A novel remitting leukodystrophy associated with a variant in FBP2

Abstract

Leukodystrophies are genetic disorders of cerebral white matter that almost exclusively have a progressive disease course. We became aware of three members of a family with a disorder characterized by a sudden loss of all previously acquired abilities around one year of age followed by almost complete recovery within two years. Cerebral MRI and myelin sensitive imaging showed a pronounced demyelination that progressed for several months despite signs of clinical improvement and was followed by remyelination. Exome sequencing did not -identify any mutations in known leukodystrophy genes but revealed a heterozygous variant in the FBP2 gene, c.343G>A, p.Val115Met, shared by the affected family members. Cerebral MRI of other family members demonstrated similar white matter abnormalities in all carriers of the variant in FBP2. The FBP2 gene codes for muscle fructose 1,6-bisphosphatase, an enzyme involved in gluconeogenesis that is highly expressed in brain tissue. Biochemical analysis showed that the variant has a dominant negative effect on enzymatic activity, substrate affinity, cooperativity and thermal stability. Moreover, it also affects the non-canonical functions of muscle fructose 1,6-bisphosphatase involved in mitochondrial protection and regulation of several nuclear processes. In patients’ fibroblasts, muscle fructose 1,6-bisphosphatase shows no colocalization with mitochondria and nuclei leading to increased reactive oxygen species production and a disturbed mitochondrial network. In conclusion, the results of this study indicate that the variant in FBP2 disturbs cerebral energy metabolism and is associated with a novel remitting leukodystrophy.

FBP2—A New Player in Regulation of Motility of Mitochondria and Stability of Microtubules in Cardiomyocytes

“…Previously, we have shown that in the cell culture, fibroblasts-to-cardiomyocytes crosstalk induced transitions that mimicked the postnatal shift from the "fetal" to "mature" mode of cardiac myocytes functioning. This included the translocation of FBP2 from the cardiomyocyte nucleus to cytoplasm where, as the dimer and/or the active R-state tetramer, it bound to mitochondrial proteins, including VDAC, and participated in the protection of the mitochondrial membrane [1,3,[9][10][11]16].…”

“…To test the FBP2-mitochondria co-localization, murine anti-FBP2 antibody [13], rabbit anti-TOMM antibody (HPA011562; Merck, Darmstadt, Germany) and appropriate secondary antibodies Alexa 633-or FITCconjugated (ThermoFisher Scientific or Merck, respectively) were used. For the analysis of FBP2 co-localization with mitochondria and microtubules with mitochondria, the Manders' coefficient (M) was determined using the JACoP plugin of the ImageJ/FIJI as we described before [11]. The coefficient varies from 0 (no co-localization) to 1 (100% of co-localization).…”

“…Mitochondrial membrane potential was measured with JC-1 dye (ThermoFisher Scientific, Waltham, MA, USA) as described in [11]. Polarized mitochondria that accumulated more JC-1 dye were red.…”

“…Under normoxic conditions, in a number of HL-1 cardiomyocytes with partially silenced FBP2 expression (HL-1 FBP2-cells), Mito-tracker-positive spherical aggregates and toroids ("donut" mitochondria) were observed (previously unpublished). Additionally, in fibroblasts of patients with the V115M-FBP2 variant that does not bind to mitochondria, the organelles aggregated around the nucleus rather than creating a network, which correlated with an increased susceptibility of the cells to stress stimuli [11]. This prompted us to search for mechanisms by which FBP2 can contribute to the observed changes in the mitochondrial network.…”

“…Most recently, we have found that a novel remitting leukodystrophy was associated with a Val115Met variant of FBP2 [11]. In fibroblasts carrying the variant, FBP2 was unable to co-localize with mitochondria which correlated with a disturbance of the mitochondrial network and an increase in ROS production.…”