related Leigh syndrome: Clinical and molecular findings of 16 patients from Turkey.

Introduction: Pathogenic variants in , a nuclear-encoded gene encoding a mitochondrial chaperone involved in COX assembly, are one of the most common causes of Leigh syndrome (LS).

Material-methods: Sixteen patients diagnosed to have -related LS between 2012 and 2020 were included in the study. Their clinical, biochemical and molecular findings were recorded. 10/16 patients were diagnosed using whole-exome sequencing (WES), 4/16 by Sanger sequencing of , 1/16 via targeted exome sequencing and 1/16 patient with whole-genome sequencing (WGS). The pathogenicity of variants was evaluated by phylogenetic studies and modelling on the 3D structure of the SURF1 protein.

Results: We identified 16 patients from 14 unrelated families who were either homozygous or compound heterozygous for pathogenic variants. Nine different variants were detected The c.769G > A was the most common variant with an allelic frequency of 42.8% (12/28), c.870dupT [(p.Lys291*); (8/28 28.5%)], c.169delG [(p.Glu57Lysfs*15), (2/24; 7.1%)], c.532 T > A [(p.Tyr178Asn); (2/28, 7.1%)], c.653_654delCT [(p.Pro218Argfs*29); (4/28, 14.2%)] c.595_597delGGA [(p.Gly199del); (1/28, 3.5%)], c.751 + 1G > A (2/28, 4.1%), c.356C > T [(p.Pro119Leu); (2/28, 3.5%)] were the other detected variants. Two pathogenic variants, C.595_597delGGA and c.356C > T, were detected for the first time. The c.769 G > A variant detected in 6 patients from 5 families was evaluated in terms of phenotype-genotype correlation. There was no definite genotype - phenotype correlation.

Conclusions: To date, more than 120 patients of LS with pathogenic variants have been reported. We shared the clinical, molecular data and natural course of 16 new SURF1 defect patients from our country. This study is the first comprehensive research from Turkey that provides information about disease-causing variants in the gene. The identification of common variants and phenotype of the gene is important for understanding SURF1 related LS.

Synopsis: SURF1 gene defects are one of the most important causes of LS; patients have a homogeneous clinical and biochemical phenotype.