AADC deficiency publications

AADC deficiency from infancy to adulthood: Symptoms and developmental outcome in an international cohort of 63 patients

Pearson TS, et al. J Inherit Metab Dis. 2020.

Publication Date | May 2020
Authors | Pearson TS, Gilbert L, Opladen T, Garcia-Cazorla A, Mastrangelo M, Leuzzi V, Tay SKH, Sykut-Cegielska J, Pons R, Mercimek-Andrews S, Kato M, Lücke T, Oppebøen M, Kurian MA, Steel D, Manti F, Meeks KD, Jeltsch K, Flint L.
Citation | J Inherit Metab Dis. 2020;43(5):1121–30.

https://pubmed.ncbi.nlm.nih.gov/32369189/

A recent retrospective study,¹ led by Toni Pearson at the Washington University School of Medicine, has highlighted the variety and complexity of motor and non-motor symptoms affecting patients with aromatic L-amino acid decarboxylase (AADC) deficiency throughout their lifespan.

AADC deficiency is a rare, autosomal recessive neurotransmitter disorder presenting early in life, in which a combined deficiency of serotonin, dopamine, norepinephrine and epinephrine leads to a complex syndrome characterised by motor, behavioural and autonomic symptoms.² Most patients exhibit a severe disease course with profound motor impairment, with little or no benefit seen with existing medical interventions,^2,3 while a smaller number of patients develop a milder disease² and in general, respond more favourably to medical treatment.^2,4–6 To further understand the evolution of symptoms throughout the disease course, the spectrum of developmental outcomes and mortality amongst patients with AADC deficiency, the international team of researchers analysed the responses to a questionnaire designed to assess the initial symptoms, symptom course, treatment response and developmental outcomes of patients with AADC deficiency.¹

Patients with AADC deficiency from across the globe were identified from the International Working Group of Neurotransmitter-Related Disorders patient registry and the AADC Research Trust. Physicians and caregivers were asked to classify a range of symptoms as major, minor, or absent at different life stages from infancy to adulthood. The analysis included 63 patients from 23 countries (60% female; age range 6 months–37 years, median age 7 years; n=58 living).

The median age at the time of symptom onset was 3 months (range 0–12 months). There was considerable variation in the time from initial symptom onset to diagnosis of AADC deficiency, with younger patients being diagnosed more quickly than those aged over 10 years. The most common initial symptoms reported were hypotonia, oculogyric crises (OGCs), developmental delay, and feeding difficulties; consistent with previous reports.⁵ Prominent non-motor symptoms included sleepiness, irritability, excessive sweating, and nasal congestion (Figure 1).

Sleep disturbances appeared to evolve with age: excessive sleepiness was more prominent in children under the age of 2 years, whereas insomnia affected a large proportion of patients aged over 2 years and presented a major issue in 50% of children aged 2–12 years. Most children (85%) aged 6–12 years were also affected by irritability. Excessive sweating was also reported in all children under two years and classified as a major symptom in 40% of children between the ages of 2 and 17 years.²

OGCs were reported by almost all patients but were more frequent, more severe, and longer lasting in younger patients (Figure 2). Episodes were particularly prevalent in those aged 2‒12 years (97%), comprising approximately half of the total study population. Of 5 older patients (≥13 years) who did not experience OGCs at the time of the survey, 4 had a milder disease and could walk independently, suggesting that the reduced prevalence of OGCs might be linked to either older age, reduced disease severity, or a combination of factors. An OGC was reported as the apparent proximate cause of death for 2 out of 5 deceased subjects, supporting the life-threatening potential of OGCs.

49 patients aged 12 months or older reported information about the attainment of motor developmental milestones. Across these patients, only a third (16/49) exhibited head control, and only 22% (11/49) could sit and walk independently. Of those patients who could sit and walk independently, none were aged under 6 years. This reflects the notable difference in motor function across the age groups, with younger patients exhibiting more severe motor impairment than older patients. Indeed, most patients (7/11) who could walk were aged over 12 years. Investigators considered this to be indicative of a greater proportion of older patients in the cohort having a milder disease course, rather than reflective of a delay in the attainment of walking. Interestingly, early regression in motor or developmental skills was reported in 24% of 63 patients, which typically occurred during infancy at the onset of OCGs and other disease symptoms. Data from 53 patients showed that past or current feeding difficulties were experienced by 75% of patients. 45% of patients required a gastrostomy tube for feeding support.

Of the 38 subjects aged 5 years or older, only 4 (11%) were classed as completely independent, with 7 (18%) partially independent and 27 (71%) completely dependent. 62% (18/29) of those aged 5–18 years attended school and 2 of the 8 young adults aged over 18 years participated in work outside the home.

Of 43 subjects with a known genotype, 9 (21%) had a mild* motor phenotype, all of whom carried compound heterozygous variants including at least one missense variant. 14 novel variants identified were associated with a severe* phenotype in all but one subject.

