Spinal Cerebrotendinous Xanthomatosis: An Easily Overlooked Treatable Disorder

Background: Classic cerebrotendinous xanthomatosis (CTX; OMIM #213700) manifests itself in childhood with chronic diarrhea, juvenile cataracts, tendon xanthomas and neurological symptoms. Biallelic inactivation of CYP27A1 is responsible for cholesterol 27-hydroxylation, leading to cholestanol accumulation in the central nervous system, eyes and tendons. Less commonly, the disease can present in young adults as spastic paraparesis in the absence of xanthomas. Methods: We report on a 28-year old woman diagnosed with CTX who worsened, under treatment, a spinal form of CTX. A review of spinal CTX in the literature is also described. Spinal CTX patients were identied by searching in Pubmed, EMBASE™ and Web of Science databases. Only patients with clinical features of spinal CTX and/or with a typical spinal MRI were included. A woman presented with chronic diarrhea and progressive spastic paraparesis in her twenties. Brain magnetic resonance imaging (MRI) showed cerebral atrophy with diffuse periventricular white matter hyperintensities. Spinal MRI was normal. CYP27A1 gene sequencing conrmed the diagnosis of CTX. Chenodeoxycholic acid (CDCA) treatment was introduced with remission of diarrhea. Treatment was discontinued several times and patient developed psychosis and an ataxospastic gait. Spinal MRI revealed new linear hyperintensities of the corticospinal and gracile tracts, compatible with spinal CTX. Thirty-three patients with spinal CTX were identied in the literature. All patients presented pyramidal signs and 48% dorsal column signs. Juvenile cataracts were described in 78% of patients, chronic diarrhea in 65%, and tendon xanthomas in 31%. Disease improvement or stabilization with chenodeoxycholic acid was observed in 69% of patients. A higher prevalence of the Arg395Cys allele was observed in patients with spinal CTX as compared to CTX in general ( (cid:0) 2 ; p<0.00001). tendinous xanthomas. two monoallelic pathogenic variants in the gene CYP27A1: a missense variant in and splicing variant (CDCA) with botulinum toxin leg injection Biochemical parameters were reported in 23 cases. All patients showed high plasma cholestanol levels (median levels of 63 µmol/L; IQR 30–89 µmol/L; N: 2– 12 µmol/L). Cerebral MRI was performed in 31 patients and was normal in only 7 patients (25%). The most reported features observed in brain MRI of spinal CTX patients were cerebellar white matter lesions (n = 16; 48%), hypersignal of dentate nuclei (n = 9; 27%) and cerebral atrophy (n = 10; 30%). A spinal cord MRI was carried out

cramps and urinary frequency. She did not take any treatment and had normal schooling. Family history was not relevant neither for walking di culties nor for cataracts. Neurological examination revealed spastic paraparesis with pyramidal signs more prominent on the left lower extremity without sensory impairment, and at feet. A neurocognitive study demonstrated severe anterograde verbal memory di culties and minor executive dysfunction. Electroneuromyography (ENMG) was normal. Brain MRI showed subtle symmetric, bilateral hyperintense T2 ground-glass appearance of the deep white matter, possible symmetric T2 hyperintensity of the cerebellar dentate nuclei and two small ischemic infarct sequelae of the right cerebellar hemisphere (Fig. 2). Medullary MRI was normal. 7 years later, due to progression of spastic paraparesis, CTX was considered. Cholestanol was increased to 64 µmol/L (3.3-12.5 µmol/L). A careful assessment found no tendinous xanthomas. Genetic analysis con rmed two monoallelic pathogenic variants in the gene CYP27A1: a missense variant (c.1183C > T; p.Arg395Cys) in exon 6 and a splicing variant (c.1184 + 1G > A; p.(?)) in a splice donor site in intron 6. Treatment with 750 mg/d chenodeoxycholic acid (CDCA) and 20 mg/d simvastatine were started together with periodic botulinum toxin leg injection to relieve spasticity. Three months later, diarrhea disappeared and no adverse effect was observed. One-year follow-up showed no progression of neurologic and radiologic signs.
As a result of CDCA withdrawal from the market one year after initiating treatment (end 2016), the patient was left untreated for a period of 16 months. Over this period, diarrhea recurred and walking worsened. She had increased stiffness with muscle pain and new apallesthesia of the lower limbs with onset of cerebellar ataxia. When CDCA treatment was reintroduced in april 2018, walking, pain and diarrhea improved. Due to renewed product shortage and patient non-compliance, CDCA treatment was repeatedly discontinued, following which the patient developed an acute psychosis and, subsequently, rapid worsening of her gait. She became rollator dependent. Neurologic evaluation showed a severe ataxospastic gait with knee recurvatum (additional les: video). ENMG was normal. Brain MRI was unchanged compared to an MRI performed 11 years before (2008) except for appearance of slight supratentorial brain atrophy, which could still be normal for age. The deep white matter abnormalities remained very subtle and the signal abnormality of the dentate nuclei was unchanged. Spinal MRI revealed extensive linear T2 weighted hyperintensities appearing bilaterally in lateral corticospinal and gracile tracts (Fig. 2). Despite normal cholestanol levels (6.18 µmol/L; normal range 3.3-12.5 µmol/L), CDCA posology was increased to 250 mg t.i.d. Six months later, the patient had resolution of diarrhea and psychiatric symptoms but no improvement of gait.
Literature review and analysis on spinal form of CTX We searched Pubmed, EMBASE™ and Web of Science databases using "spinal and xanthomatosis", "spinal and xanthoma" "spinal and cerebrotendinous xanthomatosis", "medullar and xanthomatosis", "medullar and xanthoma" and "medullar and cerebrotendinous xanthomatosis" as keywords. Patients with isolated spinal xanthomas or without biochemically and/or molecularly con rmed diagnosis of cerebrotendinous xanthomatosis were excluded. Only patients with clinical features of spinal CTX and /or with a typical spinal MRI were further evaluated. Additional studies of interest were identi ed by hand searches of bibliographies. Full text articles in English, French or Spanish with abstract in English were included. Four abstracts were included. The search was last updated on 28th April, 2020. When needed, cholestanol units were converted into µmol/L. In order to avoid confusion, nucleotide and amino acid numbering are in both new nomenclature [10] and old nomenclature, in bracket [11]. Descriptive statistical analysis was performed with SPSS 25. Results are presented, including patient number (n) and frequency (%), median and interquartile range (IQR Q25-Q75). Group comparison of categorical variables were performed using the Chi-squared test. Signi cance was set at p < 0.05.

