An autosomal recessive disorder characterized by onset of megaloblastic anemia associated with decreased serum vitamin B12 (cobalamin, Cbl) in infancy or early childhood. Low molecular weight (LMW) proteinuria is frequently present, but usually occurs later and is usually mild or subclinical. Patients often present with vague symptoms, including failure to thrive, loss of appetite, fatigue, lethargy, and/or recurrent infections. Treatment with vitamin B12 results in sustained clinical improvement of the anemia. The proteinuria is nonprogressive, and affected individuals do not have deterioration of kidney function; correct diagnosis is important to prevent unnecessary treatment. The disorder results from a combination of vitamin B12 deficiency due to selective malabsorption of the vitamin, and impaired reabsorption of LMW proteins in the proximal renal tubule. These defects are caused by disruption of the AMN/CUBN complex that forms the 'cubam' receptor responsible for intestinal uptake of B12/GIF (CBLIF).
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