Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2022 Jun 22;31(12):2049-2062.
doi: 10.1093/hmg/ddac002.

Pathogenic SLC25A26 variants impair SAH transport activity causing mitochondrial disease

Florian A Schober  1 , Jia Xin Tang  2 , Kate Sergeant  3 , Marco F Moedas  1 , Charlotte M Zierz  2 , David Moore  1 , Conrad Smith  3 , David Lewis  4 , Nishan Guha  5 , Sila Hopton  2   6 , Gavin Falkous  2   6 , Amanda Lam  7 , Angela Pyle  2 , Joanna Poulton  8 , Gráinne S Gorman  2   6 , Robert W Taylor  2   6 , Christoph Freyer  1   9 , Anna Wredenberg  1   9
Affiliations

Pathogenic SLC25A26 variants impair SAH transport activity causing mitochondrial disease

Florian A Schober et al. Hum Mol Genet. .

Abstract

The SLC25A26 gene encodes a mitochondrial inner membrane carrier that transports S-adenosylmethionine (SAM) into the mitochondrial matrix in exchange for S-adenosylhomocysteine (SAH). SAM is the predominant methyl-group donor for most cellular methylation processes, of which SAH is produced as a by-product. Pathogenic, biallelic SLC25A26 variants are a recognized cause of mitochondrial disease in children, with a severe neonatal onset caused by decreased SAM transport activity. Here, we describe two, unrelated adult cases, one of whom presented with recurrent episodes of severe abdominal pain and metabolic decompensation with lactic acidosis. Both patients had exercise intolerance and mitochondrial myopathy associated with biallelic variants in SLC25A26, which led to marked respiratory chain deficiencies and mitochondrial histopathological abnormalities in skeletal muscle that are comparable to those previously described in early-onset cases. We demonstrate using both mouse and fruit fly models that impairment of SAH, rather than SAM, transport across the mitochondrial membrane is likely the cause of this milder, late-onset phenotype. Our findings associate a novel pathomechanism with a known disease-causing protein and highlight the quests of precision medicine in optimizing diagnosis, therapeutic intervention and prognosis.

