Symptoms and impact of aromatic l-amino acid decarboxylase (AADC) deficiency: a qualitative study and the development of a patient-centred conceptual model

A position statement on the post gene-therapy rehabilitation of aromatic I-amino acid decarboxylase deficiency patients

Aromatic L-amino acid decarboxylase (AADC) deficiency is a rare genetic disorder of monoamine neurotransmitter synthesis that presents with a range of symptoms, including motor dysfunction and limited attainment of developmental motor milestones. The approval of eladocagene exuparvovec, a gene therapy for AADC deficiency with demonstrated efficacy for motor improvements, now expands the range of motor outcomes possible for patients with this disorder. However, recommendations and guidelines for therapy following treatment with gene therapy are lacking. To ensure patients can reach their full potential following treatment with gene therapy, it is essential they receive rehabilitation therapies designed specifically with their impairments and goals in mind. Therefore, we highlight specific rehabilitative needs of patients following gene therapy and propose a set of recommendations for the post-treatment period based on collective experiences of therapists, physicians, and caregivers treating and caring for patients with AADC deficiency who have been treated with gene therapy. These recommendations include a focus on periods of intensive therapy, facilitating active movements, training for functional abilities, cognitive and communication training, parent/caregiver empowerment, collaboration between therapists and caregivers to develop in-home programs, and the incorporation of supplemental forms of therapy that patients and their families may find more enjoyable and engaging. Many of these rehabilitative strategies may be employed prior to gene therapy. However, these recommendations will be valuable for therapists, caregivers, and wider treatment teams as they prepare for the post-treatment journey with these patients. Furthermore, the considerations and recommendations presented here may prove beneficial outside the AADC deficiency community as gene therapies and other treatments are developed and approved for other rare diseases.

A computational study on structural and functional consequences of nsSNPs in human dopa decarboxylase

The Dopa Decarboxylase (DDC) gene plays an important role in the synthesis of biogenic amines such as dopamine, serotonin, and histamine. Non-synonymous single nucleotide polymorphisms (nsSNPs) in the DDC gene have been linked with various neurodegenerative disorders. In this study, a comprehensive in silico analysis of nsSNPs in the DDC gene was conducted to assess their potential functional consequences and associations with disease outcomes. Using publicly available databases, a complete list of nsSNPs in the DDC gene was obtained. 29 computational tools and algorithms were used to characterise the effects of these nsSNPs on protein structure, function, and stability. In addition, the population-based association studies were performed to investigate possible associations between specific nsSNPs and arthritis. Our research identified four novel DDC gene nsSNPs that have a major impact on the structure and function of proteins. Through molecular dynamics simulations (MDS), we observed changes in the stability of the DDC protein induced by specific nsSNPs. Furthermore, population-based association studies have revealed potential associations between certain DDC nsSNPs and various neurological disorders, including Parkinson's disease and dementia. The in silico approach used in this study offers insightful information about the functional effects of nsSNPs in the DDC gene. These discoveries provide insight into the cellular processes that underlie cognitive disorders. Furthermore, the detection of disease-associated nsSNPs in the DDC gene may facilitate the development of tailored and targeted therapy approaches.Communicated by Ramaswamy H. Sarma.

Long-Term Outcomes of Eladocagene Exuparvovec Compared with Standard of Care in Aromatic L-Amino Acid Decarboxylase (AADC) Deficiency: A Modelling Study

INTRODUCTION

Aromatic L-amino acid decarboxylase (AADC) deficiency is a rare disease with symptoms including movement disorders, developmental delays, and autonomic symptoms starting from birth; further, patients with AADC deficiency are at a high risk of death in the first decade of life. Limited information on the impact of treatment with gene therapy on patients' disease trajectories and survival, quality-of-life, and resource usage benefits are available.

