Hereditary Spherocytosis: Can Next-Generation Sequencing of the Five Most Frequently Affected Genes Replace Time-Consuming Functional Investigations?

Congenital defects of the erythrocyte membrane are common in northern Europe and all over the world. The resulting diseases, for example, hereditary spherocytosis (HS), are often underdiagnosed, partly due to their sometimes mild and asymptomatic courses. In addition to a broad clinical spectrum, this is also due to the occasionally complex diagnostics that are not available to every patient. To test whether next-generation sequencing (NGS) could replace time-consuming spherocytosis-specific functional tests, 22 consecutive patients with suspected red cell membranopathy underwent functional blood tests. We were able to identify the causative genetic defect in all patients with suspected HS who underwent genetic testing (n = 17). The sensitivity of the NGS approach, which tests five genes (ANK1 (gene product: ankyrin1), EPB42 (erythrocyte membrane protein band4.2), SLC4A1 (band3), SPTA1 (α-spectrin), and SPTB (ß-spectrin)), was 100% (95% confidence interval: 81.5-100.0%). The major advantage of genetic testing in the paediatric setting is the small amount of blood required (<200 µL), and compared to functional assays, sample stability is not an issue. The combination of medical history, basic laboratory parameters, and an NGS panel with five genes is sufficient for diagnosis in most cases. Only in rare cases, a more comprehensive functional screening is required.

Severe high-molecular-weight kininogen deficiency: clinical characteristics, deficiency-causing KNG1 variants, and estimated prevalence.

BACKGROUND: Severe high-molecular-weight kininogen (HK) deficiency is a poorly studied autosomal recessive contact system defect caused by pathogenic, biallelic KNG1 variants. AIM: We performed the first comprehensive analysis of diagnostic, clinical, genetic, and epidemiological aspects of HK deficiency. METHODS: We collected clinical information and blood samples from a newly detected HK-deficient individual and from published cases identified by a systematic literature review. Activity and antigen levels of coagulation factors were determined. Genetic analyses of KNG1 and KLKB1 were performed by Sanger sequencing. The frequency of HK deficiency was estimated considering truncating KNG1 variants from GnomAD. RESULTS: We identified 48 cases of severe HK deficiency (41 families), of these 47 have been previously published (n = 19 from gray literature). We genotyped 3 cases and critically appraised 10 studies with genetic data. Ten HK deficiency-causing variants (one new) were identified. All of them were truncating mutations, whereas the only known HK amino acid substitution with a relevant phenotype instead causes hereditary angioedema. Conservative estimates suggest an overall prevalence of severe HK deficiency of approximately one case per 8 million population, slightly higher in Africans. Individuals with HK deficiency appeared asymptomatic and had decreased levels of prekallikrein and factor XI, which could lead to misdiagnosis. CONCLUSION: HK deficiency is a rare condition with only few known pathogenic variants. It has an apparently good prognosis but is prone to misdiagnosis. Our understanding of its clinical implications is still limited, and an international prekallikrein and HK deficiency registry is being established to fill this knowledge gap.

Novel GATA1 Variant Causing a Bleeding Phenotype Associated with Combined Platelet α-/δ-Storage Pool Deficiency and Mild Dyserythropoiesis Modified by a SLC4A1 Variant.

Germline defects in the transcription factor GATA1 are known to cause dyserythropoiesis with(out) anemia and variable abnormalities in platelet count and function. However, damaging variants closely located to the C-terminal zinc finger domain of GATA1 are nearly unknown. In this study, a 36-year-old male index patient and his 4-year-old daughter suffered from moderate mucocutaneous bleeding diathesis since birth. Whole exome sequencing detected a novel hemizygous GATA1 missense variant, c.886A>C p.T296P, located between the C-terminal zinc finger and the nuclear localization sequence with non-random X-chromosome inactivation in the heterozygous daughter. Blood smears from both patients demonstrated large platelet fractions and moderate thrombocytopenia in the index. Flow cytometry and electron microscopy analysis supported a combined α-/δ (AN-subtype)-storage pool deficiency as cause for impaired agonist-induced platelet aggregation (light transmission aggregometry) and granule exocytosis (flow cytometry). The absence of BCAM in the index (Lu(a-b-)) and its low expression in the daughter (Lu(a-b+)) confirmed a less obvious effect of defective GATA1 also on erythrocytes. Borderline anemia, elevated HbF levels, and differential transcription of GATA1-regulated genes indicated mild dyserythropoiesis in both patients. Furthermore, a mild SLC4A1 defect associated with a heterozygous SLC4A1 c.2210C>T p.A737V variant maternally transmitted in the daughter may modify the disease to mild spherocytosis and hemolysis.

Second MAFA Variant Causing a Phosphorylation Defect in the Transactivation Domain and Familial Insulinomatosis.

