Expanding the Phenotype Spectrum of β-Mannosidosis

Abstract

Background and Objectives

β-mannosidosis is an ultra-rare lysosomal storage disorder caused by a deficiency of β-mannosidase, which catalyzes the last step of glycoprotein degradation. Owing to the limited number of reported cases, information on the natural history of the disease and brain imaging is scarce. We report 6 new cases and review them together with all the previously reported cases in the literature.

Methods

We describe the clinical features of 6 unrelated patients with β-mannosidosis, their variants in MANBA, enzyme activity, urine oligosaccharide profiles, brain MRI findings, and longitudinal MRI changes in 2 of them. We also review previously reported patients to broaden the spectrum of clinical features and identify genotype-phenotype correlations.

Results

Developmental regression, dysphagia, obsessive-compulsive–like behavior, and erythromelalgia are newly described features associated with β-mannosidosis among the 6 patients from our cohort. Nystagmus in association with β-mannosidosis was also found in 1 patient. Half the patients have abnormal brain MRIs, showing delayed myelination before 2 years of age and diffuse hypomyelination thereafter. A new oligosaccharide was found in the urine of the 2 most severely affected patients. Including our 6 patients, a total of 46 cases from 37 different families have been reported. The mean age of diagnosis is 12.8 years, and the mean age of symptom onset is 2.4 years. Hearing loss was the initial symptom most frequently reported, and intellectual disability was the most frequent symptom overall. Across the cohort, 29 pathogenic MANBA variants were identified, 61.8% were private, and 38.2% have the recurrent c.2158-2A>G variant. Neuroimaging abnormalities were reported in 40% of published cases and included cortical and subcortical atrophy, basal ganglia and white matter calcification, hydrocephalus, periventricular and subcortical white matter hyperintensities, and delayed myelination.

Discussion

Genotype-phenotype correlation in β-mannosidosis remains elusive, likely due to phenotypic variability, disease rarity, and lack of association between the enzyme activity and the clinical severity. Modifier genes in the N-glycan degradation pathway, such as CTBS, may influence disease expression. β-mannosidosis should be considered as a differential diagnosis in patients with syndromic or apparently nonsyndromic hearing loss, unexplained brain hypomyelination, behavioral disturbances, and intellectual disability.

Introduction

β-mannosidosis (OMIM #248510) is an ultra-rare autosomal recessive lysosomal storage disorder caused by pathogenic variants in MANBA, resulting in a deficiency of lysosomal β-mannosidase activity, which catalyzes the last step of glycoprotein degradation.¹ Along with α-mannosidosis (OMIM #248500), these diseases are referred to as oligosaccharidosis because they are suspected when oligosaccharides are detected in urine. While α-mannosidosis—characterized by intellectual disability, frequent infections, immunodeficiency, hearing loss, coarse facies, and skeletal abnormalities—has been well described,^2,3 β-mannosidosis remains exceedingly rare, with an unknown incidence. Reported prevalence rates are 0.13, 0.12, and 0.16 per 100,000 live births in the Netherlands,⁴ Portugal,⁵ and the Czech Republic, respectively.⁶

First identified in 1981 in Nubian goats,⁷ β-mannosidosis has since been reported in Salers cattle,⁸ dogs,^9,10 and recently, in a domestic cat.¹¹ Human cases were first described in 1986,^1,12 and to date, 40 patients have been reported worldwide (Table 1). The rarity of β-mannosidosis and its phenotypic heterogeneity¹³—including hearing loss,¹⁴ seizures,¹⁵ angiokeratomas,¹⁶ and different levels of developmental delay¹⁷ and intellectual disability¹⁸—makes challenging to characterize the clinical spectrum and natural history of this disorder. In previously published human cases there is no mention of neuropathological studies, and many cases do not have available brain imaging.

Table 1.

