One mutation, divergent journeys: expanding the clinical spectrum of homozygous SAMHD1 deficiency in childhood¶

The finding¶

This retrospective case series describes three pediatric patients who all carry the same homozygous SAMHD1 missense variant (p.Gly209Ser) yet present with strikingly different clinical pictures — ranging from a myopathy-dominant phenotype with no CNS involvement, to classic Aicardi-Goutières-like disease with intracranial calcifications and panniculitis, to a lupus-like connective tissue disease with calcinosis and vasculopathy. All three received JAK inhibitors (mostly tofacitinib) and showed partial-to-sustained improvement, with flares reliably occurring when treatment was interrupted.

Where it fits¶

This paper is squarely about ARM 1 (interferon/JAK-STAT): it documents real-world clinical evidence that a single SAMHD1 loss-of-function genotype drives a spectrum of interferon-associated phenotypes, and that this spectrum is broadly JAK-inhibitor responsive. That's relevant background for the A565T model in two ways. First, it reinforces the core premise that SAMHD1 dysfunction converges on a treatable, IFN-driven axis rather than a single fixed syndrome — supporting the idea that a partial, heterozygous loss-of-function variant like A565T could plausibly sit on a milder point of a similar phenotypic continuum, rather than mapping to one predictable presentation. Second, the "flares on drug holiday" observation is a useful clinical signal that tonic IFN-I signaling, once established, may need continuous suppression — a consideration for any future JAK-inhibitor strategy in A565T-driven disease, since ARM 1 is the arm most likely to respond to that class of drug.

Caveats¶

These are homozygous, complete/near-complete loss-of-function cases (classic AGS5 genetics), not the heterozygous, partial loss-of-function scenario modeled for A565T — phenotype severity and mechanism may not translate directly.
This is a case series (n=3) with retrospective chart review; it establishes association between genotype, phenotype, and drug response, not mechanistic proof of the ANKIB1 node, VDAC1/mitophagy, or NLRP3 pathways.
No mitochondrial (ARM 2) or nucleotide/inflammasome (ARM 3) biomarkers were assessed — the paper speaks only to interferon pathway involvement and clinical JAK-I response, not to whether JAK-resistant arms are active in these or other SAMHD1 patients.

What to watch¶

The obvious next question is why one genotype produces such different organ-system phenotypes — modifier genes, stochastic ISG thresholds, or differential engagement of the mitochondrial/NLRP3 arms are all plausible explanations worth testing. For A565T specifically, it raises the question of whether heterozygous partial loss-of-function carriers show a similarly broad — if quieter — phenotypic range, and whether JAK-inhibitor responsiveness tracks with how much of the phenotype is ARM-1-driven versus ARM-2/3-driven.

Source: One mutation, divergent journeys: expanding the clinical spectrum of homozygous SAMHD1 deficiency in childhood — 2026.