Aspartylglucosaminuria

A number sign (#) is used with this entry because aspartylglucosaminuria (AGU) is caused by homozygous mutation in the AGA gene (613228) on chromosome 4q34.

Description

Aspartylglucosaminuria is a severe autosomal recessive lysosomal storage disorder that involves the central nervous system and causes skeletal abnormalities as well as connective tissue lesions. The most characteristic feature is progressive mental retardation. The disorder is caused by deficient activity of the lysosomal enzyme glycosylasparaginase, which results in body fluid and tissue accumulation of a series of glycoasparagines, i.e., glycoconjugates with an aspartylglucosamine moiety at the reducing end. AGU belongs to the group of disorders commonly referred to as the Finnish disease heritage (summary by Mononen et al., 1993 and Arvio and Arvio, 2002).

Clinical Features

Aspartylglucosaminuria was first reported by Jenner and Pollitt (1967) and Pollitt et al. (1968), who found urinary excretion of abnormal amounts of 2-acetamido-1-(beta-L-aspartamido)-1,2-dideoxyglucose in a 32-year-old female and her 20-year-old brother with mental retardation. An enzyme responsible for hydrolyzing this compound is normally present in seminal fluid but was absent in that of the brother. A generalized lack of this enzyme was postulated. Both sibs had thick sagging skin of the cheeks, a finding not present in normal members of the family.

Palo and Mattsson (1970) reported 11 cases in Finland. The parents of 1 patient were first cousins. The Finnish cases showed, in addition to severe mental retardation, sagging cheeks, broad nose and face, short neck, cranial asymmetry, scoliosis, periodic hyperactivity, and vacuolated lymphocytes. Diarrhea and frequent infections were problems in infancy. Aspartylglucosaminuria has also been observed in Finns living in Norway (Borud and Torp, 1976).

Gehler et al. (1981) described affected brother and sister in a consanguineous Italian sibship; one of the patients showed angiokeratoma corporis diffusum. Yoshida et al. (1991) and Vargas-Diez et al. (2002) also described the occurrence of angiokeratoma corporis diffusum in 2 Japanese patients and 1 Spanish patient, respectively, with aspartylglucosaminuria.

Stevenson et al. (1982) reported this disorder in an 18-year-old American. The family name was Scottish-Irish. The mother was said to have been aged 13 years and the father was unknown--circumstances suggesting incest. Mental retardation, recurrent infections, cardiomyopathy, and emotional lability were features.

Hreidarsson et al. (1983) reported a case in an American black and an American white of uncertain parentage. Radiographic changes in the hands were noted: thin epiphyses, broad 'poorly modeled' (undertubulated) metacarpals, and peculiarly shaped carpal bones.

Isenberg and Sharp (1975) reported the case of a girl of Mexican-Italian extraction living in the U.S.

Musumeci et al. (1984) reported a child with both enzymopathic methemoglobinemia (250800) and AGU. Since the structural genes for the enzymes deficient in these 2 disorders are on separate chromosomes, a single mutation such as a small deletion is not likely to be the basis. Furthermore, a sib had only AGU. The parents were consanguineous.

Chitayat et al. (1988) described 3 Puerto Rican brothers, with first-cousin parents, who had AGU. Two of the brothers were monozygotic twins. Macroorchidism became evident in all 3 boys at the time of puberty. This feature had not previously been noted in AGU, although other endocrinologic abnormalities had been described.

Yoshida et al. (1991) described the first Japanese patients with AGU--a brother and sister, aged 45 and 41, respectively. Both sibs had mental retardation, coarse facial features, angiokeratoma, and myoclonic seizures.

Zlotogora et al. (1997) diagnosed this disorder in 8 patients originating from 3 unrelated families, all Palestinian Arabs from the region of Jerusalem.

Gordon et al. (1998) described a Canadian family in which 4 of 12 sibs were affected, 2 brothers and 2 sisters. Though apparently normal at birth, their developmental milestones, particularly speech, were slow, and they acquired only a simple vocabulary. There was a progressive coarsening of facial features; 3 had inguinal hernia and recurrent diarrhea; all became severely retarded and by the fourth decade showed evident deterioration of both cognitive and motor skills; and 2 exhibited cyclic behavioral changes. Three of the sibs had died, at 33, 39, and 44 years of age.

Arvio et al. (1999) studied 66 Finnish patients with AGU for changes in the oral mucosa and 44 of those for changes in facial skin. Nine patients had facial angiofibromas. Edema of the buccal mucosa and gingival overgrowths were more frequent in AGU patients than in controls (P less than 0.001). Of 16 oral mucosal lesions studied histologically, 15 represented fibroepithelial or epithelial hyperplasias. Cytoplasmic vacuolization was evident in only 4. Expression of AGA in mucosal lesions of AGU patients did not differ from that seen in corresponding lesions of normal subjects.

Diagnosis

Mononen et al. (1994) described a fluorometric glycosylasparaginase assay for neonatal screening for AGU.

