 and 1990, 108 infants were diagnosed as having histidinaemia by a regional neonatal screening programme (incidence 1:11,083). A further five children were detected following diagnosis in a sibling. Of the 113, nine were lost to follow up. Infants diagnosed before 1981 (n = 47) were placed on a low histidine diet (225 mg/kg/d) for an average period of 21 months (SD 4.5). All patients were reviewed regularly, Griffiths developmental quotients (DQ) were assessed at 2 and 4 years, and WISC-R intelligence quotients (IQ) at 8, 12, and 18 years. IQ data were converted to standard deviation scores (IQ SDS) to account for increasing IQ norms with time. Neither DQ nor IQ correlated with plasma histidine at diagnosis or with the mean plasma histidine throughout life. Growth was normal in all patients. There was no apparent benefit from a low histidine diet in early childhood. In contrast to other studies, there was no excess of clinical symptoms. On the basis of these findings, histidinaemia is a benign metabolic disorder that does not require treatment.

Magnetic resonance imaging in phenylketonuria: reversal of cerebral white matter change

OBJECTIVES

To investigate the extent to which the abnormalities in cerebral white matter in adolescents and adults with phenylketonuria (PKU) are reversible.

METHOD

Magnetic resonance imaging (MRI) of the brain was repeated in 41 patients with PKU (age range, 14 to 49 years) after an interval (median, 9 months; range, 3 to 12 months) of dietary intervention. Scans were scored according to the extent of the white matter involvement. After an initial MRI, five patients returned to a strict low-phenylalanine diet with amino acid supplement; 21 patients started a low-protein diet (1 gm/kg) with amino acids supplement; and 15 patients made no dietary alteration.

RESULTS

Scans improved in all five patients who returned to a strict low-phenylalanine diet, in 5 of the 21 patients on the low-protein diet plus amino acid supplement, and in 4 of the 15 patients who made no dietary change. There was a significant association between change in the MRI findings and in the blood phenylalanine concentration (Pearson correlation: r = 0.55; p < 0.0002) and between change in the MRI and in the phenylalanine level at the time of the second scan (r = 0.58; p < 0.0001). Improvement was seen primarily in those in whom phenylalanine levels were reduced to less than 900 mumol/L. There was no obvious change in MRI score after 3 weeks of strict phenylalanine restriction for the two adults who underwent serial scanning.

CONCLUSION

The MRI changes in PKU are at least partially reversible by lowering the blood phenylalanine concentration.

Absence of acidosis in the initial presentation of propionic acidaemia

The clinical presentation and results of the initial biochemical and haematological investigations in 11 newborn term infants with propionic acidaemia are described. All patients had neurological symptoms. Only four had clinically important acidosis, but all had a raised blood ammonia. A diagnosis of propionic acidaemia should be considered in all newborn infants with unexplained neurological deterioration even in the absence of a metabolic acidosis.

Mechanisms for flexibility in DNA sequence recognition and VP16-induced complex formation by the Oct-1 POU domain

DNA binding by the Oct-1 protein is directed by its POU domain, a bipartite DNA-binding domain made up of a POU-specific (POUS) domain and a POU-homeo (POUH) domain, two helix-turn-helix-containing DNA-binding modules that cooperate in DNA recognition. Although the best-characterized DNA target for Oct-1 binding is the octamer sequence ATGCAAAT, Oct-1 also binds a number of different DNA sequence elements. For example, Oct-1 recognizes a form of the herpes simplex virus VP16-responsive TAATGARAT element, called the (OCTA-)TAATGARAT site, that lacks octamer site similarity. Our studies suggest two mechanisms by which Oct-1 achieves flexible DNA sequence recognition. First, an important arginine found in the Oct-1 POUS domain tolerates substitutions of its base contacts within the octamer site. Second, on the (OCTA-)TAATGARAT site, the POUS domain is located on the side of the POUH domain opposite from where it is located on an octamer site. This flexibility of the Oct-1 POU domain in DNA binding also has an impact on its participation in a multiprotein-DNA complex with VP16. We show that Oct-1 POUS domain residues that contact DNA have different effects on VP16-induced complex formation depending on whether the VP16-responsive element involved has overlapping octamer similarity or not.

The presenting features of mucopolysaccharidosis type IH (Hurler syndrome)

The presenting features of 39 patients with mucopolysaccharidosis (MPS) type IH are described. The mean age at diagnosis was approximately 9 months and it is difficult to see how this can be reduced without consideration of newborn screening. An earlier age at diagnosis is likely to lead to better results following therapy such as bone marrow transplantation. Clinical features which should arouse suspicion of MPS IH include frequent ENT surgery and recurrent herniae. Clinical vigilance is needed for early diagnosis.

