Glycogen storage disease, Fanconi nephropathy, abnormal galactose metabolism and mitochondrial myopathy

Inherited Fanconi syndrome

BACKGROUND

Fanconi-Debré-de Toni syndrome (also known as Fanconi renotubular syndrome, or FRST) profoundly increased the understanding of the functions of the proximal convoluted tubule (PCT) and provided important insights into the pathophysiology of several kidney diseases and drug toxicities.

DATA SOURCES

We searched Pubmed and Scopus databases to find relevant articles about FRST. This review article focuses on the physiology of the PCT, as well as on the physiopathology of FRST in children, its diagnosis, and treatment.

RESULTS

FRST encompasses a wide variety of inherited and acquired PCT alterations that lead to impairment of PCT reabsorption. In children, FRST often presents as a secondary feature of systemic disorders that impair energy supply, such as Lowe's syndrome, Dent's disease, cystinosis, hereditary fructose intolerance, galactosemia, tyrosinemia, Alport syndrome, and Wilson's disease. Although rare, congenital causes of FRST greatly impact the morbidity and mortality of patients and impose diagnostic challenges. Furthermore, its treatment is diverse and considers the ability of the clinician to identify the correct etiology of the disease.

CONCLUSION

The early diagnosis and treatment of pediatric patients with FRST improve the prognosis and the quality of life.

A secondary respiratory chain defect in a patient with Fanconi-Bickel syndrome

A North African boy, the son of consanguineous parents, presented at 8 years of age with hypophosphataemic rickets due to De Toni-Debré-Fanconi syndrome. Hepatomegaly and abnormalities of carbohydrate metabolism were suggestive of Fanconi-Bickel syndrome. This was confirmed by the detection of a mutation within GLUT2, the gene encoding the liver-type facilitative glucose transporter. The study of the respiratory chain revealed a deficiency of complexes I, III and IV in muscle. Mechanisms responsible for an impairment ofmitochondrial function, which we interpret as a secondary phenomenon, are discussed.

Renal manifestations of congenital lactic acidosis

Congenital lactic acidoses (CLAs) constitute a group of rare inborn errors of mitochondrial metabolism in which cellular energy failure is the defining biochemical abnormality. We report the principal manifestations of renal dysfunction in 35 children with CLA caused by defects in either the pyruvate dehydrogenase multienzyme complex or one or more components of the respiratory chain. The most prominent renal abnormalities included bicarbonaturia, phosphaturia, hypercalciuria, complete Fanconi's syndrome, proteinuria, and decreased glomerular filtration rate. These data were compared with those from 79 previously published cases. Clinical manifestations of renal dysfunction in CLA are common and may be the first presenting sign of the disease. The glomerulus and proximal renal tubule appear to be the anatomic sites most vulnerable to abnormal mitochondrial energy transduction. We propose that the primary defect in mitochondrial energy metabolism, together with the consequent intracellular accumulation of lactate and hydrogen ions, precipitates a state of tissue injury that, unless interrupted, becomes self-perpetuating and ultimately leads to renal cell death.

Renal involvement in mitochondrial cytopathies

Mitochondrial cytopathies have long been regarded as neuromuscular diseases. However, an oxidative phosphorylation disorder may give rise to various symptoms in other organs or tissues which are dependent upon mitochondrial energy supply. A broad spectrum of clinical symptoms have been described in these patients, including renal symptoms. The most frequent is proximal tubular dysfunction with a more or less complete de Toni-Debré-Fanconi syndrome. A few patients have been reported with tubular acidosis, Bartter syndrome, chronic tubulointerstitial nephritis, or nephrotic syndrome. The diagnosis of a respiratory chain deficiency is difficult when only renal symptoms are present but should be easier when another seemingly unrelated symptom is observed. Metabolic screening for abnormal oxidoreduction status in plasma, including lactate/pyruvate and ketone body molar ratios, can help to identify patients for further investigations. These include the measurement of oxygen consumption by mitochondria, the assessment of mitochondrial respiratory enzyme activities by spectrophotometric studies, and, when possible, the molecular analysis of mitochondrial DNA. Any mode of inheritance can be observed: sporadic, autosomal dominant or recessive, or maternal inheritance. No satisfactory therapy is presently available for mitochondrial disorders.