Adult-onset familial insulinomatosis is a rare disorder with recurrent, severe hypoglycemia caused by multiple insulin-secreting pancreatic tumors. The etiology was unclear until the variant p.Ser64Phe in the transcription factor MAFA, a key coordinator of ß-cell insulin secretion, was defined as the cause in two families. We here describe detailed genetic, clinical, and family analyses of two sisters with insulinomatosis, aiming to identify further disease causes. Using exome sequencing, we detected a novel, heterozygous missense variant, p.Thr57Arg, in MAFA's highly conserved transactivation domain. The impact of the affected region is so crucial that in vitro expression studies replacing Thr57 have already been performed, demonstrating a phosphorylation defect with the impairment of transactivation activity and degradation. However, prior to our study, the link to human disease was missing. Furthermore, mild hyperglycemia was observed in six additional, heterozygote family members, indicating that not only insulinomatosis but also MODY-like symptoms co-segregate with p.Thr57Arg. The pre-described MAFA variant, p.Ser64Phe, is located in the same domain, impairs the same phosphorylation cascade, and results in the same symptoms. We confirm MAFA phosphorylation defects are important causes of a characteristic syndrome, thus complementing the pathophysiological and diagnostic disease concept. Additionally, we verify the high penetrance and autosomal dominant inheritance pattern.

c.451dupT in KLKB1 is common in Nigerians, confirming a higher prevalence of severe prekallikrein deficiency in Africans compared to Europeans.

Essentials Prekallikrein (PK) deficiency is a recessive trait with isolated aPTT prolongation. KLKB1 c.451dupT is common in Nigerians (7/600 alleles) and absent in a European group (0/600). To date, all genotyped PK-deficient patients of African ancestry were homozygous for 451dupT. Diagnostics of isolated aPTT prolongation in African descendants should include PK testing. ABSTRACT: Background Severe prekallikrein deficiency (PK deficiency) is an autosomal-recessive condition thought to be very rare. Recently we reported that the previously unnoticed variant c.451dupT, p.Ser151Phefs*34 in KLKB1, which is listed in databases aggregating genome data, causes PK deficiency and is common in Africans according to gnomAD (allele frequency 1.43%). Patients/Methods The most common African (c.451dupT) and European (c.1643G>A, p.Cys548Tyr) PK deficiency causing KLKB1 variants were analyzed in two population-based collectives of 300 Nigerian and 300 German subjects. Genome databases were evaluated for variant frequencies and ethnicity of the subjects. The geographic origin of PK-deficient cases due to 451dupT was assessed. Results Two of five patients with PK deficiency caused by homozygous 451dupT were African, one African American, one from Oman, and one of unknown origin. The frequency of 451dupT was 1.17% in the Nigerian collective (7/600 alleles); none had Cys548Tyr. Subjects with 451dupT were found among different Nigerian ethnicities. Both variants were absent in the European collective. Database research was compatible with these findings, even though mainly data of African Americans (451dupT: 1.12%-1.78%) was accessible. A relevant number of non-American Africans are included only in the 1000Genomes collective: 451dupT frequency was 1.29% in native Africans and 1.56% in African Caribbeans. Conclusions This study underlines the higher prevalence of PK deficiency among people with African descent compared to Europeans. In order to avoid delay of necessary surgical procedures in patients of African origin, diagnostic algorithms for isolated, unexplained, activated partial thromboplastin time prolongation in these subjects should include PK deficiency screening.

Severe plasma prekallikrein deficiency: Clinical characteristics, novel KLKB1 mutations, and estimated prevalence.

BACKGROUND: Severe plasma prekallikrein (PK) deficiency is an autosomal-recessive defect characterized by isolated activated partial thromboplastin time prolongation. To date, no comprehensive methodologically firm analysis has investigated the diagnostic, clinical, and genetic characteristics of PK deficiency, and its prevalence remains unknown. PATIENTS/METHODS: We described new families with PK deficiency, retrieved clinical and laboratory information of cases systematically searched in the (gray) literature, and collected blood of these cases for complementary analyses. The Genome Aggregation Database (gnomAD) and the population-based Gutenberg Health Study served to study the prevalence of mutations and relevant genetic variants. RESULTS: We assembled a cohort of 111 cases from 89 families and performed new genetic analyses in eight families (three unpublished). We identified new KLKB1 mutations, excluded the pathogenicity of some of the previously described ones, and estimated a prevalence of severe PK deficiency of 1/155 668 overall and 1/4725 among Africans. One individual reported with PK deficiency had, in fact, congenital kininogen deficiency associated with decreased PK activity. One quarter of individuals had factor XII clotting activity below the reference range. Four major bleeding events were described in 96 individuals, of which 3 were provoked, for a prevalence of 4% and an annualized rate of 0.1%. The prevalence of cardiovascular events was 15% (6% <40 years; 21% 40-65 years; 33% >65 years) for an annualized rate of 0.4%. CONCLUSIONS: We characterized the genetic background of severe PK deficiency, critically appraised mutations, and provided prevalence estimates. Our data on laboratory characteristics and clinical course of severe PK deficiency may have clinical implications.