METHOD

A cohort-based model with a lifetime horizon has been developed, based on motor milestones, to estimate the long-term benefits for patients after treatment with eladocagene exuparvovec compared to best supportive care (BSC). The model takes a National Health Service (NHS) perspective using a UK setting. The model comprises two parts: the developmental phase, in which patients with initially no motor function can progress to other motor milestone states, and a long-term projection phase. Efficacy for eladocagene exuparvovec is derived from clinical trial data with a duration up to 120 months. As the incidence of AADC deficiency is low, data for key model inputs is lacking; therefore estimates of survival by motor milestone were based on proxy diseases. A disease-specific utility study provided quality of life inputs and a burden of illness study informed inputs for disease management.

RESULTS

The model indicates survival (25.25 undiscounted life years gained) and quality-of-life benefits (20.21 undiscounted quality-adjusted life years [QALYs] gained) for patients treated with eladocagene exuparvovec compared to BSC. Resource usage costs are greater for patients treated with eladocagene exuparvovec, mainly due to the increased life expectancy during which patients accrue additional healthcare resource usage. Scenario analyses indicate robust results.

CONCLUSION

This study assessed long-term outcomes for patients with AADC deficiency. Patients treated with eladocagene exuparvovec were found to have improved survival and quality of life benefits compared to patients treated with BSC.

Spectrum of DDC variants causing aromatic l-amino acid decarboxylase (AADC) deficiency and pathogenicity interpretation using ACMG-AMP/ACGS recommendations

Pathogenic variants in dopa decarboxylase (DDC), the gene encoding the aromatic l-amino acid decarboxylase (AADC) enzyme, lead to a severe deficiency of neurotransmitters, resulting in neurological, neuromuscular, and behavioral manifestations clinically characterized by developmental delays, oculogyric crises, dystonia, and severe neurologic dysfunction in infancy. Historically, therapy has been aimed at compensating for neurotransmitter abnormalities, but response to pharmacologic therapy varies, and in most cases, the therapy shows little or no benefit. A novel human DDC gene therapy was recently approved in the European Union that targets the underlying genetic cause of the disorder, providing a new treatment option for patients with AADC deficiency. However, the applicability of human DDC gene therapy depends on the ability of laboratories and clinicians to interpret the results of genetic testing accurately enough to diagnose the patient. An accurate interpretation of genetic variants depends in turn on expert-guided curation of locus-specific databases. The purpose of this research was to identify previously uncharacterized DDC variants that are of pathologic significance in AADC deficiency as well as characterize and curate variants of unknown significance (VUSs) to further advance the diagnostic accuracy of genetic testing for this condition. DDC variants were identified using existing databases and the literature. The pathogenicity of the variants was classified using modified American College of Medical Genetics and Genomics/Association for Molecular Pathology/Association for Clinical Genomic Science (ACMG-AMP/ACGS) criteria. To improve the current variant interpretation recommendations, in silico variant interpretation tools were combined with structural 3D modeling of protein variants and applied comparative analysis to predict the impact of the variant on protein function. A total of 422 variants were identified (http://biopku.org/home/pnddb.asp). Variants were identified on nearly all introns and exons of the DDC gene, as well as the 3' and 5' untranslated regions. The largest percentage of the identified variants (48%) were classified as missense variants. The molecular effects of these missense variants were then predicted, and the pathogenicity of each was classified using a number of variant effect predictors. Using ACMG-AMP/ACGS criteria, 7% of variants were classified as pathogenic, 32% as likely pathogenic, 58% as VUSs of varying subclassifications, 1% as likely benign, and 1% as benign. For 101 out of 108 reported genotypes, at least one allele was classified as pathogenic or likely pathogenic. In silico variant pathogenicity interpretation tools, combined with structural 3D modeling of variant proteins and applied comparative analysis, have improved the current DDC variant interpretation recommendations, particularly of VUSs.