Results
Database searches in Pubmed, Embase™ and Web of science identi ed 167 potential articles, 42 of them were duplicates. 35 articles underwent full text review and 12 articles and four abstracts met our inclusion criteria ( Fig. 3): 30 patients were included. Hand search bibliography identi ed 3 cases reports (total 33 patients).
The main ndings are summarized in Table 1. Fourteen patients were females and 11 males (n = 25; sex data was not available for 8 patients) with a median age of 36 years (IQR 30-46 yrs.) (n = 32). Median age to onset of neurological symptoms was 24 years (IQR 12-30 yrs.) (n = 31). All patients presented with a pyramidal syndrome associated in 16 patients (48%) with dorsal columns signs. Explicit mention of presence or absence of the three cardinal signs of CTX (cataracts, chronic diarrhea and xanthomas) were available for 27, 20 and 32 patients, respectively. Twenty-one patients were reported with cataracts (78%), 13 patients with diarrhea (65%) and 10 patients with xanthomas (31%). Only 2 patients (6%; n = 33) presented with the classic triad of CTX signs. Psychiatric symptoms were reported in 11 patients (33%), cerebellar signs in 10 patients (30%), seizures in 6 patients (18%), polyneuropathy in 6 patients (18%), intellectual de ciency in 4 patients (12%), dementia or cognitive decline in 6 patients (18%), dysarthria in 4 patients (12%), urinary troubles and dysphagia in 2 patients (6%). In untreated patients the disease progressed slowly, though in 2 cases the disease was more aggressive leading to wheelchair dependent patients at the age of 30 years and 35 years, respectively.   Results of genetic analysis were available for 23 patients. Allele frequencies are represented in Fig. 4. The two most frequent CYP27A1 pathogenic variants observed in spinal CTX patients were Arg395Cys and Thr339Met, with an allele frequency of 17/ 46 (36%) and 8/46 (17%), respectively. Figure 3 shows different CYP27A1 allele frequencies in the spinal CTX patients we reviewed compared with the cohort of 78 CTX patients described by Verrips in 2000 [12].

