Maternal gonosomal mosaicism in rare autosomal dominant SLFN14‐related thrombocytopenia

Inherited thrombocytopenias (ITs) are rare bleeding disorders. With the advances in genomic technologies, that is, whole exome sequencing (WES) and whole genome sequencing (WGS), in detecting new genes for IT, more cases of IT are being identified. Some ITs are at risk of developing additional disorders such as kidney failure, hearing loss, cataracts, bone marrow failure and haematological malignancies.1 Therefore, making a definite diagnosis is crucial for providing patients with the most appropriate treatment and counselling.1 We report a family of non-consanguineous parents with two affected offspring with inherited thrombocytopenia. The deceased elder brother was born at term and was noted to have a haematoma over a venepuncture site on day 5 of life with a platelet level of 8 × 109/L. His peripheral blood film showed thrombocytopenia with large platelets. Neonatal alloimmune thrombocytopenia screening was negative. A bone marrow aspiration and trephine biopsy showed increased megakaryocytes. He developed eczema at 1 month old. Wiskott–Aldrich syndrome was excluded by the detection of WAS protein (WASp) expression. He was treated for immune-mediated thrombocytopenia and responded partially to intravenous immunoglobulin and steroid. Unfortunately, he developed a spontaneous intracranial haemorrhage and succumbed at 5 months old. The proband was a full-term baby girl born 3 years later after the elder brother has passed on. At birth, she had petechiae and bruises over the trunk and limbs and a small subconjunctival haemorrhage over her right eye. She did not have hepatosplenomegaly. Her platelet level was 12 × 109/L. Peripheral blood film showed true thrombocytopenia with normal platelet size and morphology. She was screened for TORCHES (Toxoplasmosis, Rubella, Cytomegalovirus, Herpes simplex virus, Syphilis) infection, platelet antibody and Wiskott–Aldrich syndrome, which all turned out to be negative. Her platelet level improved with platelet transfusion. She was 9 months old at the time of reporting and only manifested extensive cutaneous bleeding without any other bleeding tendencies. She has been requiring weekly to twice weekly platelet transfusions since birth. As there was a family history of neonatal thrombocytopenia, IT was highly suspected although both parents were asymptomatic and had normal platelet levels. Specimens were obtained from the proband (peripheral blood), father (peripheral blood), and mother (peripheral blood, buccal and vaginal swabs and urine). Genomic DNA was extracted. WES detected a likely pathogenic heterozygous SLFN14 c.652A > G (p.Lys218Glu) mutation in proband's blood sample (Figure 1A). A polymerase chain reaction (PCR) was carried out to amplify exon 3 of the SLFN14 gene, and purified PCR products were sequenced. Sanger sequencing of the SLFN14 gene was performed on patient and both parents to identify the specific variant c.652A > G (p.Lys218Glu) located in exon 3 of SLFN14. Careful analysis of the Sanger chromatogram (reverse sequence) revealed a very low signal representing nucleotide G at the c.652 position which corresponded to the mutated allele in all maternal samples (peripheral blood, buccal swab, vaginal swab and urine) (Figures 1B,D–F). RJ Stapley concluded that mutation of SLFN14 causes dominantly inherited macrothrombocytopenia with moderate to severe bleeding phenotypes.2 The patients with SLFN14 variants have been shown to have platelet function defect due to impaired aggregation to ADP, protease-activated receptor-1 (PAR-1) and collagen with reduced ATP secretion from dense granules after PAR-1 stimulation.2 Three unrelated families with SLFN14 mutations were reported by Sarah et al. All three families were proven to have dominantly inherited moderate thrombocytopenia, but with a disproportionate bleeding type.3 Noris et al. proposed that IT be classified into three groups: disorders characterized by only a platelet defect; disorders in which IT is associated with congenital defects; and IT with an increased risk of acquiring additional disease during life.1 SLFN14-related thrombocytopenia is reported to have an impaired platelet defect. It has not been proven to be associated with additional hazardous disorders that may develop during life. Our family reported here was heterozygous for the SLFN14 c.652A > G (p.lys218Glu) mutation as found in family C reported by Sarah et al.3 Both siblings in our reported family had severe thrombocytopenia since the neonatal period. The proband was planned to undergo a haploidentical haematopoietic stem cell transplant aiming for cure and to prevent life-threatening bleeding as there was no matched donor available. The father was chosen as the donor for the stem cell transplant as the mother had low-level gonosomal mosaicism. SLFN14 mutation-associated thrombocytopenia is an autosomal dominant (AD) disorder. However, we did not detect the pathogenic mutation in both parents during the initial sequencing. The circumstance of ‘non-carrier’ parents with two children affected with rare AD thrombocytopenia was less likely to be de novo but highly suggestive of possible gonadal mosaicism in either parent. Hence, resequencing and careful analysis of the Sanger chromatograms was performed. This identified a small peak corresponding to the mutated allele in the maternal peripheral blood, urine, vaginal and buccal swab samples, which represented the three germ layers (endoderm, mesoderm and ectoderm). Therefore, it can be concluded that the mother has low-level gonosomal mosaicism and the mutation has been inherited by both offspring. Detection of maternal mosaicism is important for assessing the risk of recurrence and for offering accurate genetic counselling. Precise estimation of recurrent risk is difficult (up to 50% similar to germline mutation) as the level of mosaicism in gonads cannot be determined easily. Mosaicism in the mother also explains the lack of symptoms4 in the mother. Both of our patients have AD SLFN14 thrombocytopenia inherited from maternal gonosomal mosaicism. Precise diagnosis of IT is imperative as the disease prognosis can then be defined and the most appropriate treatment, follow-up and accurate genetic counselling can be offered. Ming-Ping Ang, Seoh-Leng Yeoh, Choo-Hau Lim and Gaik-Siew Ch'ng provided patients material. Siah-Li Foo and Yong-Wee Wong analysed and provided results. Ming-Ping Ang, Seoh-Leng Yeoh and Gaik-Siew Ch'ng wrote the paper. All authors approved the final manuscript. We are grateful to the affected family who consented to reporting the study. The authors have no conflict of interests to disclose.