PubMed Disclaimer

Figures

Figure 1
Figure 1
Genetic and biochemical characterization of patients with SLC25A26 variants. (A) Pedigree of individuals P1 and P2. (B) Sequencing chromatogram of genomic DNA aligned to SLC25A26 (NM_173471.3). (C) Diagram representing the relative positions of the SLC25A26 variants (NM_173471.3) and SLC25A26 alterations (GenBank: NP_775742.4). Amino acid alignments of five species show the regions of each variant. (D) Position of the variant amino acids in a modeled structure of human SLC25A26 (blue). Previously identified amino acids with disease association are marked in purple. (E) Standard histopathological analyses [Hematoxylin and eosin (H&E), cytochrome c oxidase (COX), succinate dehydrogenase (SDH) and sequential COX-SDH histochemistry] were undertaken in both patients, revealing a marked loss of COX activity and mitochondrial accumulation. Size bar = 100 μm. (F) Quadruple immunofluorescence analysis of NDUFB8 (complex I) and COXI (Complex IV) protein expression in patient muscle. Each dot represents the measurement from an individual muscle fiber, color co-ordinated according to its mitochondrial mass (low = blue, normal = beige, high = orange, very high = red). Gray dashed lines represent SD limits for classification of the fibers. Lines next to x- and y-axis represent the levels of NDUFB8 and COXI: beige = normal (> −3), light beige = intermediate positive (−3 to −4.5), light purple = intermediate negative (−4.5 to −6) and purple = deficient (< −6). Bold dashed lines represent the mean expression level of normal fibers. These data confirm the loss of NDUFB8 and COX1 expression in most fibers, consistent with the demonstration of multiple OXPHOS deficiencies.
Figure 2
Figure 2
Protein expression analysis in patient muscle biopsies. (A) Blue-native polyacrylamide gel electrophoresis (BN-PAGE) of isolated skeletal muscle mitochondria (5.68 μg protein) from control and Patient 1, decorated with antibodies against the indicated subunits of NADH:ubiquinone oxidoreductase (CI), succinate:ubiquinone oxidoreductase (CII), ubiquinone:cytochrome c oxidoreductase (CIV) and ATP synthase (CV). (B, C) Sodium dodecyl sulfate (SDS)-PAGE of total protein skeletal muscle (39.98 μg protein) from control and Patient 1 (B) and Patient 2 (C), decorated with antibodies as in (A), as well as SLC25A26. VDAC1 or GAPDH was used as loading control.
Figure 3
Figure 3
SAH carriage is impaired in MEF models. (A) Mitochondrial import of radiolabelled SAM with or without preincubation of mitochondria with SAH relative to Control (stable transfection of slc25a26 KO MEFs with human SLC25A26 cDNA) or E135G (slc25a26 KO MEFs expressing the p.Glu135Gly variant) or R142Q (slc25a26 KO MEFs expressing the p.Arg142Gln variant). (B) Same data as in (A) expressed as ratio. (C) Absolute levels of SAH and SAM in total cell metabolite extracts. (D) Larval phenotype at 4 days after egg laying (dae) of wDah control or larvae heterozygous for the p.Arg166Gln (R166Q) mutation balanced on Tmb6, tubby (tb), or homozygous for p.Arg166Gln (left to right). (E) Mitochondrial import of radiolabelled SAM with (grey bars) or without (white bars) preincubating mitochondria with 1 mm SAH relative to wDah controls or (F) as a ratio preincubated to untreated. I172G larvae were previously described (33) and express the Ile172Gly variant. (G) Absolute levels of SAH and SAM in fast mitochondrial enriched fractions or total larval metabolite extracts. *P < 0.05, **P < 0.01 and ***P < 0.001 with two-sided Student’s t-test (n = 3).
Figure 4
Figure 4
The Dm model expressing p.Arg166Gln is larval lethal. (A) Proteome landscape of previously established mutants at 4 dae (33) and Arg166Gln (R166Q) larvae at three timepoints after egg lay compared to wDah controls. I172G corresponds to larvae homozygous for a p.Ile172Gly mutation, P223L larvae are homozygous for p.Pro223Lys and null larvae are homozygous for non-expressed slc25a26 alleles. Functional categories were published with MitoXplorer 2.0 (42) and are here vertically sorted according to their Euclidean distance. (n = 5 for previous models, n = 3 for each Arg166Gln group including matched wDah controls). (B) Proteome volcano plot of mutant against wDah control at 4 dae. Mitochondrial proteins as annotated in (42), proteins of interest are proposed markers of mitochondrial SAM deficient fly models (n = 3). (C) Mitochondrial oxygen consumption quantified on an OROBOROS oxygraphy respirometer (n = 4). (D) Representative lipoic acid levels on PDH and α-KGDH by denaturing SDS-PAGE and western blotting. C, Control; M, R166Q mutant. (E) Quantification of (D) (n = 6). *P < 0.05, unpaired Student’s t-test.
See this image and copyright information in PMC

Similar articles

See all similar articles

Cited by

References

    1. Ruprecht, J.J. and Kunji, E.R.S. (2019) The SLC25 mitochondrial carrier family: structure and mechanism. Trends Biochem. Sci., 45, 244–258. - PMC - PubMed
    1. Palmieri, F. (2013) The mitochondrial transporter family SLC25: Identification, properties and physiopathology. Mol. Asp. Med., 34, 465–484. - PubMed
    1. Kunji, E.R.S., King, M.S., Ruprecht, J.J. and Thangaratnarajah, C. (2020) The SLC25 carrier family: important transport proteins in mitochondrial physiology and pathology. Physiology, 35, 302–327. - PMC - PubMed
    1. Palmieri, F., Scarcia, P. and Monné, M. (2020) Diseases caused by mutations in mitochondrial carrier genes SLC25: a review. Biomol. Ther., 10, 655. - PMC - PubMed
    1. Kaukonen, J., Juselius, J.K., Tiranti, V., Kyttälä, A., Zeviani, M., Comi, G.P., Keränen, S., Peltonen, L. and Suomalainen, A. (2000) Role of adenine nucleotide translocator 1 in mtDNA maintenance. Science, 289, 782–785. - PubMed

Publication types

Substances