Precise analysis of the childhood mortality risk was prohibited by the retrospective nature of the study. In addition, underreporting of some disease features due to partially incomplete responses for some subjects presented a further limitation of the study design.

The study illustrates the spectrum of disease phenotypes exhibited by patients with AADC deficiency throughout their lifespan. Whereas most patients experience severe functional and motor developmental impairment, others show milder symptoms. The authors note that the refractory nature of disease features to existing medical therapies, especially in severe cases, motivates the search for novel and effective therapeutic strategies.

Prevalence of aromatic L-amino acid decarboxylase deficiency in at-risk populations

Hyland K, Reott M. Ped Neurol. 2020;106:38–42.

Publication Date | May 2020
Authors | Hyland K, Reott M.
Citation | Ped Neurol. 2020;106:38–42. doi:10.1016/j.pediatrneurol.2019.11.022

https://pubmed.ncbi.nlm.nih.gov/32111562/

Researchers in Atlanta, US, retrospectively analysed data obtained from samples submitted to Medical Neurogenetics Laboratories, a US-based clinical diagnostics company, to estimate the prevalence of aromatic L-amino acid decarboxylase (AADC) deficiency in an at-risk population.¹

AADC deficiency is a rare, autosomal recessive metabolic disorder in which pathogenic mutations in the dopa decarboxylase (DDC) gene result in reduced AADC enzyme activity and severe combined deficiencies in dopamine, serotonin, norepinephrine and epinephrine production. This leads to a complex syndrome characterised by motor, behavioural and autonomic symptoms.² There are around 120 confirmed diagnoses of AADC deficiency worldwide documented in the literature. A founder mutation (IVS6+4A>T) has been identified in East Asia, with the prevalence of AADC deficiency in Taiwan estimated to be 1:32,000, however, little is known about the prevalence of AADC deficiency elsewhere.¹

In the 68 cases documented in the literature, the average age of symptom onset for AADC deficiency is 2.7 months. Despite early onset of symptoms, the average age of diagnosis is 3.5 years. The 2017 Consensus Guidelines for Diagnosis of AADC deficiency require at least 2 out of 3 core criteria to be met for a diagnosis of AADC deficiency: (1) low cerebrospinal fluid (CSF) levels of  5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), and 3-methoxy-4-hydroxyphenylglycol with increased CSF levels of 3-O-methyldopa (3-OMD), levodopa (L-Dopa), and 5-hydroxytryptophan (5-HTP), and normal CSF pterins; (2) decreased AADC enzyme activity in plasma; and (3) compound heterozygous or homozygous pathologic variants in the DDC gene.²

In this retrospective study,¹ investigators Keith Hyland and Michael Reott from the Department of Neurochemistry, Medical Neurogenetics Laboratories, examined the diagnostic data from almost 20,000 samples from at-risk patients presenting with neurological deficits of unknown origin. Samples were submitted from around the world between 2008 and 2016 for analysis of neurotransmitter metabolites in CSF, AADC enzyme activity in plasma, and/or Sanger sequencing of the DDC gene.

Overall, 36 new cases of AADC deficiency were identified through these analyses. From 19,577 CSF samples tested for neurotransmitter metabolite levels, the study identified 22 patients with AADC deficiency, resulting in an estimated prevalence of AADC deficiency of approximately 0.112% (1:900). Of these, 7 were confirmed by assessment of AADC enzymatic activity in plasma, 3 by genetic analysis, and 9 by both respective additional analyses, with 3 positive cases only having data for the CSF metabolite levels. A further 9 additional cases were identified through plasma AADC analysis of 81 samples, none of which tested positive for reduced CSF neurotransmitter metabolite levels but 5 of which were confirmed by genetic analysis. Sanger sequencing of the DDC gene revealed a further 5 cases (out of 59 samples tested), though none of these had data available for the other two tests (Figure 1).

In total, the retrospective analysis identified 36 new cases of AADC deficiency, with a median age at diagnosis of approximately 2.75 years. However, if only the cases that conformed to the consensus guidelines — i.e. meeting at least 2 of the 3 diagnostic criteria — were included in the retrospective analysis, it was estimated that there would have been 24 newly identified cases, a prevalence of 0.097%, or roughly 1:1,000.

The authors stated that, in their own experience, any one of the three diagnostic criteria should be sufficient for a diagnosis of AADC deficiency without the need for any accompanying analysis.

The DNA sequencing results identified 26 different variants of the DDC gene, including the founder mutation IVS6+4A>T, which was identified in 8 patients, one of whom was homozygous for this change. Another variant, p.(Arg347Gln), was identified in 7 alleles, which the authors suggested could be indicative of another founder mutation. Five of the variants identified had not previously been associated with AADC deficiency but were deemed to be likely pathogenic due to the knowledge of the plasma enzyme activity and CSF metabolite levels, illustrating the importance of functional studies when investigating the pathogenicity of variant changes in the DDC gene.