Patients Affected With β-Mannosidosis Reported in the Literature to Date

n	Author and year	Sexa	MANBA variant 1/variant 2 (reference of reported variant)b	Enzyme activity (source)c	Ancestry	Age at time of publication
1	Cooper, 198612	M	c.247 G>T, p.Glu83Ter/c.1276C>T, p.Gln426Ter (Bedilu, 2002)19	0.8 (L), absent (P), 4 (F)	Indian	44 y
2		M		0.2 (L)	Indian	19 y
3	Wenger, 19861	M		Absent (L)	White, European	46 mo
4	Dorland, 1988e26	M		Absent (L, P)	Turkish	9 y
5		M		Absent (L, P)	Turkish	7 y
6	Kleijer, 199018	F	c.2158-2A>G/c.2158-2A>G (Alkhayat, 1998)20	Absent (F)	Roma, Czechoslovakia	Died at 20 years old
7		M	c.2158-2A>G/c.2158-2A>G (Alkhayat, 1998)20	0.9 (L)	Roma, Czechoslovakia	30 y
8	Wijburg, 1992e8	F	c.1454_1455del, p.Tyr485CysfsTer27/c.1454_1455del, p.Tyr485CysfsTer27 (Bedilu, 2002)19	4.7 (L), Absent (P, F)	Not reported	5 y 10 mo
9	Cooper, 199121	F		7.0 (L), 8.7 (F)	Irish, Jamaican	Died at 15 mo
10	Poenaru, 1992e1	M	c.375_378del, p.Tyr126AlafsTer11/c.1513T>C, p.Ser505Pro (Riise Stensland, 2008)22	Absent (F), 0.5 (L), 0.1 (S)	White, European	3 y 3 mo
11	Levade, 1994e27	M		3 (L), 13 (P)	African (Cameroon)	14 y
12	Rodriguez-Serna, 199623	F	c.1175 G>A, p.Gly392Glu/c.1848del (Gort, 2006)24	3 (L), 0.009 (S), 1.9 (F)	White	22 y
13	Gourrier, 1996e28	F		Absent (L), 22 (S)	Not reported	18 mo
14	Uchino, 2002e19; Suzuki, 200425	M	c.960+1 G>A/c.960+1 G>A	5.44 (P)	Japanese	51 y
15	Cherian,15 2004	F		33 (L), 18 (F). 21 (P)	Saudi Arab	2 y
16		F		91 (L), 24 (F). 121 (P)	Saudi Arab	3.5 y
17	Sedel, 200626	M	c.563_572dup, p.Trp192Ter/c.1705-1G>A	1.08 (L), 1.4 (P)	French	21 y
18	Gort, 200624	F	c.544C>T, p.Arg182Trp/c.1398 G>A, p.Trp466Ter	Absent (L, S)	Spanish	24 y
19	Molho-Pessach, 200716	F	c.693 G>A, p.Trp231Ter/c.693 G>A, p.Trp231Ter	0.2 (L). Absent (F), 1 (S)	Arab	36 y
20	Labauge, 200927	M	c.1922G>A, p.Arg641His/c.1922G>A, p.Arg641His	4 (L)	Algerian	26 y
21	Broomfield, 201328	F	c.293T>A, p.Leu98Ter/c.293T>A, p.Leu98Ter	0.2 (L)	South Asia	23 mo
22	Blomqvist, 201913	F	Chr4q24 13.1 kb inverted duplication (g.102669696_102669764ins102664572_102677720inv), homozygous	0.06 (L)	Norwegian	15 y
23	Schrauen, 2019e3	M	c.2158-2A>G/c.2158-2A>G		Roma, Hungary	Not reported
24	Safka Brozkova, 202014	M		1.65 (L)	Roma, Czech Republic	Not reported
25-26-27		M (2), F (1)	c.2158-2A>G/c.2158-2A>G		Roma, Czech Republic	Not reported
28-29-30		M (3)	c.2158-2A>G/c.2158-2A>G		Roma, Slovakia	Not reported
31		F	c.2158-2A>G/c.2158-2A>G	Absent (L)	Roma, Slovakia	26 y
32-33-34		UNK (3)	c.2158-2A>G/c.2158-2A>G		Roma, Slovakia	Not reported
35	Alshoraim, 2021e3	M			Arab	10 y
36	Gowda, 202117	M	c.1317+1 G>A/c.1317+1 G>A	Absent (S)	Indian	3.5 y
37		M	c.1317+1 G>A/c.1317+1 G>A	Absent (S)	Indian	18 mo
38	Saleh, 202129	M	c.545 G>A, p.Arg182Gln/c.545 G>A, p.Arg182Gln		Arab	4 y
39		UNK	c.545 G>A, p.Arg182Gln/c.545 G>A, p.Arg182Gln		Arab	Not reported
40	He, 2022e2	M	c.2538del, p.Met846IlefsTer27/c.280C>A, p.Gln94Lys	25.8 (L)	Chinese Han	Died at 9 y
41	Lund, 201930,d	M	c.575 G>A, p.Trp192Ter/c.1499 G>A, p.Arg500His	2.62 (L)	German/Norwegian	13 y
42	This publication	M	c.545 G>A, p.Arg182Gln/c.545 G>A, p.Arg182Gln	Absent (L)	Italian/Mexican	8 y
43		F	c.1540_1541del, p.Val514CysfsTer10/c.2352_2356del, p.Thr785LeufsTer27	Absent (L)	Scottish	13 y
44		F	c.1430A>G p.Tyr477Cys/c.1430A>G p.Tyr477Cys	Absent (L)	Italian	2 y
45		M	c.685C>T, p.Gln229Ter/c.685C>T, p.Gln229Ter	3 (S)	Indian	19 y
46		F	c.1452_1453del, p.Tyr485CysfsTer27/c.1753C>T, p.Arg585Ter	2 (P), Absent (L)	British	31 y

