More autosomal dominant SPG18 cases than recessive? The first AD‐SPG18 pedigree in Chinese and literature review

Abstract Objective Hereditary spastic paraplegia (HSP) due to ERLIN2 gene mutations was designated as spastic paraplegia 18 (SPG18). To date, SPG18 families/cases are still rarely reported. All early reported cases shared the autosomal recessive (AR) inheritance pattern. Over the past 3 years, autosomal dominant (AD) or sporadic SPG18 cases had been continuously reported. Here, we reported the clinical and genetic features of the first autosomal dominant SPG18 pedigree in Chinese. Methods We conducted detailed medical history inquiry, neurological examinations of the proband and his family members, and charted the family tree. The proband underwent brain and cervical magnetic resonance imaging (MRI), electromyography (EMG), and whole exome sequencing. Sanger sequencing was performed to verify the genetic variation in the proband and some family members. A literature review of all reported SPG18 families/cases was carried out to summarize the clinical‐genetic characteristics of SPG18 under different inheritance patterns. Results Four patients were clinically diagnosed as chronic spastic paraplegia in three consecutive generations with the autosomal dominant inheritance model. All the patients presented juvenile‐adolescent onset and gradually worsening pure HSP phenotype. Clinical phenotypes were consistent within the family. Whole exome sequencing in the proband identified a previously reported heterozygous c.502G > A (p.V168M) mutation in exon 8 of ERLIN2 gene. This mutation was cosegregated with the phenotype in the family and was classified as likely pathogenic according to American College of Medical Genetics and Genomics (ACMG) guidelines. To date, eight AR‐SPG18 families, five AD‐SPG18 families, and three sporadic cases had been reported. Clinical phenotype of AD‐SPG18 was juvenile‐adolescent onset pure HSP, while the phenotype of AR‐SPG18 was mostly complicated HSP with earlier onset and more severe conditions. In rare cases, the initial spastic paraplegia could evolve to rapidly progressive amyotrophic lateral sclerosis (ALS). Conclusions We reported the first autosomal dominant SPG18 pedigree in Chinese Han population, which added more pathogenic evidence for V168M mutation. As more SPG18 cases reported, the essentials of SPG18 need to be updated in clinical practice. Special attentions should be given in gene test for upper motor neuron disorders in case of missing heterozygous mutations in ERLIN2.

SPG18 cases reported, the essentials of SPG18 need to be updated in clinical practice.
Special attentions should be given in gene test for upper motor neuron disorders in case of missing heterozygous mutations in ERLIN2.

ERLIN2, gene, hereditary spastic paraplegia, SPG18 INTRODUCTION
Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative upper motor neuron disorders that are clinically and genetically heterogeneous. The main clinical manifestations are gait abnormalities and spasticity in the lower extremities. Neurological examinations often revealed upper motor neuron signs including increased muscle tone, hyperreflexia, and pathologic reflexes. Clinically, HSP can be divided into the pure form and complicated form according to additional neurological and extraneurological signs. By modes of inheritance, HSP can be classified as autosomal dominant (AD), autosomal recessive (AR), X-linked recessive (XLR), and mitochondrial maternal HSP. To date, more than 80 HSP gene loci have been reported. Generally, AD-HSP is the most common type which mainly presented with pure HSP, while AR-HSP mostly present with the complicated form of HSP. Interestingly, few HSPs can be inherited in both dominant and recessive patterns, including SPG3A, SPG7, SPG9, SPG18, SPG30, and SPG72 (Boutry et al., 2019). According to OMIM and a recent review, 19 of the 83 SPG loci presented with the AD inheritance pattern, 54 loci with the AR inheritance pattern, four loci with AD/AR inheritance pattern, and six loci with the XLR inheritance pattern (Saputra & Kumar, 2021).
The causative gene for SPG18 is endoplasmic reticulum lipid raft associated protein 2 gene (ERLIN2). ERLIN2 was initially mapped in an autosomal recessive Turkish family with intellectual disability, motor impairment, and multiple joint contractures in 2011 (Yildirim et al., 2011). At the same time, a deletion mutation of ERLIN2 was also identified in an autosomal recessive family with the complicated form of HSP in Saudi Arabia, which was designated as SPG18 (Alazami et al., 2011).
Early reported families were all recessively inherited, complicated HSP (Alazami et al., 2011;Wakil et al., 2013;Yildirim et al., 2011). In 2018, two AD-HSP families caused by heterozygous ERLIN2 missense mutations were first reported in Norwegian populations (Rydning et al., 2018). So far, SPG18 families/cases reported were still limited. With more SPG18 cases discovered, clinical phenotypes also expanded. In addition to HSP, the HSP-ALS conversion phenotype and primary lateral sclerosis phenotype were also seen (Al-Saif et al., 2012;Amador et al., 2019).
Here, we reported the clinical and genetic features of the first autosomal dominant SPG18 pedigree in Chinese. We also summarized all the reported SPG18/ERLIN2 families/cases to analyze the clinical and genetic features of SPG18 under different inheritance patterns.

