Spherocytosis in Newborn Secondary to Novel Heterozygous Mutation in SPTB Gene: Case Report

Abstract

This case report describes a novel mutation of the SPTB gene as a potential pathogenic cause of spherocytosis. A 3-week-old male presented with clinical and laboratory signs consistent with hemolytic spherocytosis, including jaundice, hyperbilirubinemia, anemia, reticulocytosis, negative Coombs test, no ABO or Rh incompatibility, and a peripheral blood smear notable for numerous spherocytes. His laboratory work demonstrated persistent anemia despite daily folate prompting next-generation sequencing which revealed a novel mutation in the SPTB gene resulting in a nonfunctioning protein product. Correlation of the genetic finding with clinical presentation may help guide management for this and future patients.

Introduction

Hereditary spherocytosis (HS) is a common type of congenital hemolytic anemia, seen in 1 in every 2000 patients. It is caused by genetic variations in erythrocyte membrane and cytoskeleton proteins such as spectrin, ankyrin, band 3, and band 4.2, which then cause the red blood cell (RBC) to have an abnormal shape. This can present in patients as pallor and jaundice. Patients may have anemia, abnormal peripheral blood smears, reticulocytosis, and hyperbilirubinemia. Hereditary spherocytosis is important to identify to ensure that the patient receives timely treatment to prevent serious outcomes of this disease, such as kernicterus in neonates, cholelithiasis, and hemolytic crises. ¹

Case Description

A male neonate born at 40 weeks and 4 days via spontaneous vaginal delivery presented to the neonatal intensive care unit (NICU) 38 hours after birth with hyperbilirubinemia on phototherapy. His vital signs showed a blood pressure of 54/31 mm Hg, pulse of 140 beats/min, body temperature of 37.2°C (99°F), respiratory rate of 70 breaths/min, and oxygen saturation of 99% on room air. The patient’s head circumference, length, and weight were all within 1 standard deviation of average for his age range and sex. Physical examination revealed diffuse jaundice and scleral icterus. Both his parents identified as African American, and his family history was negative for hemoglobinopathies and hemolytic anemia.

His laboratory work was notable for a total bilirubin of 20.2 mg/dL (normal range, 0-8.2), direct bilirubin of 0.4 mg/dL (0.0-0.6), hemoglobin of 17.2 g/dL (14.7-18.6), a mean corpuscular hemoglobin concentration of 26.0 g/dL (31-34.2), mean corpuscular volume of 98.6 fL (97.3-109.8), red cell distribution width of 21.9% (12.0-15.2), and a reticulocyte count of 9.2% (2.5-6.5). These findings in combination with the patient’s jaundice on examination and downtrending hemoglobin to 12.9 g/dL suggested a hemolytic process (Table 1). The differential diagnosis focused on both intrinsic and extrinsic causes of hemolytic anemia, including membrane defects, enzyme deficiencies such as glucose-6-phosphate dehydrogenase (G6PD) and pyruvate kinase deficiencies, hemoglobinopathies, and autoimmune diseases. ²

Table 1.

Timeline of Patient Results.

Patient age	Event/results	Reference range
DOL 1	Direct antiglobulin test—negative Anti-IgG—negative
DOL 2	Total bilirubin maximum: 20.2 mg/dLReticulocyte count: 9.2%Blood smear: polychromasia with normocytes, occasional spherocytes, and rare schistocytes in the absence of bite or helmet cellsConsultants: pediatric hematology-oncology team consulted	0-8.22.5-6.5
DOL 6	Hemolytic anemia panel• Osmotic fragility—elevated• Band 3—decreased• Eosin 5-maleimide binding—decreased
2 Months	G6PD quantity: 22.1 U/g[Hb]Pyruvate kinase enzyme activity: 11.9 U/g[Hb]Consultants: seen in the pediatric hematology office and started on folic acid and vitamin D3	7.0-20.55.5-12.4
10 Months	Blood smear: numerous spherocytes, polychromasia
18 Months	Hemoglobin: 10.6 g/dLMCHC: 34.0 g/dL	10.3-13.331.9-35.2

Abbreviations: DOL, day of life; G6PD, gluocose-6-phosphate dehydrogenase; MCHC, mean corpuscular hemoglobin concentration.

On consultation with the pediatric hematology-oncology team, additional laboratory tests were suggested, revealing a negative Coombs test and no ABO or Rh incompatibility. He was found to have a G6PD quantity of 22.1 U/g[Hb] (7.0-20.5) and a pyruvate kinase enzyme activity of 11.9 U/g[Hb] (5.5-12.4). In addition, a hemolytic anemia screening panel was ordered on day of life (DOL) 6 (Table 1). This panel revealed increased osmotic fragility, decreased eosin 5-maleimide binding, and decreased protein band 3, suggesting a membrane-related disease, such as HS or hereditary elliptocytosis (HE).

Peripheral blood smears were also used to investigate the cause of the patient’s symptoms. A smear from DOL 2 showed polychromasia with normocytes, occasional spherocytes, and rare schistocytes in the absence of bite or helmet cells. White blood cells and platelets showed normal morphology. At this point, the differential included HS or HE, as the smear was significant for spherocytes on blood smear, and the patient had reticulocytosis in the absence of an immune cause. Hereditary spherocytosis and HE can present similarly on peripheral smears early in life although they have different disease courses to prepare for. ³

The patient was discharged in stable condition from the NICU after 8 days with daily outpatient folic acid and vitamin D3. He continued to have significant reticulocytosis and indirect hyperbilirubinemia. Because of the persistently abnormal laboratory test values, the negative family history, and with a goal of determining the underlying cause and inheritance risk in his siblings and in his own offspring, a next-generation sequencing panel was ordered.

The patient was evaluated using a hereditary hemolytic anemia panel, which revealed a heterozygous out-of-frame deletion in exon 2 and 3 of the SPTB gene, resulting in an abnormal and nonfunctioning spectrin protein product. Loss-of-function mutations of the SPTB gene are known to cause abnormally shaped erythrocytes and are implicated in both HS and HE. ⁴ However, the specific deletion observed in this patient has not been reported in literature and is thus potentially pathogenic.

A repeat peripheral smear performed at 10 months of age showed RBC morphology as normocytic and normochromic with significant polychromasia. Numerous spherocytes with occasional acanthocytes and echinocytes were noted. White blood cells and platelets had a normal appearance (Figure 1). This confirmed a diagnosis of HS, specifically moderate-severe HS. The patient had hemoglobin between 6 and 8, as well as a reticulocyte count between 6% and 10%. ⁵ In addition, the parents were offered an next-generation sequencing test to discover if this is a de novo or an inherited mutation.

Figure 1.

Peripheral smear at 10 months and 27 days of age. The blue arrow indicates polychromasia, and the red arrows indicate spherocytes.

Discussion

This case expands the base of knowledge in the HS field by identifying and tracking the impact of a deleterious heterozygous SPTB loss-of-function mutation not previously reported in the literature. Mutations in beta-spectrin, a principal cytoskeletal protein of the intracellular side of the plasma membrane, often show an autosomal dominant pattern of inheritance; however, this patient developed HS in the absence of a positive family history. ⁶ Patients with SPTB mutations typically present with mild to moderate-severe phenotypes/symptoms, with the possibility of blood transfusions and splenectomy. ⁷ This novel mutation guides us in tracking the patient’s clinical course over time. This also elucidates the class of HS and special considerations for future patient’s evaluation.

One limitation of this study is that the parents have not yet completed genetic testing. It is therefore unknown if this mutation has an autosomal recessive pattern or if it is a de novo mutation. The parents have been counseled about the risk of HS in future offsprings at this time.