March 2026 Research Update

What this month adds

This month, the clearest message from human studies is that hereditary spastic paraplegia (HSP) can involve more than walking and leg stiffness, and that some changes may be gradual and easy to miss without looking for them. In a small nutrition follow-up, muscle mass went down over 1 year while many routine blood and diet measures stayed similar, suggesting that keeping track of body composition may capture change even when standard labs do not. In a separate small cognitive testing study, people with HSP showed broader thinking-speed and attention difficulties compared with healthy controls, pointing to brain networks beyond the motor system. [1] [2]

In the nutrition pilot study (36 adults with SPG4, SPG7, or SPG11), relative skeletal muscle mass declined over 12 months and disease severity scores worsened slightly, even though overall diet reports and metabolic blood tests were largely unchanged. The study also reported that the SPG11 group had higher body mass index than population norms, and that worse motor severity related to lower leg muscle mass and lower reported protein and fiber intake. Put simply, these data suggest that weight, muscle, and function may shift over time in ways that are not fully reflected by standard blood tests or a single counseling session. [1]

In the cognitive study (10 people with HSP compared with 30 controls), the largest differences were in processing speed, attention, visual scanning, and social cognition, described by the authors as consistent with frontal-area brain dysfunction. The study also raised the idea that different genetic subtypes might still lead to similar thinking and behavior patterns through shared effects on frontotemporal brain networks. For patients and caregivers, this adds to the picture that non-motor symptoms can be part of HSP, although the study was small and does not show how common these problems are across all HSP. [2]

What this may mean for patients and caregivers

These studies mainly add understanding rather than changing care right now. They support the idea that tracking HSP only through routine lab tests or only through walking symptoms may miss other changes, like shifts in muscle mass or changes in thinking speed and attention. What remains uncertain is how strongly these patterns apply to different HSP genes and stages, because both studies were small and done in specific settings. [1] [2]

What this month adds

This month strengthened the idea that HSP-related genes can show wider and sometimes unexpected clinical pictures, and that genetics can clarify diagnoses that look similar on the surface. Large human datasets broadened what is reported in SPG4, including cognitive decline in some people, and a separate sequencing study suggested that carrying changes in two interacting genes (SPG7 and AFG3L2) together may contribute to motor neuron and cerebellar conditions. Smaller imaging and case reports added detail on early brain MRI findings in AP-4-HSP and expanded the known variant list for SPG48, while also reminding readers that an HSP-like presentation can sometimes be a different condition such as X-linked adrenoleukodystrophy. [3] [4] [5] [6] [7]

In an international SPG4/SPAST cohort (726 people with HSP; 284 with SPG4 variants), the authors reported 52 pathogenic SPG4 mutations, including four new variants, and described dementia in 44 people with SPG4, with neuropathology confirmation in four unrelated autopsied families. The same cohort also reported thin corpus callosum with intellectual disability in 18 mutation carriers, widening the recognized SPG4 spectrum beyond a "pure" leg spasticity picture. For families, this may help explain why symptoms can vary between people who share the same overall diagnosis label. [3]

A separate large sequencing analysis looked for people carrying potentially disease-relevant variants in both SPG7 and AFG3L2, two genes whose proteins interact in mitochondria. It found 6 such individuals among 4,817 people with motor neuron disease or ataxia and none among 1,827 controls, plus 6 more in an additional rare-disease exome set, with segregation supporting this "two-gene" (digenic) pattern in two families. This matters because it points to a possible explanation for complex motor and cerebellar symptoms in some families that may not fit a single-gene story. [4]

For AP-4-HSP (SPG47, SPG50, SPG52), a small registry-based imaging series reported that fetal or perinatal brain MRI could already show changes such as enlarged ventricles, a smaller corpus callosum, and reduced white matter, with more severe brain development differences described in SPG52. Postnatal imaging in these cases showed progressive white matter volume reduction and delayed myelination, suggesting that at least in these subtypes, brain changes may begin before birth and evolve over time. These observations are early and based on 4 individuals, but they clarify what radiologists might see when AP-4-HSP is the underlying cause. [5]