Participants
The proband was admitted to the People's Hospital of Zhengzhou University in 2018. We conducted detailed history inquiry and neurological examinations in the proband and some family members and drew the pedigree tree ( Figure 1a). Genetic testing was performed in the proband and some family members. The study was approved by the ethics committee of People's Hospital of Zhengzhou University. Written informed consent was obtained from all study participants.

Methods
The proband underwent blood tests for blood routine, serum liver and kidney function, lipids, vitamin B12 and folate levels, vitamin E, serum copper, ceruloplasmin, ammonia, lactate, and cortisol levels.
Brain and cervical-thoracic spinal magnetic resonance imaging (MRI) scans were performed at a 3.0T magnetic resonance scanner (Siemens Skyra 3.0T). Nerve conduction velocity (NCV), needle electromyography, and somatosensory evoked potentials (SEP) were examined to detect lower motor neuron damages.

Clinical presentation
The proband (III3) was a 30-year-old male farmer. He was admitted for the progressive stiffness and weakness of both lower limbs over the past 20 years. At childhood, he was noted with lower limbs weakness and abnormal gait. These symptoms gradually worsened. When he was 20, he could not run or walk fast due to leg stiffness. He was unable to flex his lower limbs freely, but was still able to walk inde-

Genetic findings
All As there is no available protein structure of ERLIN2, full-length amino acid sequences of erlin2 were used to predict the tertiary structure by online sever I-TASSER (Yang et al., 2015). Multiple alignment templates in I-TASSER contain the SPFH domain, but with identity less than 20%. Compared with the wild type, the tertiary structure of the mutant protein does not change largely. In wild-type ERLIN2, the V168 and neighboring R169 residues form the hydrogen bond. But in the mutant protein, M168 and K149 residue from adjacent α-helix form an alternative hydrogen bond. (Figure 2).
eight AR-SPG18 families, six families had the disease onset before 2 years of age, and the other two families were adolescent onset. Clinical phenotypes of AR-SPG18 were dominated by complicated HSP (7/8).
Only one Chinese family with the compound heterozygous mutations presented as pure HSP. Other superimposed manifestations include limb deformities, language and intellectual impairment, epilepsy, lower motor neuron signs, and deafness.
Among the five AD-SPG18 families, age at onset ranged from 8 to 48 years, which was older than that in AR-SPG18 families. The clinical phenotypes in the five families were mainly pure HSP with long survival, and the mutations were all heterozygous mutations. In rare cases, the initial spastic paraplegia could evolve to rapidly progressive amyotrophic lateral sclerosis (ALS) (Amador et al., 2019).
Clinical phenotypes of the three sporadic cases were also pure HSP.
Two cases had the heterozygous mutations. Another case had the homozygous missense mutation due to uniparental disomy and presented an earlier onset at 2 years of age (Srivastava et al., 2020).

DISCUSSION
Since 2011, HSP families/cases caused by ERLIN2 gene mutations have been occasionally reported. The initial cases were basically complicated AR-HSPs. In 2018, pure AD-HSP families were first reported in the Norwegian population (Rydning et al., 2018). In the present study,  The physiological function of ERLIN2 protein is not fully understood yet. It forms an erlin1/2 complex with the homologous erlin1 protein.
One well-defined function of this complex is to recruit RNF170, a ubiquitin ligase (E3), to the activated inositol 1,4,5-trisphosphate receptor (IP3R), then ubiquitylate and degrade them (Browman et al., 2006;Lu et al., 2011;Wright et al., 2015). Interestingly, mutations in the ERLIN1 and RNF170 gene could also cause HSP (Wagner et al., 2019). Three AR-SPG62/ERLIN1 families were reported presenting with the pure HSP with an onset age of 1-13 years old. Another SPG62/ERLIN1 family had the phenotype of juvenile onset, recessively inherited HSP. But the phenotype converted to slowly progressive ALS at 50-60 years of age (Tunca et al., 2018). Similarly, rare SPG18/ERLIN2 cases also presented the HSP-ALS phenoconversion (Novarino et al., 2014). RNF170 gene mutations lead to AR-HSPs. The clinical features of the four reported RNF170-HSP families were infantile onset, complicated HSP with varying degrees of optic atrophy and cerebellar ataxia (Novarino et al., 2014). Consistently, inheritance patterns and clinical presentations of ERLIN1, ERLIN2, and RNF170-related diseases were all heterogeneous.
In conclusion, we reported the first autosomal dominant SPG18 pedigree in Chinese Han population, which added more pathogenic evidence for V168M mutation. The clinical phenotypes of SPG18 are expanded and are highly heterogeneous under different inheritance patterns. As more cases reported, the essentials of SPG18 need to be updated in clinical practice and genetic testing. Special attention should be given in genetic testing of upper motor neuron disorders in case of missing heterozygous mutations in ERLIN2.

DATA AVAILABILITY STATEMENT
The data are available via contacting corresponding author, Shu-jian Li.