LAUSR

To the Editor: Low oxygen affinity hemoglobin (Hb) mutations are a rare but important consideration in patients presenting with low peripheral oxygen saturations (SpO2) and anemia without clinical evidence of hypoxia; cyanosis may also be present. Hb mutations that affect oxygen affinity alter critical molecular regions involved in the conformational transition between the relaxed (R) oxygenated, and the tense (T) deoxygenated state of Hb. Mutations that stabilize the T state or impair the R state result in lowered oxygen affinity, and a right-shifted oxygen-dissociation curve with increased p50, the point at which Hb is one-half saturated with oxygen. Lower than normal Hb concentration arises due to decreased erythropoietic drive from excellent oxygen delivery to tissues, and erythropoietin level is typically decreased or normal. Low oxygen affinity mutations of the alpha (α)-globin gene present in neonates with hypoxemia, anemia, and sometimes cyanosis, whereas mutations of the beta (β)-globin gene present after about 6 months of age attributed to delayed β-globin expression with the postnatal switch of fetal to adult Hb. Both high and low oxygen affinity Hbs are inherited in an autosomal dominant manner; de novo mutations are also seen. We present a female child who had normal SpO2 measurements at birth but at 22 months of age was found to have persistent severe hypoxemia and mild anemia that was previously undetected; parents had normal hematological parameters. She underwent extensive evaluations for cardiopulmonary disease, including cardiac catheterization and bronchoscopy, and was treated with supplemental oxygen for over 1 year. Having ruled out cardiopulmonary etiologies of hypoxemia, the patient underwent evaluation for low oxygen affinity Hb. This female was born by spontaneous vaginal delivery at 39 3/7 weeks gestation to Caucasian, nonconsanguineous, healthy parents. On day 3 of life, she had normal SpO2 measurements (≥98%) in both hands and feet. She had no clinical jaundice and appeared normal. State newborn hemoglobinopathy screening reported fetal and adult (FA) Hb by isoelectric focusing. No other laboratory testing was performed during her birth hospitalization and in subsequent routine follow-up; she exhibited normal growth and development without apparent cyanosis. At 22 months old, she presented with symptoms of an upper respiratory tract infection and was found to be severely hypoxemic with SpO2 61% on room air. During a 10-day hospitalization for bronchiolitis, although noted to have a clinical resolution of respiratory symptoms, she was persistently hypoxemic and unable to wean from supplemental oxygen. She was discharged on continuous supplemental oxygen, and in follow-up was noted to require between 0.5 and 3 L O2/min by nasal cannula to maintain a SpO2 > 90%. Over the next 10 months due to the persistence of unexplained hypoxemia, an extensive pulmonary and cardiac workup was undertaken. Her pulmonary evaluation included computerized tomography (CT) of the chest, flexible bronchoscopy, and abdominal ultrasound to rule out hepatopulmonary syndrome. Cardiac evaluation included electrocardiogram, echocardiogram with bubble study, and cardiac catheterization. These studies ruled out pulmonary hypertension, intracardiac shunts, and arteriovenous malformations but noted low SO2 in the pulmonary veins (81%–86% at 30% FiO2). All other studies were normal. An arterial blood gas showed low arterial oxygen saturation (SaO2) of 82% with an elevated PaO2 of 111 mm Hg, and was negative for methemoglobinemia and carboxyhemoglobinemia. In the setting of negative cardiopulmonary work-up, she was referred for hematology evaluation after being found to have mild normocytic anemia (Hb 10.1 g/dL, MCV 89 fL) and the possibility of a low oxygen affinity Hb variant was entertained. Hb oxygen affinity was estimated from a venous blood sample and p50 calculated from pH, O2, and Hb/O2 saturation. We used a previously reported mathematical formula converted to Microsoft Excel program for rapid calculation of p50 (Methods-online supplement). The calculated p50 was elevated at 41.9 mm Hg (normal 24–29 mm Hg), suggesting presence of a low oxygen affinity Hb variant. The initial screening Hb fractionation tests by capillary electrophoresis (CE) and high-performance liquid chromatography (HPLC) failed to show distinct abnormal peaks. However, an unusually high and broadened Hb A2 peak was appreciated on the chromatogram. Discrepancy between the Hb A2 peak values was suggestive of a variant Hb measuring approximately 10%, which coeluted with Hb A2 on HPLC and migrated together with Hb A by CE (Figure 1A,B). Sequencing of α-globin genes revealed no abnormalities; β-globin (HBB) sequencing discovered a novel variant HBB:c.317T>A; p. Leu106His. HBB sequencing of the child's parents (paternity confirmed using short tandem repeat markers) was negative, indicating a de novo mutation. Full Hb oxygen-dissociation curve was performed at UCSF Benioff Children's Hospital Oakland and confirmed a right-shifted Hb oxygen-dissociation curve (Figure 1C). The p50 was lower than the single point estimate from venous blood; possibly from shipping delay that may have resulted in decreased red cell adenosine triphosphate (ATP) and 2,3-bisphosphoglycerate (BPG). Received: 14 August 2021 Revised: 14 September 2021 Accepted: 20 September 2021