Polycythemia vera (PV), the most common form of primary polycythemia, is caused by somatic mutation in a single hematopoietic stem cell leading to clonal hematopoiesis. PV is a myeloproliferative disorder characterized predominantly by erythroid hyperplasia, but also by myeloid leukocytosis, thrombocytosis, and splenomegaly. Familial cases of PV are very rare and usually manifest in elderly patients (Cario, 2005). PV is distinct from the familial erythrocytoses (see, e.g., ECYT1, 133100), which are caused by inherited mutations resulting in hypersensitivity of erythroid progenitors to hormonal influences or increased levels of circulating hormones, namely erythropoietin (EPO; 133170) (Prchal, 2005).

Schimke immunoosseous dysplasia (SIOD) is characterized by spondyloepiphyseal dysplasia (SED) resulting in short stature, nephropathy, and T-cell deficiency. Radiographic manifestations of SED include ovoid and mildly flattened vertebral bodies, small ilia with shallow dysplastic acetabular fossae, and small deformed capital femoral epiphyses. Nearly all affected individuals have progressive steroid-resistant nephropathy, usually developing within five years of the diagnosis of growth failure and terminating with end-stage renal disease. The majority of tested individuals have T-cell deficiency and an associated risk for opportunistic infection, a common cause of death. SIOD involves a spectrum that ranges from an infantile or severe early-onset form with a greater risk of death during childhood to a juvenile or milder later-onset form with likely survival into adulthood if renal disease is appropriately treated.

Cerebral amyloid angiopathy, or cerebroarterial amyloidosis, refers to a pathologic process in which amyloid protein progressively deposits in cerebral blood vessel walls with subsequent degenerative vascular changes that usually result in spontaneous cerebral hemorrhage, ischemic lesions, and progressive dementia. APP-related CAA is the most common form of CAA (Revesz et al., 2003, 2009).