Two Japanese cases with aspartylglycosaminuria: clinical and morphological features

Aspartylglucosaminuria: Clinical Presentation and Potential Therapies

Aspartylglucosaminuria (AGU) is a recessively inherited neurodegenerative lysosomal storage disease characterized by progressive intellectual disability, skeletal abnormalities, connective tissue overgrowth, gait disturbance, and seizures followed by premature death. AGU is caused by pathogenic variants in the aspartylglucosaminidase (AGA) gene, leading to glycoasparagine accumulation and cellular dysfunction. Although more prevalent in the Finnish population, more than 30 AGA variants have been identified worldwide. Owing to its rarity, AGU may be largely underdiagnosed. Recognition of the following early clinical features may aid in AGU diagnosis: developmental delays, hyperactivity, early growth spurt, inguinal and abdominal hernias, clumsiness, characteristic facial features, recurring upper respiratory and ear infections, tonsillectomy, multiple sets of tympanostomy tube placement, and sleep problems. Although no curative therapies currently exist, early diagnosis may provide benefit through the provision of anticipatory guidance, management of expectations, early interventions, and prophylaxis; it will also be crucial for increased clinical benefits of future AGU disease-modifying therapies.

Aspartylglucosaminuria caused by a novel homozygous mutation in the AGA gene was identified by an exome-first approach in a patient from Japan

BACKGROUND

Aspartylglucosaminuria (AGU) is an autosomal recessive lysosomal storage disorder caused by a deficiency of the lysosomal enzyme, aspartylglucosaminidase (AGA). This disorder is rare in the general population except in Finland. Since the most characteristic feature of this disorder is a progressive developmental regression, patients often show no specific symptoms in the initial stages, and thus early diagnosis is often challenging.

CASE REPORT

We encountered a 16-year-old boy who began to show difficulties in his speech at the age of 6years. Due to a mild regression in his development, he gradually lost common daily abilities. His diagnosis was first obtained through exome sequencing that identified a novel homozygous mutation in the AGA gene. This result was reasonable because of parental consanguinity. Reduced enzymatic activity of AGA was then confirmed. His urine was retrospectively screened by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and a specific pattern of abnormal metabolites was identified.

CONCLUSIONS

Because both exome sequencing and MALDI-TOF-MS screening are adaptable and comprehensive, future combinatory use of these methods would be useful for diagnosis of rare inborn errors of metabolism such as AGU.

Aspartylglucosaminuria: unusual neonatal presentation in Qatari twins with a novel aspartylglucosaminidase gene mutation and 3 new cases in a Turkish family

Aspartylglucosaminuria is a rare autosomal recessive lysosomal storage disorder leading early to a progressive intellectual disability. Monozygous Qatari twins presented with an unusual perinatal manifestation characterized by severe muscular hypotonia, scarce spontaneous movements, multiple contractures, and respiratory insufficiency. Biochemical investigations suggested aspartylglucosaminuria, and a novel homozygous mutation c.439T>C (p.S147P) was found in the aspartylglucosaminidase gene. However, it cannot be excluded that the unusual neonatal presentation is due to an additional autosomal recessive disease in this multiply consanguineous family. The classical aspartylglucosaminuria phenotype (progressive speech delay, psychomotor retardation, and behavioral abnormalities) was observed in 3 Turkish siblings. Although aspartylglucosaminuria was suspected early, the definite diagnosis was not confirmed until the age of 18 years. A novel homozygous mutation c.346C>T (p.R116W) was found. These 5 cases emphasize that aspartylglucosaminuria is panethnic and may possibly present with prenatal manifestation. Screening for aspartylglucosaminuria should be done in all patients with unexplained psychomotor retardation.

Startle epilepsy complicating aspartylglucosaminuria

A 21-year-old right-handed man with definite diagnosis of aspartylglucosaminuria (AGU) presented with a 5-year history of progressive severe gait disturbance with frequent falls and generalized epileptic seizures triggered by unexpected stimuli. At one time, he was confined to a wheelchair because of the frequent falls. Electromyogram recording showed a large, excessive and not habituating motor startle response, with the classical and stereotyped order of muscle recruitment. During video-polygraphic recording, we recorded a reflex generalized tonic seizure triggered by a loud, unexpected acoustic stimulus. Brain magnetic resonance (MR) revealed no structural abnormality. A diagnosis of abnormal startle and startle epilepsy (SE) was made. The addition of clonazepam to valproate and phenobarbital led to a dramatic improvement in his abnormal startle and SE, and the patient was able to walk alone unaided. This report illustrates, for the first time, that abnormal startle and SE may occur in AGU and complicate its clinical picture. Recognition of this entity in AGU is important, as progressive gait disorder with frequent falls could be easily misinterpreted as an additional irreversible manifestation of the ongoing neurological deterioration characteristic of AGU.