Discussion
Since the description of CTX in 1937 [14], more than 300 patients have been described worldwide. The classic form of CTX is the most frequent phenotype. A small proportion of patients develop the spinal form in which spastic paraparesis is the main clinical symptom and possibly the sole expression of the disease for many years. Because the presentation is not speci c, spinal CTX patients are often initially misdiagnosed, especially when tendon xanthomas are not present or remain unnoticed. Among the 33 identi ed cases of spinal CTX, 69% did not have xanthomas, which possibly explains a diagnostic delay of 10 or more years (as in our case). The "CTX suspicion index" developed by Mignarri et al [15] emphasizes the importance of cataracts and diarrhea (rather than xanthomas): these signs were present in the majority of spinal cases we reviewed, including our case.
The rst neurological signs in spinal CTX patients was spastic paraparesis with stiffness, hyperre exia and positive Babinsky signs, associated with proprioceptive symptoms in approximately half of the cases. About one third of spinal CTX patients developed psychiatric disturbances. However, most of the patient did not present developmental delay nor intellectual de ciency, which are frequent in the juvenile form of CTX.
Early treatment with CDCA had positive impact on disease evolution and symptoms including bowel function, spasticity, psychiatric disease and/or cognitive function. Although in a minority of cases, CDCA was able to improve the spastic paraparesis, in the majority of cases CDCA seemed to have a limited impact on the spinal cord syndrome. Our patient showed improvement of her chronic diarrhea and stabilization of neurological symptoms under CDCA treatment. However, after treatment was discontinued she rapidly developed psychiatric symptoms and worsening of gait (spastic paraparesis and gait disturbance).
Noteworthy, Luyckx et al described 2 brothers with stable CTX, treated with CDCA and statins during 11 years, who developed pyramidal signs and speech disturbances when CDCA treatment was discontinued because of product withdrawal [16]. This raised the hypothesis that abrupt interruption of CDCA could lead to a rebound of cholesterol biosynthesis with accumulation of toxic bile metabolites, which increases permeability of the blood brain barrier leading to brain and spinal accumulation of cholestanol [17,18]. We raise the hypothesis of an existing compensatory mechanism (such as feedback inhibition) that might become deactivated during CDCA treatment, leading to an increased vulnerability of the central nervous system to toxic metabolites. Whatever the mechanism, continuity of treatment with CDCA appears to be an important target to avoid rebound effect and disease progression in CTX patients.
Imaging ndings on brain MRI are non-speci c and include periventricular white matter, basal ganglia, cerebral peduncles and dentate nuclei signal abnormalities and brain atrophy [6,19]. In the present patient, brain MRI revealed questionable periventricular white matter "ground-glass" T2 hyperintensity and T2 hyperintensity of the dentate nuclei. These signal abnormalities were stable over a period of 11 years, during which a slight brain atrophy developed that could, however, correspond to normal ageing of the brain.
More characteristic for spinal CTX are signal abnormalities such as those observed in our patient involving the lateral cortico-spinal and gracilis tracts in the spinal cord [6,20].
To date, there are 70 con rmed pathogenic variants causing CTX and 39 likely pathogenic variants in CYP27A1 [21]. No clear genotype-phenotype correlation has been observed. However, the p.Arg395Cys allele seems to be signi cantly more frequent in spinal CTX patients than in classic CTX. The p.Arg395Cys substitution affects a highly conserved sequence of the adrenodoxin binding site and was shown to strongly reduce CYP27A1 enzyme activity [22].
Tridimensional protein modeling showed that Arg395 is located within the ERR triad (the glutamine-arginine-arginine motif conserved in all cytochrome P450 sequence) and its substitution to cysteine was suggested to favour misfolding and possibly affects adrenodoxin binding [23].
In summary, our review of the literature highlighted features of spinal CTX (as opposed to classic CTX) such as later age at presentation (early adulthood vs pediatric age), absence of xanthomas in two-thirds of patients and absence of development delay and intellectual disability in most patients. Unfortunately, absence of xanthomas may be the cause of a signi cant delay in making the diagnosis and installing a disease-speci c treatment with CDCA. We suggest that systematic use of the clinical "suspicion index " proposed by Mignarri et al. might be helpful in recognizing CTX without xanthomas [15]. Spinal MRI studies seem to be useful as they may demonstrate signal abnormalities of the spinal cord suggestive of CTX and should be offered routinely. The CYP27A1 p.Arg395Cys allele is signi cantly more frequent in spinal CTX patients than in classic CTX patients; the mechanistic basis of this association remains to be ascertained. The dramatic rebound effect seen after treatment interruption, possibly related to the complex feedback inhibition between CDCA and CYP7A1, highlights the importance of not interrupting CDCA treatment. More generally, this case also illustrates the fragility of relying on orphan drugs for which the supply may not be guaranteed. Ethical review and approval was not required for the study on human participants in accordance with the local legislation and institutional requirements.

Consent for publication
The patient provided its written consent to participate in this publication.

Availability of data and supporting materials section
The authors con rm that the data supporting the ndings of this study are available within the article and its supplementary materials. weighted images (a-d) at the level of the dentate nuclei and periventricular white matter showing stable minimal increased signal in the dentate nuclei (arrowheads) and questionable slightly abnormal periventricular white matter T2 hyperintensity ("ground-glass appearance"). The rest of brain MRI was unremarkable except for two small old infarcts in the right cerebellar hemisphere (one lesion shown in a and b*). Spinal cord MRI from 2008 (e) was unremarkable, though no axial plane images were performed. Spinal cord MRI in 2019 revealed subtle longitudinal high signal (white arrow-heads) of the posterior columns at the cervico-dorsal junction and middle dorsal region on sagittal T2-weighted images (f). Axial T2-weighted images con rmed bilateral, symmetric signal abnormalities corresponding to the gracilis tracts (g, black arrowhead) and the lateral cortico-spinal tracts (g, white arrows) at different cervical and dorsal levels, without spinal cord atrophy or contrast material (gadolinium) uptake. Abbreviations: C: cervical