Magnetic resonance imaging of the brain in phenylketonuria

Abnormalities of magnetic resonance imaging (MRI) of the brain occur in some patients with phenylketonuria but the clinical importance of this finding is not clear. In order to determine the frequency and functional significance of changes on MRI we investigated 77 adolescent and adult patients with phenylketonuria. Patients aged 14-49 years and taking a restricted diet of 1 g/kg protein underwent clinical examination, IQ testing, neurophysiological investigation, and MRI of the brain. Patients aged between 10-14 years taking a low phenylalanine diet with amino acid supplements had MRI of the brain only. Biochemical control was assessed from: the lifetime blood phenylalanine determined from the mean blood concentration throughout life; the accumulated time for each patient that phenylalanine was < 120 mumol/L; the accumulated time for each patient that phenylalanine was > 1200 mumol/L); mean blood concentration in the first 4 years of life; and the mean blood phe concentration in the 5 years prior to imaging. MRI changes, compatible with a disturbance in the water content of white matter, were present in all but 1 patient. The severity of abnormality was most strongly associated with the blood phenylalanine concentration at the time of imaging. Clinical and neurophysiological abnormalities were less common and usually mild. 3 patients had prolonged central motor conduction time, 7 had prolonged visual evoked potentials, and 5 had impaired peripheral sensory nerve conduction. There was no significant association between the extent of MRI abnormalities and IQ, and the presence of neurophysiological, or clinical abnormalities. An abnormal brain scan in PKU may reflect present biochemical control rather than indicate significant neurological damage. As yet there is little evidence that in most patients with PKU these MRI changes are of clinical importance.

Management of mucopolysaccharidosis type III

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Differential positive control by Oct-1 and Oct-2: activation of a transcriptionally silent motif through Oct-1 and VP16 corecruitment

Transcriptional regulation by the ubiquitous human POU homeo domain protein Oct-1 and the related B-cell protein Oct-2 is a model for understanding how proteins that recognize the same regulatory site elicit different programs of gene transcription. Here, we describe a mechanism for differential promoter activation whereby only Oct-1, through selective corecruitment with the herpesvirus trans-activator VP16, acquires the ability to stimulate transcription from a TAATGARAT-containing site that responds to neither Oct-1 nor Oct-2 alone. To measure differential in vivo activation by human Oct-1 and Oct-2 in response to VP16, we have developed a transient assay in murine NIH-3T3 cells. Surprisingly, murine Oct-1 associates with VP16 much less effectively than its human counterpart, most likely because the murine Oct-1 homeo domain differs at four positions from the human Oct-1 homeo domain. The murine cell transient assay shows directly that human Oct-1, but not human Oct-2, can respond to VP16 in vivo. The Oct-1 DNA-binding POU domain is sufficient and the Oct-1 homeo domain is critical for this response, because an Oct-1 POU domain containing the Oct-2 homeo domain fails to respond to the VP16-induced positive control of transcription. Thus, by selective homeo domain interaction and corecruitment to an otherwise silent regulatory element, VP16 expands the repertoire of sites responsive to Oct-1 without affecting the activity of its close relative Oct-2.

A single amino acid exchange transfers VP16-induced positive control from the Oct-1 to the Oct-2 homeo domain

The selective association of the herpesvirus trans-activator VP16 with the human Oct-1 homeo domain is a model for differential positive transcriptional control by homeo domains. VP16 discriminates between the closely related homeo domains of Oct-1 and Oct-2 by distinguishing among their seven amino-acid differences; these differences lie on the surface that is thought to be accessible when the homeo domain is bound to DNA. Only two of these seven differences are recognized by VP16, one in each of the first two alpha-helices of the tri-alpha-helical homeo domain. The major determinant for selective association with VP16 in vitro and VP16-induced positive control in vivo is a single glutamic acid residue at position 22 in the first alpha-helix of the Oct-1 homeo domain, but the acidic properties of this residue are not critical for association with VP16 in vitro or in vivo, because it can be replaced by glutamine with little or no deleterious effect. Mere replacement of the single corresponding alanine residue in the Oct-2 homeo domain with the key glutamic acid residue is sufficient to confer on the Oct-2 homeo domain the ability to associate with VP16 in vitro and respond to VP16-induced positive control in vivo. Thus, the specificity of homeo domain positive control can be conferred by a single amino acid difference.