NovelPEX1 coding mutations and 5′ UTR regulatory polymorphisms

Abstract

Zellweger syndrome and its milder variants-neonatal adrenoleukodystrophy and infantileRefsum disease-comprise a clinical continuum of diseases referred to as the Zellweger spectrum. Mutations in the PEX1 gene, which consists of 24 exons and encodes a AAA ATPase protein required for peroxisomal protein import, account for approximately twothirds of the known Zellweger spectrum patient mutations. In this paper, we report on four novel PEX1 mutations and two polymorphisms in an Australasian cohort. Two of the mutations-c.1108_1109insA and c.2391_2392delTC-that lead to the introduction of a premature termination codon in exons 5 and 14, respectively, are associated with the severe Zellweger phenotype. One patient with a milder disease phenotype was a compound heterozygote for two missense mutations (I989T and R998Q), both affecting amino acids in the second, C-terminal AAA domain of the protein. PTS1 protein import levels in cultured skin fibroblasts from this patient were almost 20% of normal control levels. We have also characterized two co-segregating polymorphisms in the 5' UTR of the PEX1 gene. Based on reporter assays, the c.-137T>C polymorphism leads to reduced PEX1 expression, whereas the c.-53C>G polymorphism leads to increased expression. When present together, these regulatory polymorphisms lead to near-normal PEX1 expression. Altered PEX1 expression due to the presence of either the c.-137T>C or the c.-53C>G variant could impact on residual PEX1 function if another co-allelic mutation was present which did not completely abolish PEX1 function. It also follows that the presence of polymorphisms in the PEX1 promoter region could have implications for patients with mutations in other PEX proteins known to interact with PEX1, such as PEX6. Thus, although not deleterious in control individuals, these polymorphisms could contribute to phenotypic heterogeneity among Zellweger spectrum patients

PEX1mutations in the Zellweger spectrum of the peroxisome biogenesis disorders

“…In agreement with this general finding on survival, Poll-The et al [2004] did not find any c.2097 -2098insT homozygotes in a group of PEX1-deficient patients who were older than 4 years. These three studies and a recent work by Maxwell et al [2005] also investigated 15 other individuals with either one c.2097 -2098insT allele and another null allele, or two other null alleles. The clinical data on all of these patients also indicated a ZS phenotype.…”

“…A total of 41 mutations and five significant polymorphisms in PEX1 have been reported to date from studies on Zellweger spectrum patient cohorts representing a wide spectrum of ethnic backgrounds from Australasia [Maxwell et al, 1999[Maxwell et al, , 2002[Maxwell et al, , 2005, Europe [Gärtner et al, 1999;Walter et al, 2001;Preuss et al, 2002], Japan [Imamura et al, 1998a;Tamura et al, 1998aTamura et al, , 2001, and the United States [Portsteffen et al, 1997;Reuber et al, 1997;Collins and Gould, 1999;Steinberg et al, 2004]. Table 1 summarizes all of the putative disease-causing mutations identified and the five polymorphisms.…”

“…These findings were interpreted to indicate that in addition to the PEX1 genotype, other unknown factors may determine the disease phenotype. Of interest in this regard is the recent identification of two common regulatory polymorphisms in the PEX1 5 0 UTR: the c.-137T4C variant which reduces, and the c.-53C4G variant which increases PEX1 expression in cultured skin fibroblasts [Maxwell et al, 2005]. These polymorphisms cosegregated in most, but not all, control individuals tested.…”

“…The first group, categorized as regulatory polymorphisms, consists of two recently identified, cosegregating polymorphic sites located 5 0 of the reported PEX1 translation start site [Maxwell et al, 2005]. The c.-137T4C transition generates an upstream, in-frame AUG that is present in the PEX1 mRNA in a favorable Kozak context.…”

The PEX1 ATPase Stabilizes PEX6 and Plays Essential Roles in Peroxisome Biology

“…An Ile-989-to-Thr mutation in the human PEX1 residue that is analogous to the Leu-993 residue mutated in pex1-2 (Fig. 1D) results in peroxisome biogenesis disorders when compounded with other PEX1 mutations (Maxwell et al, 2005;Smith et al, 2016). PEX1 D2 mutations can confer general peroxisomal defects (Birschmann et al, 2005) and decrease ATPase activity (Gardner et al, 2015) and PEX5 export in yeast (Platta et al, 2005), reduce PEX1 binding to PEX6 in yeast (Birschmann et al, 2005) and humans (Tamura et al, 2006), and impair PEX26 disassociation from PEX14 in humans (Tamura et al, 2014).…”