The results of this study add to the growing evidence base surrounding the diagnosis and management of AADC deficiency, supporting the current understanding of the epidemiology and genetics of the condition, as well as adding to the expanding list of known pathogenic variants of the DDC gene.

High throughput newborn screening for aromatic L-amino-acid decarboxylase deficiency by analysis of concentrations of 3-O-methyldopa from dried blood spots

Brennenstuhl H, et al. J Inherit Metab Dis. 2020;43(3):602–610.

Publication Date | May 2020 (epub 6 Jan 2020)
Authors | Brennenstuhl H, Kohlmüller D, Gramer G, Garbade SF, Syrbe S, Feyh P, Kölker S, Okun JG, Hoffmann GF, Opladen T.
Citation | J Inherit Metab Dis. 2020;43(3):602–610.

https://pubmed.ncbi.nlm.nih.gov/31849064/

Researchers in Heidelberg, Germany, have developed a novel high-throughput method for the detection and measurement of 3-O-methyldopa (3-OMD) in dried blood spots (DBS). It represents a non-invasive, simple, rapid and valid method for diagnosing aromatic L-amino-acid decarboxylase (AADC) deficiency in newborn screening (NBS).¹

AADC deficiency is a rare autosomal recessive disorder of biogenic amine metabolism. AADC catalyses levodopa (L-Dopa) to dopamine, 5-hydroxytryptophan (HTP) to serotonin, and L-tryptophan to tryptamine. Genetic variants of the AADC-coding DDC gene result in reduced synthesis of catecholamines and serotonin. In AADC-deficient patients, the phenotypic spectrum ranges from mild courses with predominantly autonomous symptoms to severe cases with early-onset muscular hypotonia, movement disorders, and developmental delay.

Positive treatment outcomes are correlated with early, pre-symptomatic diagnosis and treatment initiation, emphasising the need for early and reliable diagnostic approaches, e.g. in NBS.^2,3 The characteristic cerebrospinal fluid (CSF) profile reveals low concentrations of the dopamine and serotonin degradation products homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA), and elevated concentrations of L-Dopa and 3-OMD. 3-OMD accumulates in AADC-deficient patients due to the inability to utilise L-Dopa generated by tyrosine hydroxylase. Its concentration is elevated in dried blood spots of AADC-deficient patients and is therefore a candidate biomarker for NBS.

Measurement of 3-OMD in DBS using LC-MS/MS was proposed in 2016 as a suitable method to identify AADC-deficient patients pre-symptomatically in the newborn period.⁴ The novel approach proposed by Brennenstuhl, et al. eliminates the use of column separation, thereby shortening the measurement time per sample. They established a tandem mass spectrometry method to quantify 3-OMD in DBS and successfully tested it in 38,888 unaffected newborns, 14 heterozygous DDC variant carriers, 7 known AADC-deficient patients, and 1,079 healthy control subjects.¹

The group obtained 38,888 newborn DBS samples after parental consent during regular NBS, and DBS filter cards from 7 confirmed AADC-deficient patients between the age of 1 and 28 years (n = 2 male, n = 5 female). To rule out that heterozygous variant carriers were detected, 14 DBS samples were obtained from confirmed carriers of monoallelic genetic variants of the DDC gene. No clinical symptoms which could be related to AADC deficiency were reported in any of these subjects. Cut-off values were adjusted according to age (5 µmol/l for NBS, 3 µmol/l for children ranging from 28 days to 10 years, and 2 µmol/l for children 10 to 18 years).¹

3-OMD concentrations in the healthy newborns revealed a mean of 1.16 µmol/l (SD = 0.31; range 0.31–4.6 µmol/l). Non-AADC control subjects showed a mean 3-OMD concentration of 0.78 µmol/l (SD = 0.34; range 0.24–2.36 µmol/l) with a negative correlation with age. The highest concentration of 3-OMD was found in the NBS filter card of a confirmed AADC-deficient patient, with a mean 3-OMD of 35.95 µmol/l (SD = 0.77 µmol/l). The 3-OMD measurement in the DDC variant carriers showed a mean concentration of 0.69 µmol/l with an SD of 0.19 µmol/l (Figure 1).¹

The diagnostic gap between initial symptoms and the confirmed diagnosis remains large for rare neurotransmitter related disorders such as AADC deficiency, where clinical symptoms are unspecific or diagnostic procedures are invasive and limited to highly specialised laboratories. A valid, stable, and reliable method to identify newborns suffering from AADC deficiency is necessary for pre-symptomatic diagnosis and early treatment initiation. Brennenstuhl, et al. propose their novel high-throughput method to measure 3-OMD concentrations in DBS be integrated into existing NBS programs and included in the diagnostic workflow to investigate unexplained movement disorders, developmental delay or intellectual disability in patients aged 0–18 years, enabling early diagnosis of AADC deficiency. In case of an elevated DBS 3-OMD concentration, confirmatory diagnostics using an enzymatic assay, or genetic testing is recommended.¹