Report on three additional patients and genotype-phenotype correlation in SLC25A22-related disorders group

Abstract

Early infantile epileptic encephalopathy (EIEE) is a heterogeneous group of severe forms of age-related developmental and epileptic encephalopathies with onset during the first weeks or months of life. The interictal electroencephalogram (EEG) shows a "suppression burst" (SB) pattern. The prognosis is usually poor and most children die within the first two years or survive with very severe intellectual disabilities. EIEE type 3 is caused by variants affecting function, in SLC25A22, which is also responsible for epilepsy of infancy with migrating focal seizures (EIMFS). We report a family with a less severe phenotype of EIEE type 3. We performed exome sequencing and identified two unreported variants in SLC25A22 in the compound heterozygous state: NM_024698.4: c.[813_814delTG];[818 G>A] (p.[Ala272Glnfs*144];[Arg273Lys]). Functional studies in cultured skin fibroblasts from a patient showed that glutamate oxidation was strongly defective, based on a literature review. We clustered the 18 published patients (including those from this family) into three groups according to the severity of the SLC25A22-related disorders. In an attempt to identify genotype-phenotype correlations, we compared the variants according to the location depending on the protein domains. We observed that patients with two variants located in helical transmembrane domains presented a severe phenotype, whereas patients with at least one variant outside helical transmembrane domains presented a milder phenotype. These data are suggestive of a continuum of disorders related to SLC25A22 that could be called SLC25A22-related disorders. This might be a first clue to enable geneticists to outline a prognosis based on genetic molecular data regarding the SLC25A22 gene.

Fig. 1

a Visualization of variants by integrative genomics viewer (IGV). The sequence is the complementary sequence of genomic DNA. The first two lines correspond to patients 2 and 3. They shared compound heterozygous variants NM_0246989.4: c.[813_814delTG];[818G>A]. The third line corresponds to the father of patient 2 who carries the variant c.818G>A. The fourth line corresponds to the mother of patient 3 who carries the variant c.813_814delTG. b Pedigree

Fig. 2

Respiration and mitochondrial substrate oxidation in cultured skin fibroblasts from one patient and one control. After polarographic measurement of intact cell respiration, digitonin-permeabilized cells were loaded with adenosine diphosphate (ADP) and amino oxyacetate. This compound inhibits the aspartate-amino transferase enzyme activity. A subsequent addition of glutamate allowed estimation of mitochondrial glutamate oxidation under phosphorylation conditions (i.e., presence of ADP). Note the lack of glutamate-triggered oxygen uptake in the patient’s cells. A similar succinate oxidation rate was measured in control’s and patient’s cells. The dotted line corresponds to the speed of consumption of O2 by cells. After addition of glutamate, the speed rises by 41% in the control’s cells and decreases by 12% in the patient’s cells. Numbers along the traces are nmol O2 consumed per minute per milligram of protein. AOA = amino oxyacetate; Asp = aspartate; Glut = glutamate; α-KG = α-ketoglutarate; OAA = oxaloacetate. Experimental conditions were as described in the “Materials and methods” section

Fig. 3

Visualization of variants in SLC25A22. a Linear visualization of patient’s variants. Htz = heterozygote; Hmz = homozygote. b 2-D visualization of variants in SLC25A22 protein