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    Confirmation of TACO1 as a Leigh Syndrome Disease Gene in Two Additional Families
    (Ios Press, 2020) Oktay, Yavuz; Gungor, Serdal; Zeltner, Lena; Wiethoff, Sarah; Schoels, Ludger; Sonmezler, Ece; Yilmaz, Elmasnur
    Background: In 2009, we identified TACO1 as a novel mitochondrial disease gene in a single family, however no second family has been described to confirm the role of TACO1 in mitochondrial disease. Objective: In this report, we describe two independent consanguineous families carrying pathogenic variants in TACO1, confirming the phenotype. Methods: Detailed clinical investigations and whole exome sequencing with haplotype analysis have been performed in several members of the two reported families. Results: Clinical phenotype of the patients confirms the originally reported phenotype of a childhood-onset progressive cerebellar and pyramidal syndrome with optic atrophy and learning difficulties. Brain MRI showed periventricular white matter lesions with multiple cystic defects, suggesting leukoencephalopathy in both patients. One patient carried the previously described homozygous TACO1 variant (p.His158ProfsTer8) and haplotype analysis suggested that this variant is a rare founder mutation. The second patient from another family carried a homozygous novel frame shift variant (p.Cys85PhefsTer15). Conclusions: The identification of two Turkish families with similar characteristic clinical presentation and an additional homozygous nonsense mutation confirms that TACO1 is a human mitochondrial disease gene. Although most patients with this clinical presentation undergo next generation sequencing analysis, screening for selected founder mutations in the Turkish population based on the precise clinical presentation may reduce time and cost of finding the genetic diagnosis even in the era of massively parallel sequencing.
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    Severe neurodevelopmental disease caused by a homozygous TLK2 variant
    (Nature Publishing Group, 2020) Topf, Ana; Oktay, Yavuz; Balaraju, Sunitha; Yilmaz, Elmasnur; Sonmezler, Ece; Yis, Uluc; Laurie, Steven
    A distinct neurodevelopmental phenotype characterised mainly by mild motor and language delay and facial dysmorphism, caused by heterozygous de novo or dominant variants in the TLK2 gene has recently been described. All cases reported carried either truncating variants located throughout the gene, or missense changes principally located at the C-terminal end of the protein mostly resulting in haploinsufficiency of TLK2. Through whole exome sequencing, we identified a homozygous missense variant in TLK2 in a patient showing more severe symptoms than those previously described, including cerebellar vermis hypoplasia and West syndrome. Both parents are heterozygous for the variant and clinically unaffected highlighting that recessive variants in TLK2 can also be disease causing and may act through a different pathomechanism.
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    y COL4A1-related autosomal recessive encephalopathy in 2 Turkish children
    (Lippincott Williams & Wilkins, 2020) Yaramis, Ahmet; Lochmueller, Hanns; Topf, Ana; Sonmezler, Ece; Yilmaz, Elmasnur; Hiz, Semra; Yis, Uluc
    Objective This study presents the neurologic phenotypes of 2 brothers with a novel homozygous COL4A1 mutation that was identified in a large Turkish consanguineous cohort of neurogenetic diseases. Methods Whole-exome sequencing and bioinformatic analysis of consanguineous families with children affected by early-onset, neurogenetic disorders was performed using the RD-Connect Genome-Phenome Analysis Platform. We also performed clinical, EEG, and neuroimaging analyses in unaffected siblings and parents. Results We have identified a homozygous missense mutation in COL4A1 (p.Gly1278Ser, NM_ 001845.5:c.3832G>T) in 2 siblings affected by small vessel brain disease with periventricular leukoencephalopathy and ocular defects. Presenting symptoms included mild weakness, hemiparetic gait, pyramidal findings, and seizures, whereas their intellectual and behavioral functions were normal. Both parents and 5 of the siblings (3 boys and 2 girls) were heterozygous for the variant. They did not show any clinical or laboratory signs of small vessel disease. Conclusions COL4A1 has previously been associated with dominant small vessel disease of the brain and other organs, manifesting with high penetrance in heterozygous mutation carriers. Our findings provide evidence that COL4A1-related encephalopathy can be inherited in an autosomal recessive manner, which is important for counseling, prognosis, and treatment. Genotypephenotype correlations remain to be established.