Historical note and nomenclature
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By Raphael Schiffmann MD

Adrenoleukodystrophy was first described in 1923 by Siemerling and Creutzfeldt (Siemerling and Creutzfeldt 1923). In 1963 Fanconi and colleagues proposed an X-linked mode of inheritance on the basis of pedigree analysis (Fanconi et al 1963). In 1981 the gene was mapped to Xq28, the terminal segment of the long arm of the X-chromosome (Migeon et al 1981), and the putative gene was isolated in 1993 (Mosser et al 1993). The identity of the gene and the defective protein, which is referred to as ALDP, is now established firmly because all patients who have been studied in sufficient detail have a mutation in this gene (Dodd et al 1997; Kemp et al 2001), and transfection of mutant cells with the normal gene corrects the biochemical defect (Cartier et al 1995). The principal biochemical abnormality, namely the abnormal accumulation of saturated very long-chain fatty acids in the brain white matter and adrenal cortex, particularly hexacosanoic acid (C26:0), was identified by Igarashi and colleagues (Igarashi et al 1976). Singh and colleagues showed that this accumulation is due to the impaired capacity to degrade very long-chain fatty acids (Singh et al 1984a), a reaction that normally takes place in the peroxisome (Singh et al 1984b). Lazo and colleagues pinpointed the defect more precisely by showing that patients with adrenoleukodystrophy have an impaired capacity to form the coenzyme A derivative of very long-chain fatty acids (Lazo et al 1988). The enzyme that catalyzes this reaction, very long-chain Acyl-Coenzyme A synthetase, has been cloned (Uchiyama et al 1996). It came as a surprise that the gene that has been proven to be deficient in adrenoleukodystrophy does not code for this enzyme, but rather for a peroxisomal membrane protein, ALDP, which is a member of the ATP binding cassette protein family, a group of proteins that is involved in a variety of transport functions (Higgins 1992). The relationship between ALDP and very long-chain fatty acid metabolism is not yet understood.

The nomenclature of adrenoleukodystrophy has evolved over time. The patient with "encephalitis periaxialis diffusa" described by Paul Schilder in 1913 almost certainly had what we now refer to as adrenoleukodystrophy (Schilder 1913), and sometimes the designation of Schilder disease is still applied to adrenoleukodystrophy patients. This may lead to confusion because Schilder disease is heterogeneous (Poser and van Bogaert 1956). In the German literature adrenoleukodystrophy was sometimes termed "bronzed Schilder disease," referring to the pigmentation associated with the frequently associated adrenal insufficiency. Up to the mid 1970s the disorder was often referred to as "Addison-Schilder disease," or "melanodermic type of leukodystrophy." The term "adrenoleukodystrophy" was coined by Blaw in 1970 and is now generally accepted (Blaw 1970). In 1976, Budka and colleagues and Griffin and colleagues showed independently that an adult form of the disease, now referred to as "adrenomyeloneuropathy," is a variant of adrenoleukodystrophy (Budka et al 1976; Griffin et al 1977), and the whole group of disorders is at times referred to under the umbrella term of "adreno-testiculo-leukomyeloneuropathy-complex" (Powers 1985). The condition referred to as "neonatal adrenoleukodystrophy" represents a possible source of confusion and must be differentiated sharply (Kelley et al 1986). Neonatal adrenoleukodystrophy is a disorder with an autosomal recessive mode of inheritance, and its basic defect involves the import of a variety of proteins into the peroxisome (Lazarow and Moser 1994). Its clinical manifestations are totally different from the X-linked form of adrenoleukodystrophy described in this chapter. In spite of this plethora of historically-based designations, the classification and nomenclature can now be applied precisely. We recommend that the designation adrenoleukodystrophy be applied to all males who have a defect in the x-linked gene that codes for ALDP. All of these persons have demonstrable defects in very long-chain fatty acid metabolism. Their clinical manifestations may vary from asymptomatic, to isolated primary adrenal insufficiency, to various degrees of neurologic involvement. When there is clinical and radiological evidence of involvement of the cerebral hemispheres we refer to them as childhood, adolescent, or adult cerebral forms of adrenoleukodystrophy. When nervous system involvement affects mainly the spinal cord and peripheral nerves, the condition is referred to as adrenomyeloneuropathy. The term neonatal adrenoleukodystrophy refers to a totally unrelated disorder that has an autosomal recessive mode of inheritance, and that nearly always manifests in the neonatal period, with multiple and distinct biochemical abnormalities due to a defect in peroxisome biogenesis (Lazarow and Moser 1994).

Studies have defined the molecular basis of neonatal adrenoleukodystrophy, and these are summarized in a review (Moser 1999a). Neonatal adrenoleukodystrophy is now considered to be member of a clinical continuum, with Zellweger syndrome the most severe, neonatal adrenoleukodystrophy of intermediate severity, and infantile Refsum disease the least severe. Each of these disorders can be associated with 10 separate genetic defects, all of which impair the import of proteins into the peroxisome. The severity of disease expression relates to some extent with the nature of the mutation. Mutations that abolish import completely are most likely to be associated with the Zellweger syndrome phenotype, whereas in patients with milder phenotypes the import function is partially retained. Mosaicism has been demonstrated in some other patients with milder phenotypes.