Diabetes-like renal glomerular disease in Fanconi-Bickel syndrome

The Fanconi-Bickel syndrome is a rare inherited disorder of metabolism characterized by hepatic glyconeogenesis, galactose intolerance, renal Fanconi syndrome with nephromegaly, and glycogen accumulation in proximal renal tubular cells. An 8-year-old patient with this disease and severe rickets due to medically resistant hypophosphatemia was found to have the previously unrecognized complication of renal glomerular hyperfiltration, microalbuminuria, and diffuse glomerular mesangial expansion. Similar to patients with glucose-6-phosphatase deficiency, the glomerular disease in this patient resembles incipient diabetic nephropathy. The Fanconi syndrome may be due to the defective transport of glucose at the proximal tubular basolateral membrane, which results in accumulation of glucose and secondarily glycogen within tubular cells. Since the metabolic defect, as evidenced by glycogen accumulation, selectively involves proximal renal tubular cells in the kidney of patients with Fanconi-Bickel syndrome and glucose-6-phosphatase deficiency, the abnormalities in renal glomerular hemodynamics and mesangial construct in these rare diseases are likely due to renal tubular factors, if the mechanism originates in the kidney. A delineation of these phenomena may further our understanding of the pathogenesis of diabetic nephropathy.

Combined deficiencies of the pyruvate dehydrogenase complex and enzymes of the respiratory chain in mitochondrial myopathies

In six patients with mitochondrial (encephalo-) myopathy investigations of skeletal muscle revealed a defect of pyruvate dehydrogenase complex (PDHC) in combination with one or more respiratory chain complex deficiencies. A combination of defects of this kind has not been reported previously. Five of the six patients presented within the 1st year of life and had a severe clinical course. Intrafamilial variability of the clinical course in dizygotic twins both suffering from a cytochrome c oxidase deficiency and one of them also from a PDHC deficiency suggests an additional effect of PDHC deficiency on the clinical symptoms. Immunoblot studies of PDHC in five of the patients revealed no abnormalities in their subunit pattern, rendering a defect of mitochondrial protein import or assembly unlikely. The finding of a combined PDHC and respiratory chain deficiency has implications for the diagnostic approach, for therapy and genetic counselling. The exact pathogenetic mechanism of this combination of defects remains to be elucidated.

Type I glycogen storage disease: kidney involvement, pathogenesis and its treatment

Type I glycogen storage disease (GSD-I) is due to the deficiency of glucose-6-phosphatase activity in the liver, kidney and intestine. Although kidney enlargement occurs in GSD-I, renal disease has not been considered a major problem until recently. In older patients (more than 20 years of age) whose GSD-I disease has been ineffectively treated, virtually all have disturbed renal function, manifested by persistent proteinuria; many also have hypertension, renal stones, altered creatinine clearance or a progressive renal insufficiency. Glomerular hyperfiltration is seen in the early stage of the renal dysfunction and can occur before proteinuria. In younger GSD-I patients, the hyperfiltration is usually the only renal abnormality found; and, in some patients, microalbuminuria develops before clinical proteinuria. The predominant underlying renal pathology is focal segmental glomerulosclerosis. Renal stones and/or nephrocalcinosis are also common findings. Amyloidosis and Fanconi-like syndrome can occur, but rarely. The risk factors for developing the glomerulosclerosis in GSD-I include hyperfiltration, hypertension, hyperlipidemia and hyperuricemia. Dietary therapy with cornstarch and/or nasogastric infusion of glucose, aimed at maintaining normoglycemia, corrects metabolic abnormalities and improves the proximal renal tubular function. Long-term trial will be needed to assess whether the dietary therapy may prevent the evolution or the progression of the renal disease.