Plastin 3 is upregulated in iPSC-derived motoneurons from asymptomatic SMN1-deleted individuals

Abstract

Spinal muscular atrophy (SMA) is a devastating motoneuron (MN) disorder caused by homozygous loss of SMN1. Rarely, SMN1-deleted individuals are fully asymptomatic despite carrying identical SMN2 copies as their SMA III-affected siblings suggesting protection by genetic modifiers other than SMN2. High plastin 3 (PLS3) expression has previously been found in lymphoblastoid cells but not in fibroblasts of asymptomatic compared to symptomatic siblings. To find out whether PLS3 is also upregulated in MNs of asymptomatic individuals and thus a convincing SMA protective modifier, we generated induced pluripotent stem cells (iPSCs) from fibroblasts of three asymptomatic and three SMA III-affected siblings from two families and compared these to iPSCs from a SMA I patient and control individuals. MNs were differentiated from iPSC-derived small molecule neural precursor cells (smNPCs). All four genotype classes showed similar capacity to differentiate into MNs at day 8. However, SMA I-derived MN survival was significantly decreased while SMA III-and asymptomatic-derived MN survival was moderately reduced compared to controls at day 27. SMN expression levels and concomitant gem numbers broadly matched SMN2 copy number distribution; SMA I presented the lowest levels, whereas SMA III and asymptomatic showed similar levels. In contrast, PLS3 was significantly upregulated in mixed MN cultures from asymptomatic individuals pinpointing a tissue-specific regulation. Evidence for strong PLS3 accumulation in shaft and rim of growth cones in MN cultures from asymptomatic individuals implies an important role in neuromuscular synapse formation and maintenance. These findings provide strong evidence that PLS3 is a genuine SMA protective modifier.

PLS3 Deletions Lead to Severe Spinal Osteoporosis and Disturbed Bone Matrix Mineralization

“…A waddling gait was previously reported in 2 subjects with PLS3 osteoporosis, suggesting that myopathy may occasionally be part of the phenotypic presentation. Moreover, recent observations of Plastin 3 being a protective modifier in spinal muscular atrophy and having a role in neuromuscular synapse maintenance could support such a connection . Nishi and colleagues reported facial dysmorphism in two boys with suspected damaging variants in PLS3 , but the features (hypertelorism, upslanted palpebral fissures, and broad nasal tip) were different from those in our patients and the contribution of aberrant PLS3 therefore remains unknown …”

Infants Diagnosed with Spinal Muscular Atrophy and 4 SMN2 Copies through Newborn Screening – Opportunity or Burden?1

“…Whereas most patients with 4 and more copies of SMN2 develop either type 2 or type 3 SMA, there are 32 reported individuals belonging to 22 families in the literature who show no signs despite a homozygous deletion in the SMN1 gene [32][33][34][35][36][37][38]. Most of these patients have 3-5 SMN2 copies and other known but separate SMA modifiers as well.…”

Effect of genetic background on the phenotype of theSmn2B/-mouse model of spinal muscular atrophy

“…A similar increase in FVB Smn 2B/- mice was not observed. PLS3 has been proposed to be a genetic modifier of SMA in both human and animal studies (14,34,35). In SMA patients, higher levels of Plastin 3 correlated with less severe SMA types (36).…”