Burden and severity of disease of aromatic L-amino acid decarboxylase deficiency: a systematic literature review

OBJECTIVE

The objective was to investigate the severity of aromatic L-amino acid decarboxylase deficiency (AADCd) as reported in the published literature and to collate evidence of the clinical manifestations of AADCd, and the impact of the disease on patients, caregivers, and healthcare systems.

METHODS

Published articles reporting severity of disease or disease impact were eligible for inclusion in this review. Articles were searched in MEDLINE, EMBASE, Cochrane CENTRAL, TRIP medical, and CRD databases in October 2021. The quality of the included studies was investigated using a modified version of the grading system of the Centre for Evidence-Based Medicine (CEBM). Descriptive data of the literature was extracted and a narrative synthesis of the results across studies was conducted. This review is reported according to the PRISMA reporting guidelines for systematic reviews.

RESULTS

The search identified 970 unique reports, of which 59 met eligibility criteria to be included in the review. Of these, 48 included reports provided details on the clinical manifestations of AADCd. Two reports explored the disease impact on patients, while four described the impact on caregivers. Five reports assessed the impact on healthcare systems. Individuals with AADCd experience very severe clinical manifestations regardless of motor milestones achieved, and present with a spectrum of other complications. Individuals with AADCd present with very limited function, which, in combination with additional complications, substantially impact the quality-of-life of individuals and their caregivers. The five studies which explore the impact on the healthcare system reported that adequate care of individuals with AADCd requires a vast array of medical services and supportive therapies.

CONCLUSIONS

Irrespective of the ambulatory status of individuals, AADCd is a debilitating disease that significantly impacts quality-of-life for individuals and caregivers. It impacts the healthcare system due to the need for complex coordinated activities of a multidisciplinary specialist team.

Aromatic L-amino acid decarboxylase deficiency: a systematic review

Aim: To gain greater knowledge regarding the natural history of aromatic L-amino acid decarboxylase (AADC) deficiency, a genetic disorder that causes severe deficits in motor and cognitive development. Materials & methods: A systematic literature review was performed of all case reports and clinical studies published through December 2019 of patients with AADC deficiency. The data were summarized descriptively. Results: The search identified 94 publications that described 237 unique patients. Mean (standard deviation) age at diagnosis was 3.2 (±5.7) years and 16 deaths were reported. Most patients (57%) received the standard of care therapies, which showed limited efficacy in this patient population. Conclusion: AADC deficiency is a devastating disease and prospectively defined natural history studies are warranted to further understand this disease.

Burden of illness of aromatic L-amino acid decarboxylase deficiency: a survey of physicians in Southern Europe

BACKGROUND

Aromatic L-amino acid decarboxylase deficiency (AADCd) is an ultra-rare genetic neurometabolic disorder caused by mutations in the DDC gene.

OBJECTIVE

This retrospective, noninterventional study was designed to describe the burden of AADCd including the associated healthcare resource utilization in Southern Europe.

METHODS

Eleven clinicians completed a patient case study survey for patients with AADCd currently or previously under their care, followed by an interview with each clinician to assess healthcare resource utilization, patient characteristics, and symptoms.

RESULTS

Clinicians provided data for 20 patients with AADCd, of whom 60% were male. All patients experienced movement disorders, 90% exhibited developmental delay, 85% reported sleeping problems, and 80% experienced gastrointestinal problems. The symptoms varied with disease severity. Patients with AADCd received care from more than 16 different specialists including both medical and paramedical healthcare professionals. Hospitalizations and visits to accident and emergency departments were also frequent.

CONCLUSION

In terms of symptoms and healthcare resource utilization, the burden of illness of AADCd is substantial. This study provides insights into several aspects of the disease that are difficult to ascertain from published case reports.

