Pathogenic, criteria provided, single submitter | clinical testing | Quest Diagnostics Nichols Institute San Juan Capistrano | Sep 17, 2024 | The HBB c.364G>A (p.Glu122Lys) variant (also known as Hb O-Arab or p.Glu121Lys) has been described in individuals with normal clinical presentation when in heterozygosity and mild anemia when in homozygosity, and with structural hemoglobinopathy association (HbVar (http://globin.bx.psu.edu/) and ITHANET (http://www.ithanet.eu/)). It has also been identified in compound heterozygosity with the Hb S variant to cause severe sickling hemoglobinopathy with laboratory and clinical manifestations similar to those homozygous sickle cell anemia and higher severity relative to Hb SC disease, and described to cause increased red cell mean corpuscular hemoglobin concentration (MCHC) similar to that of Hb C (PMIDs: 20788973 (1960), 5481775 (1970), 10203101 (1999), 10583251 (1999), 22975760 (2013), 24880717 (2014), 32371413 (2020), 35064169 (2022), and 37554704 (2023)). Furthermore, when the variant is on the same chain as the Hb variant, these two changes are collectively known as HbS-Oman which can present from no to a dominant expression of a sickle cell anemia clinical syndrome (PMIDs: 9834244 (1998) and 10583251 (1999)). This variant has also been reported along with Hb D-Los Angeles or Hb C in individuals with hemoglobinopathies including a mild microcytic anemia (PMIDs: 31973650 (2020) and 33091040 (2020)). In vitro studies have shown that this variant results in the impaired hemoglobin function in Hb S compound heterozygous individuals (PMIDs: 5481775 (1970), 3859465 (1985), and 10583251 (1999)). The frequency of this variant in the general population, 0.000014 (4/282706 chromosomes (Genome Aggregation Database, http://gnomad.broadinstitute.org)), is consistent with pathogenicity. - |
Pathogenic, criteria provided, single submitter | clinical testing | ARUP Laboratories, Molecular Genetics and Genomics, ARUP Laboratories | Aug 22, 2023 | The Hb O-Arab variant (HBB: c.364G>A; p.Glu122Lys, also known as Glu121Lys when numbered from the mature protein, rs33946267, HbVar ID: 510) is not associated with clinical manifestations in heterozygous carriers but can cause sickling disease when found in trans to Hb S or Hb C (Milner 1970, Ramot 1960, Rossi 2011, Zimmerman 1999, HbVar database and references therein). This variant has also been reported in cis to Hb S in a doubly substituted variant known as Hb S-Oman (HbVar ID: 687, Langdown 1989, HbVar database and references therein) which causes red cell sickling and has been reported in heterozygous individuals either with sickle cell disease or that were asymptomatic, though individuals with less severe clinical symptoms often had alpha thalassemia trait (Al Balushi 2017, Nagel 1998). Functional characterization of the Hb O-Arab variant globin in the presence of Hb S indicates that the Hb S/Hb O-Arab hemoglobin precipitates at a lower concentration that Hb S/Hb S or HbS/Hb A, suggestive of a strong sickling effect (Milner 1970). Hb O-Arab is listed as pathogenic by multiple laboratories in ClinVar (Variation ID: 15292), and it is found in the general population with an overall allele frequency of 0.001% (4/282706 alleles) in the Genome Aggregation Database. Based on available information, the variant is considered to be pathogenic. References: Link to HbVar database: https://globin.bx.psu.edu/hbvar/menu.html Al Balushi HWM et al. The super sickling haemoglobin HbS-Oman: a study of red cell sickling, K+ permeability and associations with disease severity in patients heterozygous for HbA and HbS-Oman (HbA/S-Oman genotype). Br J Haematol. 2017 Oct;179(2):256-265. PMID: 28699687. Langdown JV et al. A new doubly substituted sickling haemoglobin: HbS-Oman. Br J Haematol. 1989 Mar;71(3):443-4. PMID: 2930724. Milner P et al. Hemoglobin O arab in four negro families and its interaction with hemoglobin S and hemoglobin C. N Engl J Med. 1970; 283(26):1417-25. PMID: 5481775 Nagel RL et al. HbS-oman heterozygote: a new dominant sickle syndrome. Blood. 1998 Dec 1;92(11):4375-82. PMID: 9834244. Ramot B et al. Haemoglobin O in An Arab Family. Br Med J. 1960; 2(5208):1262-4. PMID: 20788973. Rossi P et al. Bone marrow necrosis and sickle cell crisis associated with double heterozygosity for HbS and HbOARAB. Am J Hematol. 2011; 86(3):309-10. PMID: 20954261. Zimmerman S et al. Hemoglobin S/O(Arab): thirteen new cases and review of the literature. Am J Hematol. 1999; 60(4):279-84. PMID: 10203101. - |
Pathogenic, criteria provided, single submitter | clinical testing | Labcorp Genetics (formerly Invitae), Labcorp | Jan 24, 2024 | This sequence change replaces glutamic acid, which is acidic and polar, with lysine, which is basic and polar, at codon 122 of the HBB protein (p.Glu122Lys). The frequency data for this variant in the population databases is considered unreliable, as metrics indicate poor data quality at this position in the gnomAD database. This missense change has been observed in individual(s) with sickle cell anemia (PMID: 3859465, 5481775). In at least one individual the data is consistent with being in trans (on the opposite chromosome) from a pathogenic variant. It has also been observed to segregate with disease in related individuals. This variant is also known as HbO-Arab or p.Glu121Lys. ClinVar contains an entry for this variant (Variation ID: 15292). Advanced modeling of protein sequence and biophysical properties (such as structural, functional, and spatial information, amino acid conservation, physicochemical variation, residue mobility, and thermodynamic stability) has been performed at Invitae for this missense variant, however the output from this modeling did not meet the statistical confidence thresholds required to predict the impact of this variant on HBB protein function. This variant disrupts the p.Glu122 amino acid residue in HBB. Other variant(s) that disrupt this residue have been determined to be pathogenic (PMID: 24245819, 24616059, 25666204). This suggests that this residue is clinically significant, and that variants that disrupt this residue are likely to be disease-causing. For these reasons, this variant has been classified as Pathogenic. - |