Congenital Disorder of Glycosylation Type Ia Revealed by Hypertransaminasemia and Failure To Thrive in a Young Boy with Normal Neurodevelopment

INTRODUCTION Congenital disorders of glycosylation (CDG), formerly called carbohydrate deficient glycoprotein syndromes, are a group of inherited metabolic errors resulting in defective synthesis or processing of N-glycoproteins or, in some cases, of O-glycans. Hypoglycosylation of proteins leads to variable, generally severe, problems in affected individuals. Since CDG was first described in 1980 (1) several subtypes have been identified. Following recent recommendations (2), congenital disorders of N-glycosylation are divided into two groups. CDG type I comprises defects in the assembly of N-glycans in cytosol and endoplasmic reticulum, and CDG type II refers to defects in the processing of the protein-bound glycans in the endoplasmic reticulum or in the Golgi compartment. The most frequent subtype is CDG Ia, which originates from a phosphomannomutase 2 deficiency caused by a mutation of the PMM gene locus on chromosome 16p13 (3,4). We report the case of a 6-year-old boy who presented at 2 years of age with abnormal serum transaminases and failure to thrive as the main signs of a type Ia CDG syndrome. Neurologic signs were detected only on careful examination of the patient, as he showed only slight motor clumsiness and his neurodevelopment quotient (Brunet-Lézine) was normal. CASE REPORT The patient is the first child of healthy, non-consanguineous parents. He was born prematurely at 34 weeks. His birth weight was 1770 g, body length was 41 cm and head circumference was 31 cm. A subependymal hemorrhage was diagnosed in the first week of life. An upper tract urinary infection and frequent respiratory infections occurred in the 2 first years of life with no complications. Psychomotor development was normal. At 2 years of age failure to thrive was evident and laboratory tests showed alanine aminotransferase of 157 U/L and aspartate aminotransferase 105 U/L. Screening antibody titers for hepatitis A, hepatitis B and hepatitis C were negative. Antitransglutaminase and antigliadin antibody titers were positive. A small bowel mucosal biopsy was normal. He was examined in our hospital at the age of 2 years and 8 months. Clinical examination showed a poorly grown boy (weight and height at −2 standard deviations below normal for age) with no dysmorphic features. His liver edge was palpable 1-2 cm below the right costal margin. He had convergent strabismus and motor clumsiness when running. Studies obtained included hemogram, serum glucose, bilirubin, gamma glutamyl transpeptidase, fluorescent antibody, ammonium, lactate, pyruvate, creatine phosphokinase, α-1-antithrombin, immunoglobulin, plasma amino acids, thyroid-stimulating hormone, T4, insulin and peptide C. All of these tests were within normal limits. Mild elevation of serum transaminases remained with aspartate aminotransferase 82 U/L and alanine aminotransferase 66 U/L. Coagulation factors were reduced (antithrombin III 39, 6% and coagulating C protein 22%). A liver biopsy showed nonspecific inflammation. Echocardiography, abdominal ultrasound, renal function, brainstem auditory evoked potentials and electroencephalogram were normal. Ophthalmologic exploration revealed retinopathy consistent with retinitis pigmentosa, and position effect variegation and electroretinography were abnormal. Magnetic resonance imaging studies showed cerebellar vermis atrophy (Fig. 1). A quotient of development test was performed at 3 years with normal results (Brunet-Lezine quotient of development 91). Serum concentration of carbohydrate-deficient transferrin was 16% (normal value <7%). Isoelectric focusing of transferrin revealed a CDG type I pattern. Deficient PMM activity was found in fibroblasts (48%) and to a significantly greater extent in leukocytes (6%). The boy is compound heterozygous for the 278 A to C (E93A) and 422 G to A (R141H) mutations in the PMM2 gene. The first one is a known mutation and the second is the most common mutation found in this syndrome.FIG. 1: Brain magnetic resonance imaging scan shows slight atrophy of cerebellar vermis (arrow).At his most recent assessment at 5.5 years of age, clinical examination revealed slight dysmetria and minimal motor clumsiness without ataxia. Deep tendon reflexes are present. He has no hepatomegaly and his overall growth pattern is at −2 standard deviations. Serum transaminases concentration, coagulation factors and hepatomegaly factors had normalized at 3.5 years. The results of echocardiography, abdominal ultrasound and renal function testing remain within normal limits. His only complaint is poor night vision, and his electroretinography remains abnormal. He attends school and is doing well in a normal classroom environment. The minimal dysmetria does not limit his ability to write or draw. He has no other neurologic or systemic problems to date. DISCUSSION Glycosylation is the most common post-translational protein modification. It is a complex process in which sugars are added to proteins as they pass through the endoplasmic reticulum and Golgi apparatus. These carbohydrates play an essential role in modulating protein stability, conformation and interactions with other proteins involved in cell adhesion, differentiation and development. The clinical spectrum of CDG has expanded since it was first described by Jaeken et al. (1), and numerous patients have been described. CDG type Ia is the most common form. Patients have a defective activity of the PMM enzyme, which is coded by the PMM2 gene on chromosome 16p13. The clinical presentation of CDG syndrome type Ia is characterized by severe neurologic