^aNumber in parentheses is the reported patients in the correspondent publication. UNK: unknown.

^bRefSeq NM_005908.4, GRCh38.

^cFor β-mannosidase enzyme activity in leukocytes (L) and fibroblasts (F), the units are in nmol/h/mg. For serum (S) and plasma (P), the units are in nmol/h/mL.

^dThis patient corresponds to patient B described in this study.

We describe 6 unrelated patients with confirmed diagnosis of β-mannosidosis, with varying clinical manifestations and severity, and brain MRI findings. One previously published individual (patient B) is revisited with extended clinical and imaging follow-up.^30,31 Finally, we review the reported patients to date, their clinical features, and the associated MANBA variants for genotype-phenotype correlations.

Methods

Standard Protocol Approvals, Registrations, and Patient Consents

This study was approved by the University of California Irvine Institutional Review Board (IRB #2024-3829). Written informed consent was obtained from the parents or legal guardians of all participants, including consent for the use of medical records, biochemical and genetic test results, brain MRI data, and patient photographs.

Identification of Patients With β-Mannosidosis

One patient was evaluated at University of California, Irvine. Three additional patients from the United States and 2 patients from the United Kingdom were recruited through the International Society of Mannosidosis and Related Disorders (ISMRD). Clinical, imaging, and laboratory data were extracted from medical records.

Analysis of MANBA Variants

Pathogenicity of MANBA variants was assessed using Mutation Taster³² and PolyPhen 2,³³ and their classification was based on the ACMG/AMP criteria.³⁴ The missense variants were aligned across species using Clustal Omega.³⁵ The protein sequences used for alignment are listed in the eAppendix. The variants were mapped to a schematic diagram created with IBS 2.0,³⁶ and structurally modelled using Alphafold³⁷ and UCSF Chimera X³⁸ based on the published databases (Uniprot entry access O00462 Beta mannosidase).³⁹

Brain MRI Analysis

MRI images of brain and changes over time obtained for each patient were reviewed by a pediatric radiologist (LHG).

Enzyme Activity Assay

β-mannosidase enzyme activity assay was measured at different laboratories for each patient as part of clinical care, using a fluorometric measurement of 4-methylumbelliferone derived from a β-mannosidase-specific artificial substrate catalysis.

Urine Oligosaccharide Analysis

Urine oligosaccharide profiles (UO) were performed for patients A, B, C, and D by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry, as previously described.⁴⁰

Review of Literature

A systematic search was conducted using PubMed Advanced Search Builder with the terms “beta mannosidosis,” “MANBA,” “beta mannosidase,” and “mannosidosis,” combined with “human,” “patient,” and “case report” using “AND,” and excluding “alpha” and “alfa” using “NOT.” Articles were included if they reported human cases with confirmed β-mannosidosis by enzyme assay. Cross-referencing was used to consolidate duplicate patient reports. In total, 25 references were included, and a list of reported patients was obtained (Table 1).

