Pathogenic variants & ALD Protein Stability

Johannes Berger, Ph.D. and Stephan Kemp, Ph.D

The effect of missense (likely) pathogenic variants on ALDP stability

This picture shows a hypothetical model of adrenoleukodystrophy protein (ALDP). Individual amino acids are represented by circles. Missense pathogenic variants may affect the stability of the ALD protein. The effect of a missense pathogenic variants on ALDP stability by means of immunofluorescence (IF) in primary fibroblasts was investigated for >200 independent (non-recurrent) missense pathogenic variants. The results are presented in this figure. Green circles indicate missense pathogenic variants that do not affect protein stability; red circles those that result in no detectable ALDP (see remark below); orange circles represent pathogenic variants that result in reduced ALDP expression; blue circles indicated amino acids residues at which multiple missense pathogenic variants have been reported that result in different outcomes. The black circles indicate amino acids at which a missense pathogenic variants was identified, but for which no expression data is available. Overall, >65% of all missense pathogenic variants result in reduced levels or absence of detectable ALDP (Kemp et al. (2001) Hum Mutat 18(6):499-515).

All other pathogenic variants (including in frame amino acid deletions and truncations near the carboxyl terminus like p.Gln672X) result in the absence of detectable levels of ALDP.

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To be absent or not?

In 2011, using a novel detection method based on Western blot, Zhang et al. (2011) established that residual protein is actually present in many patient fibroblast lines harboring a missense pathogenic variant, although often below the level of detection by IF (see Table). This indicates that only few newly synthesized mutant ALDP molecules fold correctly and incorporate into the peroxisomal membrane, while most are misfolded or assume their topology so slowly that they are rejected by the cell’s quality control machinery.

variant immunofluorescence immunoblot (% of control)
p.Arg113fsX84 absent 0%
p.Arg74Trp absent 7%
p.Arg554His absent 1%
p.Glu609Gly absent 4%
p.Glu609Lys absent 2%
p.Ala616Thr absent 5%
p.Leu654Pro absent 2%
p.Arg660Trp absent 3%
p.His667Asp absent 2%
p.Arg104Cys reduced 35%
p.Leu220Pro reduced 26%
p.Ser606Leu present 25%
p.Ser149Asn present 77%
p.Asp194His present 45%
p.Arg389His present 43%

See Zhang et al. (2011) Biochem J 436(3):547-57 for experimental details.

Last modified | 2022-01-10