Molecular Characteristics
ISCA2 stands for “Iron Sulfur Cluster Assembly 2 protein”, previously known as HBLD1 and ISA2. According to Ensembl the gene encodes for “A-type iron sulfur cluster protein”, located on long arm of chromosome 14 (14q24.3, Genomic coordinates: 74,493,720 – 74,497,106 on forward strand) and has 4 transcripts, three of which produce proteins with different lengths. ISCA2 is ubiquitously expressed in nearly all major organs and tissues according to publicly available RNAseq data sets (mRNA expression in normal human tissues from GTEx and illumina). ISCA2 is mostly located in mitochondria. The function of the protein is to involve in assembly and maturation of rhombic Iron Sulphur clusters, known as [4Fe-4S] clusters. This process is vital for iron-Sulfur cluster (ISC) biogenesis which plays a critical role for essential functions such as glycine cleavage and the formation of lipoic acid, an essential cofactor of respiratory chain complexes. Defects in ISC biogenesis and related members, such as ISCA1 and IBA57, lead to various mitochondrial dysfunction syndromes including ISCA2 with infantile onset leukodystrophy. Together with ISCA1 and IBA57, ISCA2 catalyzes the cluster’s assembly and transfer it to specific apoproteins. The main outcomes discovered from this step are the formation of aconitase, lipoic acid synthase (LAS), succinate dehydrogenase (SDH). Other proteins are essential for the formation of these end products such as NFU1, BOLA3, NUBPL and others.
Initial Genetic Study
Alhassnan et al. (2015) reported five consanguineous Saudi families having six affected individuals with similar white matter abnormalities accompanied by leukodystrophy, neuroregression, nystagmus and optic atrophy during early infancy. The patients found to have a homozygous missense founder mutation (c.229G >A:p.Gly77Ser) in ISCA2. Fibroblasts from one of the affected individuals were tested for complex I activity that showed reduced enzyme activity using Dip Stick assays. The same fibroblasts had reduced number of mtDNA copy numbers. The mRNA expression for ISCA1, ISCA2, and IBA57 were also reduced. Skeletal muscle of the same individual showed mild to moderate variation in myofibre size and presence numerous moderately to severely atrophic fibres having a random distribution.
Follow-up Studies
Alaimo et al. (2016) reported two unrelated Saudi cases having hyperglycinemia, leukodystrophy involving the brainstem and spinal cord, and mtDNA deficiency. Both patients had the founder mutation reported by Alhassnan et al. (2015). Cultured fibroblasts from the first patient revealed significantly reduced membrane potential. The fibroblats were also tested for all four complexes revealing a marginally increased complexes I and III activities in contrast to markedly diminished complexes II and IV activities. Subject 2 was expired at the age of 9-month.
Lebigot et al. (2017) reported two male cases with ISCA2 mutations (PMID: 28803783). The patients’ ethnicity was not reported. The individuals had two different missense mutations in the gene (ISCA2:NM_194279). The first mutation is located in exon 2 leading to non-progressive spastic parapleagia and white matter atrophy starting at 12 years. The second mutation is positioned on exon 4 causing hypotonia, apnea and leukodystrophy. The symptoms started since 12th days of the infancy. The first patient had a reduced pyruvate dehydrogenase (PDHc) and α-ketoglutarate dehydrogenase (α-KGDHc) activities in the patient’s fibroblasts. The patient did not found to have reduced complex I activity in his fibroblasts. The second patient had reduced complex II activity in his fibroblasts.
Alfadhel et al. (2018) reported additional 10 patients (six females and four males from nine unrelated consanguineous Saudi families) having the same founder mutation reported in AlHassnan et al. (2015) work. Five families reported the death of other siblings with the same clinical presentation and unclear etiology. All the patients deteriorated and ended up in a vegetative state with a gastrostomy tube or a nasogastric feeding tube. During the course of the disease, the patients developed recurrent chest infections, which was the main cause of death for 5/10 patients at ages ranging from 11 to 28 months.
Toldo et al. (2018) reported a compound-heterozygous mutation in ISCA2 in a presumably Italian infant with 2-month-old age. The heterozygous mutations included a frameshift mutation c.295delT, causing an early stop codon (p.Phe99Leufs*18) and a missense mutation c.334A >G:p.Ser112Gly. The girl had presented with severe hypotonia and nystagmus and quickly deteriorated and died at the age of three months. Interestingly, the patient had increased cerebral spinal fluid level of lactate, indicated by brain spectroscopy. She also had restricted diffusion of white and gray matter abnormalities, sparing of the corpus callosum and extensive involvement of the spinal cord. Muscle biopsy revealed reduced activities in mitochondrial respiratory chain complexes II and IV.
Mutations
Currently, there are 18 patients reported with Saudi ethnicity. All the affected individuals have the founder mutation (p.Gly77Ser). The other 3 patients had different mutations, a compound heterozygous mutation (c.295delT: p.Phe99Leufs*18 and c.334A >G:p.Ser112Gly) in a presumably Italian patient, and two homozygous missense mutations (p.L52F and p.R105G) in two other patients with unknown ethnicity. Cultured fibroblasts and muscles biopsies from the selected patients revealed reduced complex II and IV activities.