Novel genetic causes of severe neurological and multi-organ disorders in children

During the recent decade it has clearly been shown that mitochondrial diseases form a group of the most common neurometabolic diseases, but specific treatments are still lacking. Therefore, it is important to identify the pathogenic origins of these deficiencies.

The aim of this research is to find the genetic causes and disease mechanisms related to mitochondrial respiratory chain deficiencies in a cohort of children with encephalomyopathies.

Mitochondria have a critical role in the cell metabolism. They generate ATP molecules which provides energy to drive many processes in living cells. ATP is produced via electron transport chain during a process called oxidative phosphorylation. The significance of the mitochondria in energy yielding reactions is observed in diseases caused by mitochondria dysfunction. It is estimated that OXPHOS disorders occur once in 5000 live births and they can be considered as one of the most common inborn errors.

The incidence and the clinical symptoms of OXPHOS disorders vary from severe disease in early childhood to mild muscle weakness in adulthood. Most of the patients are suffering from multiorgan disorder in which many high-energy-demanding organs like brain, skeletal muscle, heart, liver etc. are affected. Consequently, mitochondrial dysfunction in these patients leads to variety of highly heterogeneous clinical phenotypes. These features make challenges to diagnose mitochondrial diseases at prenatal and postnatal stages and to find appropriate therapeutic strategies.

In our study, molecular genetics including whole exome sequencing and Sanger sequencing followed by functional studies have been performed on patient-derived cell lines from children with mitochondrial encephalomyopathies with recently identified novel genetic etiology. We studied the assembly of mitochondrial respiratory chain enzyme complexes and identify novel disease pathomechanisms.

In Study I, in a patient with Leigh syndrome and CI deficiency, molecular genetic studies level showed that a variant affects the splicing of BLZF1 mRNA. The role of BLZF1 on the assembly of mitochondrial complex I was further characterized. Later, the younger sibling of the index patient showed similar symptoms, and trio plus exome sequencing was carried out on the family. A variant was identified in the NDUFS7 gene from both siblings as a cause for Leigh syndrome.

In Study II, a homozygous splice donor variant was identified in DIAPH1 in four patients from three unrelated families who presented with seizures, cortical blindness, and microcephaly syndrome (SCBMS). Fibroblast cell cultures from two patients with DIAPH1 mutations and control fibroblasts overexpressing the wild-type DIAPH1 showed a decrease in mitochondrial complex IV levels.

In Study III, mitochondrial dysfunction was found to co-occur with variants in P4HTM from patients with hypotonia¬¬–intellectual disability–eye abnormalities (HIDEA).

As an outcome of our study, the genetic causes and the pathomechanisms behind diseases in these children are resolved. These results can be utilized in designing the medical treatments of these and other patients with mitochondrial encephalomyopathies, or in the genetic counselling and prenatal diagnostics. Furthermore, the findings from this study will broaden our understanding on molecular mechanisms behind mitochondrial dysfunction behind these devastating disorders in children. The results of this study lead to identification of the effective mutations in our patients, which is crucial for understanding the molecular and genetic background of their disease. This can improve the early diagnosis, and by utilizing the knowledge in personalized medicine, can prevent the progression of the disease dramatically in these cases. Overall, our findings will have high impact on understanding the molecular mechanisms behind mitochondrial associated diseases.