A Novel Homozygous CTC1 Germline Mutation Associated with Bone Marrow Failure
Authors: Wartiovaara-Kautto Ulla , Hirvonen Elina A. M. , Kettunen Kaisa , Nordberg Anna , Saarela Janna , Porkka Kimmo , Kivelä Tero , Linnankivi Tarja , Kilpivaara Outi 
Published: October 2019

Abstract
Iron deficiency in young adult males is rare and necessitates prompt investigations. Here we describe a previously healthy 22-year-old male suffering from gastrointestinal bleeding leading to iron deficiency anemia and explained by a constitutional disease. His diagnosis was obtained after detecting a homozygous CTC1 mutation in his germline DNA. Conserved telomere maintenance component 1 (CTC1) is a part of the conserved telomere-associated complex (CST) participating in telomere replication. CST seems to have a role in resolving replication stress also in non-telomeric regions, but little is known about the exact mechanisms. Biallelic, typically compound heterozygous mutations in CTC1 are known to cause cerebroretinal microangiopathy with calcifications and cysts(CRMCC), which is considered as a telomere disease. It is a highly pleiotropic multi-organ disease characterized by abnormalities e.g. in the brain, retina, intestinal vasculature, as well as bone marrow failure (BMF). Some CRMCC patients have dyskeratosis congenita (DC) -like skin, nail, mucosal, and hair changes. Most patients are diagnosed in early childhood and die before teens. To date 37 patients with CRMCC and CTC1 mutations have been reported.
The patient was referred to hematology due to iron deficiency anemia with coinciding thrombocytopenia. He also had a notable lymphopenia but no tendency for viral infections or other signs of immunodeficiency. Imaging revealed a patent umbilical vein, widened portal vein, and hepatosplenomegaly without parenchymal changes or lymphadenopathy. Gaucher's disease was excluded by glucocerebrosidase and genetic tests. Histology of a liver biopsy was normal. Based on these findings an idiopathic portal hypertension was diagnosed. Endoscopy revealed gastric antral vascular ectasia. The bone marrow was hypoplastic and pancytopenia persisted after supplementation with iron. He had no known family history of BMF. The patient's telomere length analyzed by a semi-quantitative PCR-test was within the normal range compared to healthy controls of the same age group. We then sequenced the patient's germline exome. Roche MedExome kit was used to target the coding regions of the genome and sequencing conducted using an Illumina HiSeq1500 sequencer. The quality of the data was assessed and the sequence data analyzed using an in-house developed analysis pipeline. The mutation analysis was targeted to genes previously reported to cause immune deficiencies, myelodysplastic syndrome and BMF. The average sequence coverage was 59x, translating into 95% of the exome being covered with a minimum of 20x.
Common polymorphisms were excluded from the analysis. The patient was identified to carry a homozygous variation in CTC1 exon 12; c.1994T>G, p.V665G (rs199473676). The variant has previously been reported pathogenic, but all previously reported cases of p.V665G are compound heterozygous (in the majority of cases V665G and a frameshift mutation). The minor allele frequency for this variant in Finns is 0.0015 (10/6612 alleles) and 0.0001 (13/120608) in total (ExAC database) suggesting this mutation is a Finnish founder mutation. We validated the finding by capillary sequencing. The variant was detected heterozygous in both parents ruling out a larger deletion and verifying the mode of inheritance. A healthy sister did not carry the mutation. Identification of CTC1 p.V665G homozygosity raised a high suspicion of CRMCC and further investigations were indicated. Retinal examination as well as brain MRI/MRA revealed multiple lesions characteristic for CRMCC.
This is the first report on homozygous CTC1 p.V665G and the second describing CTC1-homozygosity associated with CRMCC, strengthening the disease-causality of biallelic missense mutations in CTC1. The identification of a homozygous CTC1 p.V665G may also open new prospects for studying the effects of the mutation. Our case shows the importance and capacity of new genomic techniques in clinical management, as well: The patient's primary phenotype with iron deficiency anemia, thrombopenia and hepatosplenomegaly could have been explained by only an idiopathic portal hypertension. However, with exome sequencing we were able to reveal a more complex disease entity with major impact on patient's future care and genetic counseling, highlighting the value of germline sequencing in modern diagnostics.

Disclosures
No relevant conflicts of interest to declare.