abnormalities, dysmorphism and variable involvement of other organs. The neurologic features are age-related (5). Hypotonia, muscular weakness, developmental retardation, alternating exotropia and hyporeflexia can be observed in the neonatal and infantile periods. After infancy, motor skills develop slowly. Cerebellar ataxia and coordination difficulties are common findings. Contractures gradually appear. Stroke-like episodes occur, sometimes in combination with infections, after the age of 4 to 5 years. Retinitis pigmentosa develops after 3 to 8 years. Mental development is retarded to varying degrees and is frequently moderate to severe. Possible non-neurologic features include slight facial dysmorphism, abnormal subcutaneous fat distribution, inverted nipples, mild hepatomegaly and hypogonadism. Some patients have a severe systemic illness that includes intestinal dysmotility, protein-losing enteropathy, hepatic dysfunction, pericardial effusions and cardiomyopathy. Most of the multi-organ manifestations are present in infancy and appear to regress during childhood (5). In CDG Ia, hepatic and gastroenterological symptoms have a high prevalence and are often severe in the neonatal period. However they are normally associated with a multisystem disorder and do not occur in isolation. By contrast, in older children neurologic involvement is the main sign. Gastrointestinal symptoms and liver disease are more characteristic of CDG type Ib (as a result of phosphomannose isomerase enzyme deficiency) and also of type Ih (as a result of dolichyl-P-glucose deficiency) (10,11). In these two cases clinical symptoms appear in the neonatal period (6,7). Our patient has an atypical CDG Ia phenotype. He presented initially at 2 years of age with gastrointestinal abnormalities (slight hypertransaminasemia with failure to thrive) suggestive of a hepatitis-like episode. Liver histologic studies supported this hypothesis. In the majority of cases, hepatodigestive symptoms present in the neonatal period and resolve after a few years. Low weight for gestational age and recurrent infections are common findings in neonates and young infants. As these are nonspecific signs, we were not led to suspect the diagnosis of CDG Ia in this patient. Our clinical examination revealed a near-total absence of neurologic involvement in our patient which suggests only the possibility of minor cerebellar dysfunction. The results of developmental testing (Brunet-Lezine and quotient of development) at 3 years of age and neurodevelopment and school performance at 5.5 years are normal. To our knowledge, all the reported patients with CDG type Ia have mental impairment that is mild in a few cases and severe in the majority of cases. There are no references in the literature to patients with normal intelligence and an absence of neurologic signs. Minimal neurologic signs, whether intellectual or motor, are extremely rare (8,9). Our patient's neurologic signs not only are mild but also have not worsened over 3.5 years of follow-up. Other features, such as strabismus and cerebellar hypoplasia, low levels of protein C and antithrombin III and retinitis pigmentosa are common in this disorder and were present in our patient. Numerous mutations, mostly missense, have been found in the PMM2 gene (10). R141H is a severe, common mutation present in the white population (75% of caucasian patients but only 43% to 50% in our geographical area) (11). Homozygosity of this mutation has not been reported and would most likely result in spontaneous abortion of the fetus. Usually heterozygous patients with R141H and other alleles have a severe neurologic or multisystem form of the disease (R141H/V231M, R141H/F119L) (12). Our patient is a compound heterozygote for the E93A/R141H mutations with mild clinical manifestations. We hypothesise that the E93A mutations modify the classic severe presentation of the well-known R141H mutation in our case. Molecular studies in a series of 26 Spanish patients with CDG Ia have demonstrated marked genetic heterogeneity in this population. This fact could explain the very diverse phenotypes, generally milder than in other series (11). In any case, it is not always possible to correlate genotype with phenotype, and it is difficult to predict the clinical features of an individual patient on the basis of a given genotype. Measurement of PMM activity in leukocytes correlates better with diagnosis than with values in fibroblasts. Grünewald et al (13) have already reported that these particular findings are mostly found in patients with mild to moderate clinical expression. In conclusion, disorders of glycosylation may manifest in a wide variety of clinical presentations. Our case demonstrates that well-preserved psychomotor development does not rule out CDG Ia syndrome. It is important for pediatricians to consider this diagnosis in children with failure to thrive or elevated serum transaminases of unclear origin. In particular, pediatricians should consider this group of metabolic/genetic conditions in children with mild clinical findings and in children with any kind of unexplained multisystem disorder, especially when hepatic and neurologic involvement is present. Acknowledgments: We are grateful to Dr. A. Chabás and Dr E. van Schaftingen for their assistance in performing enzymatic assays, and to Dr E Matthijs for molecular assays. Their collaboration took place in the context of EURO-GLYCAN, a European action on glycan defects, funded by a grant from the European Union's Quality of Life programme. (Fifth Framework Programme). Thanks to Susan M. DiGiacomo, Ph.D., editor in chief of the Fundació Sant Joan de Déu, for English language editing of the manuscript.