For each reported patient, a thorough review of listed symptoms was made and classified into present (+), absent (−), and not reported (NR) (eTable 1). Included symptoms were based on those listed in IEMbase⁴¹: developmental delay, intellectual disability, hearing loss, angiokeratoma, facial dysmorphism, skeletal dysplasia, behavioral abnormalities, organomegaly, ataxia, and seizures. Recurrent infection was included as a previously reported symptom associated with β-mannosidosis.⁴² All MANBA variants were updated to the GRCh38 assembly (RefSeq NM_005908.4) and validated accordingly.⁴³ Descriptive statistics (mean, SD, median, interquartile range, frequencies, and correlation) were computed using SPSS version 29.0.0.0. The reported variants were also displayed schematically using IBS 2.0 (Figure 1).³⁶

Figure 1. Beta Mannosidase Protein and MANBA Gene.

The red asterisks mark the variants found in patients A to F. (A) Schematic illustration of beta mannosidase protein domains and location of the reported variants: frameshift variants in orange, nonsense variants in red, and missense variants in yellow. The residues in the aqua boxes are the active sites, and the blue boxes are the binding sites of the protein. (B) Schematic illustration of the MANBA gene, and the splice site variants reported in the literature.

Data Availability

Data supporting the findings of this study are available on request.

Results

Description of the Patients

Patient A

Patient A is a 7-year-old boy with bilateral sensorineural hearing loss detected at 5 years of age (Figure 2A). Pregnancy was uneventful, and he was born by a C-section at 38 weeks gestational age due to heartbeat decelerations. Birth weight and head circumference were normal; length was 45.7 cm (first centile). Newborn screening was unremarkable. Motor milestones were normal; language was delayed. There is no history of metabolic decompensation or neurologic regression. He has learning difficulties associated with anxiety, obsessive-compulsive traits, aggressive ideation, and perseverance. He had precocious puberty noted at 4–5 years. Family history was unremarkable. Although both parents belonged to the same geographic region, consanguinity was denied. Brain MRI was normal. On examination, his height was 132 cm (83rd centile), weight was 43.1 kg (99th centile), and there were no dysmorphic features, angiokeratoma, organomegaly, or skeletal malformations. The Tanner stage was 1. The left testicle was 1–2 mL, and the right testicle was 3 mL in volume.

Figure 2. Phenotypical Features of Affected Patients With β-Mannosidosis.

Patient A does not show dysmorphic features. Patients B and C show mild facial dysmorphic features and slender fingers. Patient D has mildly coarse face, fair hair and skin, and angiokeratomas in face, trunk, and limbs. Patient E has multiple angiokeratomas in palms and dorsum of fingers. Patient F at different ages, with no dysmorphic features. Both palms and fingers have mild angiokeratomas.

A hearing loss genetic panel showed a homozygous missense variant in MANBA (c.545G>A, p.Arg182Gln, eFigure 1A) initially classified as a variant of uncertain significance (VUS). β-mannosidase enzyme activity in leukocytes was absent. In silico prediction classifies this variant as disease-causing (Mutation Taster) and probably damaging (PolyPhen 2, score 1.0). Arg182 is highly conserved and contributes 6 hydrogen bonds to the protein structure; substitution with Gln182 results in a significant loss of bonding, affecting the catalytic domain (eFigure 1A). Two patients with this same variant have been reported as affected,²⁹ and another patient was previously reported with the variant Arg182Trp.²⁴ These data supported reclassification of the variant as likely pathogenic. Urine showed excretion of mannosyl-β(1 → 4)-N-acetylglucosamine (ManGlcNAc) and sialyl-α(2 → 6)-mannosyl-β(1 → 4)-N-acetylglucosamine (NeuManGlcNAc, Figure 3).

Figure 3. Urine Oligosaccharide Profiles of Patients A to D With β-Mannosidosis.

Characteristic peaks are labeled, and theoretical structures are provided in the inset. Patient B is posttransplant, and only 1 characteristic peak (m/z = 879) is present in the profile. Patients C and D have a third peak (m/z = 1,328), corresponding to a Neu-Man2-GlcNac2 structure.