Using the EQ-5D to measure caregiver health-related quality of life in rare pediatric disease: a study in aromatic L-amino acid decarboxylase deficiency

Aims: To evaluate how well the EQ-5D-5L, a generic preference-based measure of health-related quality of life, captures caregiver burden in a rare pediatric neurotransmitter disease. Materials & methods: Caregivers (n = 14) of individuals with aromatic L-amino acid decarboxylase (AADC) deficiency completed qualitative interviews on their experience as a caregiver, the EQ-5D-5L and a background questionnaire. Qualitative and quantitative data were compared to determine whether there was concordance or discordance in the findings. Results: No caregivers reported problems with mobility and self-care in either the qualitative interviews or on the EQ-5D-5L, and there was general concordance for pain/discomfort and anxiety/depression. However, discordance was found for usual activities, with 79% reporting no problems with this dimension on the EQ-5D-5L, compared with 100% describing substantial limitations during the interviews. Conclusion: The EQ-5D-5L may not be appropriate to evaluate caregiver burden in AADC deficiency, where caregivers' perceptions of “usual activities” differ substantially from the general population.

Symptoms and impacts of aromatic l-amino acid decarboxylase (AADC) deficiency among individuals with different levels of motor function

Background

Aromatic l-amino acid decarboxylase (AADC) deficiency is a rare neurological disorder associated with a range of symptoms and functional impairments. The aim of this study was to describe the experience of AADC deficiency across five different motor milestone health states.

Methods

Qualitative interviews were conducted with caregivers of individuals with AADC deficiency in Italy, Spain, Portugal and the United States. An interview guide was developed with input from clinical experts and caregivers, and explored the symptoms and impacts of AADC deficiency. Interviews were conducted by telephone and were recorded and transcribed. Data were analysed using thematic analysis and the symptoms and impacts were compared across health states.

Results

Fourteen caregivers took part, who provided care to 13 individuals with AADC deficiency aged 1–15 years. Six individuals were in the ‘no motor function’ health state, one in the ‘sitting unsupported’ health state, one in the ‘standing/stepping when fully supported’ health state and five in the ‘walking with minimal support’ health state. The results highlight a substantial impact of AADC deficiency, even among those who were able to walk with minimal support. Overall, those with better motor function also had better functional hand use, communication skills, ability to eat and perform other activities independently, and interact with their peers. The burden of caring was high across all health states, but caregivers of individuals in the walking health state were better able to participate in social and leisure activities.

Conclusion

Individuals with higher levels of motor function had less severe symptoms and were better able to perform their daily, leisure and social activities. Treatments which improve motor function have the potential to improve other aspects of the lives of individuals with AADC deficiency and their caregivers.

The impact of caring for an individual with aromatic l-amino acid decarboxylase (AADC) deficiency: a qualitative study and the development of a conceptual model

BACKGROUND

Aromatic l-amino acid decarboxylase (AADC) deficiency is a rare neurological condition, associated with a wide range of symptoms and functional issues, such as profound motor impairment and learning disability. Most individuals with AADC deficiency are completely dependent on their caregivers. This study explored the impact of caring for an individual with AADC deficiency.

METHODS

Qualitative interviews were conducted with caregivers of individuals with AADC deficiency in Italy, Portugal, Spain and the United States. An interview guide was developed with input from clinical experts and caregivers and included questions on the impact of caring for an individual with AADC deficiency. Interviews were conducted by telephone/videoconference and were recorded and transcribed. Data were analysed using thematic analysis.

RESULTS

Fourteen caregivers took part who provided care to 13 individuals with AADC deficiency aged 1-15 years. Caregivers reported that their lives centred around the individual with AADC deficiency, due to their need for 24-hour care and regular healthcare appointments. They reported both proximal impacts (impact on time, planning, physical health and emotional wellbeing), and distal impacts (impact on social/leisure activities, relationships, work and finances). These concepts and relationships were illustrated in a conceptual model.

CONCLUSIONS

This is the first qualitative study to report on the experience of caring for an individual with AADC deficiency. Caregivers reported that caring had a substantial multifaceted impact on their lives. These findings highlight the importance of considering the caregiver experience when evaluating the burden of AADC deficiency.