Homozygous mutation in SAMHD1 gene causes cerebral vasculopathy and early onset stroke

Abstract

We describe an autosomal recessive condition characterized with cerebral vasculopathy and early onset of stroke in 14 individuals in Old Order Amish. The phenotype of the condition was highly heterogeneous, ranging from severe developmental disability to normal schooling. Cerebral vasculopathy was a major hallmark of the condition with a common theme of multifocal stenoses and aneurysms in large arteries, accompanied by chronic ischemic changes, moyamoya morphology, and evidence of prior acute infarction and hemorrhage. Early signs of the disease included mild intrauterine growth restriction, infantile hypotonia, and irritability, followed by failure to thrive and short stature. Acrocyanosis, Raynaud's phenomenon, chilblain lesions, low-pitch hoarse voice, glaucoma, migraine headache, and arthritis were frequently observed. The early onset or recurrence of strokes secondary to cerebral vasculopathy seems to always be associated with poor outcomes. The elevated erythrocyte sedimentation rate (ESR), IgG, neopterin, and TNF-α found in these patients suggested an immune disorder. Through genomewide homozygosity mapping, we localized the disease gene to chromosome (Chr) 20q11.22-q12. Candidate gene sequencing identified a homozygous mutation, c.1411-2A > G, in the SAMHD1 gene, being associated with this condition. The mutation appeared at the splice-acceptor site of intron 12, resulted in the skipping of exon 13, and gave rise to an aberrant protein with in-frame deletion of 31 amino acids. Immunoblotting analysis showed lack of mutant SAMHD1 protein expression in affected cell lines. The function of SAMHD1 remains unclear, but the inflammatory vasculopathies of the brain found in the patients with SAMHD1 mutation indicate its important roles in immunoregulation and cerebral vascular hemeostasis.

Fig. 1.

Partial pedigree of the family with SAMHD1 gene mutation associated with cerebral vasculopathy. Filled symbols represent affected individuals, and open symbols represent unaffected individuals. Circles and squares denote females and males, respectively. A double line identifies consanguinity. Arrows indicate affected individuals included in the genetic mapping study and sequence analysis.

Fig. 2.

Spectrum of neuroimaging abnormalities associated with the cerebral vasculopathy caused by SAMHD1 gene mutation. (A) Axial FLAIR image from X-4 shows moderate nonspecific white matter signal hyperintensity, which is abnormal for age. The territory matches the internal–external watershed, which is vulnerable to chronic small vessel disease as often seen in elderly patients. (B) Axial FLAIR image from X-16 shows encephalomalacia from a prior right parietal transcortical infarct (arrow), with subcortical gliosis. Additional white matter change is present in the left periventricular regions, likely from chronic small vessel disease. (C) Axial noncontrast CT in X-15 shows encephalomalacia in the left anterior temporal lobe (arrow) from a preceding hemorrhagic stroke. (D) Coronal CTA from X-1 shows a large aneurysm arising from the left MCA bifurcation (arrow), projecting inferiorly. (E) Coronal maximal-intensity projection (MIP) from an MRA from X-28 shows extensive stenoses, severe in the right MCA territory with an acute cutoff (Left arrow) and minimal distal flow-related enhancement. There are similar stenoses in the bilateral A1 segments and a moderate stenosis of the distal left internal carotid artery (Right arrow).

Fig. 3.

Identification of the disease-causing mutation in SAMHD1 gene. (A) Sequence electropherograms showing the homozygous c.1411-2A > G mutation from affected individuals compared with a normal control. (B) Agarose gel electrophoresis of RT-PCR products from lyphoblastoid cells showing a shorter abnormal transcript as a consequence of c.1411-2A > G mutation. (C) Sequencing analysis of the RT-PCR products showing the splicing out of exon 13 in the aberrant transcript. (D) Western blot analysis of SAMHD1 protein expression in cell line samples. Cell lysates of lymphoblastoid cell lines from a normal control, a carrier, and an affected individual were immunoblotted with anti-SAMHD1 antibody. An arrow indicates the trace amount of mutant SAMHD1 detected. A diamond shows a smaller weak band suggesting protein degradation of the mutant SAMHD1. β-Actin levels were monitored by immunoblotting as a control of protein loading.