Developmental delay and non-phenylketonuria (PKU) hyperphenylalaninemia in DNAJC12 deficiency: Case and approach

Impaired cognitive function and decreased monoamine neurotransmitters in the DNAJC12 gene knockout mouse model

BACKGROUND

Hyperphenylalaninemia, a prevalent amino acid metabolism disorder, often results in cognitive impairment. Recent studies have identified a rare variant of this disorder caused by mutations in the DNAJC12 gene. The specific mechanisms by which DNAJC12 mutations lead to hyperphenylalaninemia and the lack of an animal model for study remain significant gaps in understanding.

PURPOSE

This study aims to elucidate the role of DNAJC12 in intellectual disability and explore the mechanisms by which DNAJC12 deficiency leads to hyperphenylalaninemia through developing a DNAJC12 gene knockout mouse model.

METHODS

We thoroughly examined the clinical features and genetic mutations evident in two patients with biallelic mutations in the DNAJC12 gene. Based on the patient's mutation locations, we determined the target site for the knockout utilizing CRISPR/Cas9 technology. To assess the impact of these mutations on DNAJC12 expression, we used quantitative real-time PCR and Western blotting techniques to measure mRNA and protein levels, respectively. The Morris water maze test was administered to evaluate the cognitive functions of the mice. Additionally, we utilized High-Performance Liquid Chromatography (HPLC) to measure serum aromatic amino acids and brain monoamines, facilitating an investigation into the metabolism of phenylalanine and tyrosine.

RESULTS

We reported two patients with mutations in the DNAJC12 gene. Case 1 carried the mutations c.158-1G > A and c.262 C > T in the DNAJC12 gene. He presented with nocturnal eye closure, crying, and arching back in reverse tension before treatment, suggesting a neurotransmitter metabolism disorder. Case 2 carried the mutations c.473 C > G, and c.102 deletion in the DNAJC12 gene. He showed elevated blood phenylalanine levels, although further clinical details were not available. Based on the patients' mutation locations, exons 2-4 of the DNAJC12 gene were targeted and eliminated. In our animal model experiments, DNAJC12 gene knockout mice exhibited a complete absence of DNAJC12 expression at both mRNA and protein levels. These knockout mice showed significant deficits in learning and memory performance as assessed by the Morris water maze test. Quantitative real-time PCR analysis indicated no significant differences in the levels of aromatic amino hydroxylases between knockout and wild-type mice. However, Western blot analysis revealed a substantial reduction in phenylalanine hydroxylase (PAH) protein levels in the liver of knockout mice, while tyrosine hydroxylase (TH) and tryptophan hydroxylase 2 (TPH2) expression remained unchanged. HPLC analysis demonstrated increased serum Phe concentrations and decreased levels of brain neurotransmitters in the knockout group.

CONCLUSIONS

We report two patients with four novel DNAJC12 mutations (c.158-1G > A, c.262 C > T, c.473 C > G, c.102delT), expanding the mutation spectrum. Based on the patients' mutation location, we established the first DNAJC12 gene knockout mouse model. The knockout mice exhibit hyperphenylalaninemia, impaired cognitive function, and decreased monoamine neurotransmitters. DNAJC12 deficiency may contribute to the clinical phenotype via the PAH pathway, potentially at the post-transcriptional level.

DNAJC12 in Monoamine Metabolism, Neurodevelopment, and Neurodegeneration

Recent studies show that pathogenic variants in DNAJC12, a co-chaperone for monoamine synthesis, may cause mild hyperphenylalaninemia with infantile dystonia, young-onset parkinsonism, developmental delay and cognitive deficits. DNAJC12 has been included in newborn screening, most revealingly in Spain, and those results highlight the importance of genetic diagnosis and early intervention in combating human disease. However, practitioners may be unaware of these advances and it is probable that many patients, especially adults, have yet to receive molecular testing for DNAJC12. Hence, this review summarizes genotype-phenotype relationships and treatment paradigms for patients with pathogenic variants in DNAJC12. It provides an overview of the structure of DNAJC12 protein, known genetic variants, domains, and binding partners, and elaborates on its role in monoamine synthesis, disease etiology, and pathogenesis. © 2023 International Parkinson and Movement Disorder Society.

DNAJC12 deficiency: Mild hyperphenylalaninemia and neurological impairment in two siblings

Background

DNAJC12 co-chaperone protein deficiency has been recently described as a stand-alone metabolic disorder explaining many cases of mild hyperphenylalaninemia (HPA) that are not caused by variants in the PAH gene, which encodes for the hepatic enzyme phenylalanine hydroxylase (PAH), or inGCH1, PTS, QDPR, PCBD1 and DHPR, involved in tetrahydrobiopterin (BH4) biosynthesis and activity.

Results

We describe two sisters born to consanguineous parents. The youngest sister (Patient 1), initially asymptomatic, tested positive at NewBorn Screening (NBS) for mild HPA. After variants in the PAH and BH4 related-genes were excluded, we performed DNAJC12 genetic analysis and found a previously described homozygous deletion [NM_021800.3: c.58_59del p.(Gly20Metfs*2)]. The older sister (Patient 2), homozygous for the same variant and exhibiting mild HPA, was diagnosed subsequently and presented with ataxia and repeated falls, upper limb dyskinesia, intentional tremor, and mild intellectual disability. Patient 1 was started on treatment with low Phenylalanine (Phe) diet, BH4, l-3,4-dihydroxyphenylalanine/carbidopa (L-DOPA) and 5-OH-Tryptophan, soon after diagnosis, and despite poor adherence to the dietary regimen, only manifested language impairment at last follow-up (age 5 years and 4 months). Patient 2, who started the same treatment at school age, experienced a minimal progression of neurological symptoms, with some improvement in her motor skills.

Conclusions

These two new patients with DNAJC12-associated HPA, in addition to previous reports, point to DNAJC12 deficiency as a new metabolic syndrome that must be considered in patients with unexplained HPA.