Two single-patient reports added focused diagnostic lessons. One expanded the known variant and symptom range of SPG48 (AP5Z1) by reporting a Turkish patient with compound heterozygous AP5Z1 variants confirmed by family testing. Another described a woman previously labeled as having HSP whose testing instead showed elevated very long-chain fatty acids and an ABCD1 variant, supporting X-linked adrenoleukodystrophy presenting as adrenomyeloneuropathy, despite normal brain and spine MRI in that case. [6] [7]

What this may mean for patients and caregivers

This group of studies mainly affects understanding of diagnosis and why symptoms can vary, rather than pointing to a new treatment. The SPG4 cohort suggests that cognitive problems and certain brain imaging findings can occur in some people with SPG4, which may help families make sense of a wider symptom range. The SPG7 plus AFG3L2 results raise the possibility that more than one gene can sometimes contribute to a motor disorder, but this is still an emerging explanation that needs confirmation in more families. The prenatal AP-4-HSP imaging and the two case reports highlight that genetics and metabolic testing can refine a diagnosis when the clinical picture is complex or unusual, while also reminding readers that single cases do not tell how often this happens. [3] [4] [5] [6] [7]

What this month adds

This month included several laboratory studies that make the biology of HSP-related genes more concrete by showing specific cellular problems that could contribute to symptoms. Across these studies, the repeated picture is that changes in proteins that move cargo along nerve fibers (like KIF5A and KIF1A) can disrupt transport, and that SPG4 (SPAST) changes may also affect myelin support around nerve fibers. A separate enzyme study identified small molecules that block FicD activity in a test tube, which is early-stage work that may eventually relate to stress responses in cells, but it is not yet connected to patient outcomes. [8] [9] [10] [11]

In an iPSC-derived motor neuron model from a SPG10 family, a new KIF5A variant (p.R17Q) reduced forward movement of mitochondria and lysosomes in axons and increased axonal swellings that contained mitochondria. The authors interpreted this as support that the variant is disease-causing by directly disturbing axonal transport, a process neurons rely on to keep long nerve fibers healthy. This is cell-based work, so it shows what can go wrong in neurons, not how severe symptoms will be in a person. [8]

A structural study of KIF1A mutations (R350G and R350W) found that these changes disrupt a specific salt-bridge contact between the motor protein and microtubules, and that this loss aligned with faster movement but shorter travel distance and weaker microtubule binding in certain conditions. In plain language, the motor may move differently but less reliably along its track, which could help explain nerve cell stress over time. This work helps pinpoint a physical "weak spot" in how the motor attaches to microtubules. [9]

An experimental study focused on SPG4 biology reported that pathogenic SPAST mutations lowered the amount of myelination in a neuron and oligodendrocyte co-culture system. In a mouse demyelination model, Spastin levels were decreased in demyelinated white matter, and adding normal Spastin in a culture model protected neurons from demyelination. Together, these findings support the idea that SPG4 may involve not only the nerve fiber itself but also the myelin support around it, at least in these experimental settings. [10]

A separate biochemical study identified geranyl-pyrophosphate and farnesyl-pyrophosphate as potent inhibitors of FicD, an enzyme that regulates a protein involved in the unfolded protein response (a cell stress system). The researchers showed how one inhibitor binds in the FicD active site using a crystal structure and tested effects on disease-linked FicD variants in vitro. The authors noted that it is still unclear how relevant these molecules are inside the body, so this work is best viewed as early drug-discovery groundwork rather than an HSP-directed therapy study. [11]

What this may mean for patients and caregivers

These studies are early research and do not predict how a specific person will do, but they sharpen the biological story of HSP. They help clarify that disrupted transport inside long nerve fibers is a recurring problem in multiple HSP-related genes, and they add experimental support that SPG4 changes may also influence myelin stability in certain models. The FicD inhibitor work is even earlier and is not yet tied to HSP symptoms, but it shows how researchers identify and test possible molecular starting points for future medicines. What remains uncertain is how directly each lab finding translates into human symptoms and which changes are most important to target. [8] [9] [10] [11]