Zlotogora et al. (1997) stated that the clinical diagnosis of AGU is difficult, in particular early in the course of the disease; most of the patients are diagnosed after the age of 5 years. They noted that since patients with AGU excrete large amounts of aspartylglucosamine in urine, biochemical detection is easy by urine chromatography.

Clinical Management

Arvio et al. (2001) described the state of health, intellectual skills, and dysmorphic features of 19 young patients with aspartylglucosaminuria. Of the 19, 5 had undergone a successful bone marrow transplantation between 1991 and 1997. The first 2 patients who received transplants were, after 7 and 5 years' follow-up, more severely mentally retarded than the nontransplanted patients. The general health of the latter group was quite good, whereas the 5 patients who underwent bone marrow transplantation had posttransplant complications. Arvio et al. (2001) concluded that bone marrow transplantation should not be encouraged for the treatment of patients with aspartylglucosaminuria after infancy.

Mapping

In 12 AGU families with 15 affected persons and 50 carriers (determined by reduced activity of enzyme in lymphocytes), Gron et al. (1989, 1990) studied linkage to chromosome 4 markers and concluded that the locus is distal to MNS (111300). They suggested the order cen--ADH--EGF--FG--MNS--AGU.

Population Genetics

Aspartylglucosaminuria occurs worldwide, but is enriched in the Finnish population (Arvio and Arvio, 2002).

Palo and Mattsson (1970) estimated that there are at least 130 cases in the total population of 4.5 million in Finland.

Autio (1980) estimated the frequency at 1 in 26,000 in Finland. A total of 128 cases in 97 families had been identified.

Mononen et al. (1991) found a frequency of 1 in 3,643 in a study of children in eastern Finland. This frequency is consistent with a carrier rate of 1 in 30 and indicates that this disorder, after trisomy 21 and the fragile X syndrome, is the most common genetic cause of mental retardation.

Molecular Genetics

In Finnish patients with aspartylglucosaminuria, Ikonen et al. (1991) and Fisher and Aronson (1991) independently identified homozygosity for a cys163-to-ser (C163S; 613228.0001) mutation in the AGA gene. The C163S mutation is responsible for 98% of the cases of AGU in Finland (Isoniemi et al., 1995).

Ikonen et al. (1991) described the spectrum of 10 AGU mutations found in 12 unrelated patients of non-Finnish origin. Since 11 of the 12 were homozygotes, consanguinity appears to be a common denominator in most AGU families, although consanguinity could be confirmed in only 2 of the families. Screening for the unknown gene defects was done using single-strand conformation polymorphism (SSCP) analysis. The mutations were distributed over the entire coding region of the AGU cDNA, except in the carboxyl-terminal 17-kD subunit in which they were clustered within a 46-amino acid region. Based on the character of the mutations, Ikonen et al. (1991) concluded that most of the mutations probably affected the folding and stability of the molecule and did not directly affect the active site of the enzyme. There were 3 non-Finnish patients who had the 'Finnish' cys163-to-ser mutation (613228.0001) but 2 of them were Norwegian and 1 was Swedish. These patients presumably had Finnish ancestry (Borud and Torp, 1976).

Tollersrud et al. (1994) reported on 9 patients from 7 families identified in northern Norway. All were homozygous for the Finnish C163S founder mutation. Genealogic investigation of 9 parents proved Finnish ancestry in all pedigrees. These Finnish immigrants originated in the main from the Tornio valley in northern Finland in a continuous immigration movement from 1700 to 1900.

Ikonen and Peltonen (1992) reviewed a total of 11 AGA mutations causing AGU published to that time.

Animal Model

Through targeted disruption of the mouse Aga gene in embryonic stem cells, Kaartinen et al. (1996) generated mice that completely lack Aga activity. At the age of 5 to 10 months, a massive accumulation of aspartylglucosamine was detected in Aga-null mice along with lysosomal vacuolization, axonal swelling in the gracile nucleus, and impaired neuromotor coordination. A significant number of older male mice had massively swollen bladders, which was not caused by obstruction, but was most likely related to the impaired function of the nervous system. The findings were considered consistent with the pathogenesis of AGU and provided further data explaining the impaired neurologic function in AGU patients.

Gonzalez-Gomez et al. (1998) reported that after the age of 10 months the general condition of the null mutant mice created by Kaartinen et al. (1996) gradually deteriorated. They suffered from progressive motor impairment and impaired bladder function and died prematurely. A widespread lysosomal hypertrophy in the central nervous system was detected. The oldest animals (20 months old) displayed neuronal loss and gliosis, particularly in the regions where the most severe neuronal vacuolation was found. The severe ataxic gait of the older mice was probably due to the dramatic loss of Purkinje cells, intensive astrogliosis and vacuolation of neurons in the deep cerebellar nuclei, and the severe vacuolation of the cells in vestibular and cochlear nuclei. The impaired bladder function and subsequent hydronephrosis were secondary to involvement of the central nervous system. The mice thus appeared to be a suitable animal model for testing therapeutic strategies in AGU.