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Hereditary proximal spinal muscular atrophy (SMA) is caused by survival motor neuron (SMN) protein deficiency due to homozygous loss of SMN1 gene function. Residual SMN protein levels are produced by the SMN2 gene and SMN protein is expressed ubiquitously. Its deficiency causes alpha motor neuron loss.1
Observations in animal models suggest other tissues, for example heart, peripheral vascular system, liver and pancreas, may also require SMN protein above specific threshold levels. This is further supported by case reports of severely affected patients but it remains to be established whether multisystem pathology is actually part of the patient phenotype.2 3 This is even more relevant now that SMN-augmenting therapies have become available.
We here report a systematic characterisation of coagulation in patients with SMA, which we studied for several reasons. First, we observed abnormal coagulation screening results frequently during the course of clinical trials and nusinersen treatment. Second, local thrombotic small vessel occlusions and peripheral vascular dysfunction have been suggested to play a role in causing some of the observed non-neuromuscular pathology. Third, relatively high levels of SMN protein are found in platelets.2
Here, we find activated partial thromboplastin time (APTT) to be significantly and consistently prolonged in patients with SMA. Prothrombin time, platelet count, von Willebrand Factor (vWF) antigen and activity also differ significantly from reference values. These findings represent a common functional defect outside the nervous system in patients with SMA.
We enrolled patients with SMA types 1–4 and analysed blood samples to evaluate coagulation parameters. Primary analyses were all conducted in treatment-naïve patients, secondary analyses in a small group of patients treated with nusinersen. A full description of all methods is available in the online supplementary file.
We obtained data from 98 patients (table 1). Median age was 7.4 years (IQR: 3.8–32.8).
Analyses of plasma samples from treatment-naïve patients showed that APTT was significantly prolonged (p=1.45×10–14). Additionally, prothrombin time (PT) and platelet count were also significantly elevated (p=2.4×10–7 and p=0.014, respectively) and vWF antigen and vWF activity were significantly decreased (p=0.03 and p=0.008, respectively) although generally just within normal reference ranges. We found no significant differences for coagulation factors II, V, VII, VIII and X (table 1, online supplementary figure S1). We also found no correlation between SMN2 copies or clinical SMA type and coagulation parameters, except for vWF activity: lower vWF activity was associated with increased disease severity (Jonckheere-Terpstra test [JT]=781, p=0.017). However, this likely reflects age differences in our cohort, as vWF activity levels are known to increase with age and more severely affected patients in our cohort were younger (table 1).
Secondary analyses to evaluate APTT prolongation
We further investigated possible causes of the identified prolonged APTT by means of APTT 1:1 mixing studies in 17 patients who were treated with nusinersen. All had a prolonged APTT prior to nusinersen use (SMA types 1b–3a, median age: 4.3 years, IQR: 3.2–7.0).
In 4/17 patients (24%), APTT normalised in mixing studies, indicating a coagulation factor deficiency. A factor XII deficiency was present in two of these four patients while we found no cause in the remaining two (table 1).
In the other 13/17 patients (76%), APTT did not normalise in mixing studies, indicating an inhibitor interfering with one or more coagulation factors or a lupus anticoagulant (LAC). LAC was not found in any of the patients tested. By contrast, a factor VIII (n=1) or XII (n=1) deficiency was present in two patients. We found no cause in the remaining 11 of 13 patients (table 1).
Here, we have shown that abnormally prolonged APTT is common in patients with SMA, irrespective of age and disease severity, and that PT, platelet count and vWF also differ significantly from reference values. APTT prolongation was explained by deficiencies of coagulation factors VIII and XII in only four cases but remained unexplained in the majority of patients. This may point towards the presence of other, non-detectable inhibitors or a multifactorial aetiology. Our findings illustrate the presence of multisystem involvement in patients with SMA.
Abnormalities outside the peripheral nervous system have been suspected in patients with SMA, following descriptions in small case series and observations in animal models.2 3 By contrast, our findings are based on data from a large, treatment-naïve patient cohort and indicate consistent abnormalities across all SMA types and ages.
Multiple causes may underlie the most prominent finding in our cohort, that is, prolonged APTT. In a minority of patients, we found specific coagulation factor deficiencies. Possibly, SMN deficiency interfered with production or release of these factors. To some extent, this is supported by the observation that plasma vWF levels normalised after 1-deamino-8-D-arginine vasopressin treatment in five patients prior to planned invasive procedures (data not shown). However, we cannot exclude that the deficiencies are merely coincidental. We believe the aetiology of APTT prolongation is likely diverse, with multiple contributing effects secondary to SMN deficiency.
As recently introduced therapies and some experimental treatment strategies are delivered intrathecally, SMN restoration may be confined to the central nervous system. Systemically administered drugs are currently also in various stages of clinical development,4 for which measurements of coagulation may serve as a biomarker. Longitudinal data of treated patients are required to establish its biomarker value.
The current standards of care do not contain recommendations regarding tests of (abnormal) coagulation.5 Based on our results, we propose to measure APTT, PT and platelet count in patients with SMA at least once, particularly prior to invasive procedures. Findings outside reference ranges warrant further haematological follow-up. If initial tests are normal, repeated measurements or routine screening of coagulation parameters seem unnecessary for treatment-naïve patients. A lack of longitudinal reference data, however, precludes specific recommendations for patients receiving nusinersen or other experimental treatments. In this context, it is worth mentioning that, so far, we have not encountered clinically relevant haemorrhagic events following lumbar punctures in patients with SMA with prolonged APTT.
We thank all patients with SMA who have been participating in our ongoing study and the Dutch organization for Neuromuscular Diseases (Spierziekten Nederland) for their continuing support of our research. The authors wish to thank Fay-Lynn Asselman, BSc and SMA Research Coordinator, Louise AM Otto, MD and Féline Scheijmans, MD, all from the Department of Neurology at the University Medical Center Utrecht, for their assistance during the course of this study and their helpful comments. We also thank Ruben PA van Eijk, MD, from the department of Biostatistics & Research Support at the Julius Center for Health Sciences and Primary Care (University Medical Center Utrecht), for his statistical advice.
Contributors CAW, AH, RW, REGS and W-LvdP were involved in the study concept and design. CAW, AH, RIW, IC, MS, KMJH-P, EJNG, REGS and W-LvdP were involved in acquisition, analysis and interpretation of data. CAW, AH, EJNG and W-LvdP were involved in drafting a significant portion of the manuscript and figures. All authors were involved in revising the manuscript for intellectual content and have approved its final version.
Funding Our work was financially supported by the Prinses Beatrix Spierfonds (WAR 14-26) and stichting Spieren voor Spieren.
Competing interests The competing interests statement reads 'nothing to report'. This option was chosen as none of the conflicts are relevant financial activities/relevant interests to the submitted work. Full details are provided in the ICMJE forms that were uploaded to the journal.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
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