Neonatal adrenoleukodystrophy (NALD) is an autosomal recessive disorder that is a member of a family of disorders that result from defects in the biogenesis and/or functioning of the peroxisomes and are referred to as peroxisome biogenesis disorders, PBDs. NALD belongs to the Zellweger spectrum PBDs which includes Zellweger syndrome and infantile Refsum disease (IRD). Zellweger syndrome represents the extreme of the clinical manifestation of peroxisome biogenesis dysfunction with patients rarely surviving their first year of life. Zellweger syndrome is associated with either severe, moderate or mild defects in all peroxisome functions. An additional phenotypic spectrum in PBDs is represented by rhizomelic chondrodysplasia punctata, RCDP. RCDP is distinguished from the Zellweger spectrum PBDs by manifesting with more severe skeletal involvement as well as specific biochemical characteristics.
NALD can result from mutations in several genes involved in peroxisome biogenesis (PEX genes encoding proteins termed peroxins). These include the PEX1 gene on chromosome 7q21–q22, the PEX5 gene on chromosome12p13.3 (this gene encodes the receptor for proteins containing a PTS1 sequence for targeted to the peroxisomes, see below), the PEX10 gene on chromosome 1, The PEX13 gene on chromosome 2p15, and the PEX26 gene on chromosome 22q11.21. The PEX1 gene encodes a protein that is a member of the AAA-type ATPases (AAA = ATPases associated with diverse cellular activities). The protein encoded by the PEX13 gene is an integral membrane protein that interacts with the PEX5 encoded PTS1 receptor. The PEX26 encoded protein interacts with the PEX6 encoded protein and may have catalase activity.
The peroxisomes are a single membrane organelle, similar to lysosomes, present in virtually all eukaryotic cells. The peroxisome is a specialized enzyme "factory" that contains in excess of 50 different enzymes involved in a variety of metabolic processes including β-oxidation of very long chain fatty acids, α-oxidation of fatty acids and synthesis of ether-lipids. Proteins that are involved in and necessary for correct peroxisome biogenesis are called peroxins (PEX). At least 15 PEX genes have been identified in humans. Enzymes that are targeted to the peroxisomes contain either of two amino acid consensus elements called peroxisome targeting sequences (PTS). The PTS1 is a C-terminal consensus sequence of –(S/A/C)(K/R/H)(L/M) referred to as the SKL motif. This sequence element is recognized by a cytosolic PTS1 receptor encoded by the PEX5 gene. There are two isoforms of PEX5 encoded proteins in humans identified as Pex5pS and Pex5pL (for short and long forms, respectively). The Pex5pL protein has an internal 37 amino acid insertion, hence the "long" designation. The PTS2 is an N-terminal consensus sequence of –(R/K)(L/V/I/Q)XX(L/V/I/H/Q)(L/S/G/A/K)X(H/Q)(L/A/F)–, (where X represents any amino acid). The PTS2 receptor is encoded by the PEX7 gene and the encoded protein is referred to as Pex7p. Proteins that are targeted to the membrane of the peroxisome (called peroxisome membrane proteins, PMPs) contain a consensus sequence identified as the PEX19 binding site (PEX19BS) and this site is recognized by the membrane protein receptor encoded by the PEX19 gene.
Pex5pS, Pex5L, and Pex7p interact with newly synthesized target proteins in the cytosol and direct them to the peroxisome. On the membrane of the peroxisome is a component of the protein import machinery encoded by the PEX14 gene called Pex14p. Following interaction of Pex5pS or Pex5pL, bound to a protein containing a PTS1 sequence, with Pex14p, the PTS1 containing protein is transferred into the peroxisome. The activity of Pex7p in peroxisome protein import actually requires Pex5pL as well. PTS2 containing proteins interact with Pex7p and then, in conjunction with Pex5pL, the complex interacts with Pex14p and the PTS2 containing protein is transferred into the peroxisome. Very few proteins contain a PTS2 sequence but one enzyme of note is phytanoyl-CoA hydroxylase (PHYH) which is defective in classic Refsum disease.
NALD bears many clinical and biochemical similarities to X-linked adrenoleukodystrophy (X-ALD) and Zellweger syndrome including the accumulation of very long chain fatty acids (VLCFA), particularly hexacosanoic acid (C26:0). Given that NALD is inherited as an autosomal recessive disorder and X-ALD is an X-linked disorder it is easy to distinguish these two disorder based upon inheritance patterns. However, biochemically they are distinguishable as well given that NALD results in the accumulation of VLCFAs as a result of defects in numerous peroxisomal enzymes necessary for peroxisomal β-oxidation and X-ALD is due to a defect in only the initial step of peroxisomal VLCFA β-oxidation. In addition, NALD patients elevated levels of intermediates in bile acid synthesis and elevated plasma levels of pipecolic acid (piperidine-2-carboxylic acid). Fibroblasts isolated from NALD patients are impaired in their ability to oxidize phytanic acid and to synthesize plasmalogens.
The clinical course of NALD can be quite rapid and is associated with no psychomotor progression followed by death within several months after birth. Alternatively, some patients have a less severe involvement and survive into their mid teens. However, these latter patients are severely retarded with sensorineural deafness and are blind due to retinopathy. The mental age of surviving NALD patients never exceeds that of a 10 to 12 month old and developmental regression will appear at around 3 to 5 years of age due to the onset of leukodystrophy.