Angiokeratoma corporis diffusum in a Spanish patient with aspartylglucosaminuria

Angiokeratoma corporis diffusum (ACD), initially considered to be synonymous with Fabry's disease, represents a well-known cutaneous marker of some other lysosomal enzyme disorders. Aspartylglucosaminuria (AGU) is a rare hereditary disorder mostly affecting the Finnish population, with only a few sporadic patients of non-Finnish origin. To date, only three patients with AGU have been reported with cutaneous lesions of ACD. A 19-year-old Spanish woman presented with a 10-year history of progressive ACD affecting the limbs, buttocks and trunk. After the age of 6 years she had developed progressive mental deterioration, coarse facies and macroglossia with a scrotal appearance. Peripheral blood smears showed many vacuolated lymphocytes. Enzyme analysis in cultured fibroblasts revealed a decreased activity of aspartylglucosaminidase. By the age of 31 years the patient had developed a bipolar psychosis, polycystic ovarian disease and severe impairment of cognitive skills. This is the first case of AGU detected in a Spanish patient presenting with cutaneous lesions of ACD. To our knowledge, macroglossia with a scrotal appearance and polycystic ovarian disease have not been reported in previous cases of AGU.

Aspartylglycosaminuria: biochemistry and molecular biology

Aspartylglucosaminuria (AGU, McKusick 208400) is an autosomal recessive lysosomal storage disease caused by defective degradation of Asn-linked glycoproteins. AGU mutations occur in the gene (AGA) for glycosylasparaginase, the enzyme necessary for hydrolysis of the protein oligosaccharide linkage in Asn-linked glycoprotein substrates undergoing metabolic turnover. Loss of glycosylasparaginase activity leads to accumulation of the linkage unit Asn-GlcNAc in tissue lysosomes. Storage of this fragment affects the pathophysiology of neuronal cells most severely. The patients notably suffer from decreased cognitive abilities, skeletal abnormalities and facial grotesqueness. The progress of the disease is slower than in many other lysosomal storage diseases. The patients appear normal during infancy and generally live from 25 to 45 years. A specific AGU mutation is concentrated in the Finnish population with over 200 patients. The carrier frequency in Finland has been estimated to be in the range of 2.5-3% of the population. So far there are 20 other rare family AGU alleles that have been characterized at the molecular level in the world's population. Recently, two knockout mouse models for AGU have been developed. In addition, the crystal structure of human leukocyte glycosylasparaginase has been determined and the protein has a unique alphabetabetaalpha sandwich fold shared by a newly recognized family of important enzymes called N-terminal nucleophile (Ntn) hydrolases. The nascent single-chain precursor of glycosylase araginase self-cleaves into its mature alpha- and beta-subunits, a reaction required to activate the enzyme. This interesting biochemical feature is also shared by most of the Ntn-hydrolase family of proteins. Many of the disease-causing mutations prevent proper folding and subsequent activation of the glycosylasparaginase.

Characterization of four monosialo and a novel disialo Asn N-glycosides from the urine of a patient with aspartylglycosaminuria

We previously reported for the first time two Japanese patients with aspartylglycosaminuria (AGU). A novel disialo Asn N-glycoside (AG-5) has been isolated from the urine of one of the patients in addition to four known monosialo Asn N-glycosides (AG-1 to AG-4) by gel filtration and anion exchange chromatography in this study. Final purification of AG-5 was achieved by an electrochemical chromatographic method, high performance liquid chromatography with pulsed amperometric detector (HPLC-PAD). The yield of AG-5 was approximately 1 mg l-1 urine. The chemical structures of AG-1 to AG-5 were characterized by gas-liquid chromatography, a permethylation study, fast atom bombardment-mass spectrometry (FAB-MS), and nuclear magnetic resonance (NMR). Based on the structural analysis, AG-5 had the following novel structure: NeuAc alpha 2-->8NeuAc alpha 2-->3Gal beta 1-->4GlcNAc beta 1-->Asn.

Histopathologic and ultrastructural studies of angiokeratoma corporis diffusum in Kanzaki disease

A novel metabolic disease, angiokeratoma corporis diffusum (Kanzaki), was the subject of an extensive histopathologic and ultrastructural study. Findings included dilated lymph and blood vessels in the upper dermis with an orthokeratortic, thickened, horny layer in well developed angiokeratoma. In the early papules, a few sporadic dyskeratotic keratinocytes were present in the epidermis with or without a thickened horny layer. Vesicular clear vacuolation was clearly observed in the cytoplasm of the secretory portion of the eccrine sweat glands, but none was observed in the vascular endothelial cells with hematoxylin-eosin staining. Using electron microscopy, lysosomal vacuolation was observed in many cell types, including eccrine sweat gland cells, vascular endothelial cells, dermal fibroblasts, dermal neural cells, lymphocytes of peripheral blood in the skin, and glomerular endothelial cells, but none was noted in the epithelial cells of the kidney. Widely dilated vacuoles were found to contain only a small amount of fuzzy filamentous material in the vascular endothelial cells, filamentous or electron-dense granular substances in fibroblasts, and electron-dense, lamellated or homogeneous structures in eccrine sweat gland cells and in neural cells. Ultracytochemical examination revealed glycoconjugates in dilated lysosomes. Characteristics of Kanzaki Disease were shown to differ from those of Fabry disease or any other lysosomal storage disease.