Patient B

This male patient received an HLA-matched umbilical cord blood transplant at 4.5 years of age, and the medical history from birth to the umbilical cord blood transplant has been previously described.³⁰ (Figure 2B). In this study, we describe the progression of the disease after his transplantation until present time. The diagnosis on this patient was confirmed through an enzyme activity panel for lysosomal storage disorders that showed low β-mannosidase activity (2.62 nmol/mg/h, normal range 10–162.4). MANBA sequencing showed 2 heterozygous variants in trans: a likely pathogenic variant (c.575G>A, p.Trp192Ter) paternally inherited and a VUS (c.1499G>A, p.Arg500His) maternally inherited that was predicted as deleterious by Mutation Taster and PolyPhen2. Arg500 residue is highly conserved. Although Arg and His are positively charged, His500 disrupts the catalytic domain of the protein because of its imidazole group (eFigure 1B).

After the transplantation, the β-mannosidase enzyme activity levels normalized in leukocytes, and NeuManGlcNAc remained detectable in urine (Figure 3). Four weeks after transplantation, elliptical nystagmus in both eyes, loss of independent feeding, worsening ataxia, and increased anxiety were noticed. These symptoms stabilized 2 years after transplantation, with residual intention tremor, leg spasticity, and gait imbalance.

At 10 years of age, he developed progressive regression in gross motor skills, urinary incontinence, low muscle bulk, axial hypotonia, end point dysmetria, and dysarthria, accentuated by infectious and febrile episodes. Spasticity and ataxia have worsened, requiring assistance for mobilization. At 13 years of age, his weight was 44.5 kg (29th centile), height was 172.5 cm (91st centile), and BMI was 14.94 (first centile). Examination showed thick lips, bilateral epicanthal folds, bilateral elliptical nystagmus, optic nerves with temporal pallor, slender fingers, and scoliosis. Brain MRI (Figure 4) confirmed abnormal mild T2 hyperintensity in the periventricular and subcortical white matter corresponding to isointense T1 signal suggestive of hypomyelination, without substantial change over time.

Figure 4. Brain MRI of Patient B.

Brain MRI of patient B at 4 years (before transplant), 5 years, 6 years, 9 years, and 13 years of age. The sequences from left to right for each row of images are sagittal T1, axial T1, axial T2, and axial fluid attenuated inversion recovery (FLAIR).

Patient C

This 14-year-old girl had an uneventful prenatal and birth history, with normal hearing and metabolic newborn screening (Figure 2C). Between 1 and 3 years of age, she experienced recurrent upper respiratory infections, requiring adenoidectomy and tonsillectomy at 2 years of age. Language delay was noted at age 3 years, and bilateral sensorineural hearing loss was diagnosed, requiring hearing aids. Motor milestones were normal. Behavioral disturbances emerged between 3 and 4 years of age. Seizures began at 6 years, characterized by leftward gaze deviation, eye twitching, drooling, and unresponsiveness. Strabismus and bilateral optic nerve damage were diagnosed at 7 years. Brain MRI revealed features suggestive of metabolic white matter disease (Figure 5), prompting lysosomal enzyme testing that showed absent β-mannosidase activity. Sequencing of MANBA showed 2 frameshift variants: c.1540_1541del, p.Val514CysfsTer10 maternally inherited, and c.2352_2356del, p.Thr785LeufsTer27 paternally inherited (Figure 1). At 14 years, she developed recurrent generalized tonic-clonic seizures, necessitating emergency care and antiepileptic regimen adjustment. She returned to her baseline health status but developed urinary incontinence after the episode.

Figure 5. Brain MRI of Patient C.

Brain MRI of patient C at 6 years, 9 years, 11 years, and 13 years of age. The sequences from left to right for each row of images are sagittal T1, axial T1, axial T2, and axial fluid attenuated inversion recovery (FLAIR).

At 14 years old, she is dependent for daily activities and exhibits aggressive behavior. Weight was 63.7 kg (86th centile), height was 162.6 cm (59th centile), and BMI was 27.5 (87th centile). Physical examination showed mild dysmorphic features: esotropia, broad nose bridge, flat philtrum, microretrognathia, and mildly prominent cheeks. Hands anomalies included slender fingers with bilateral clinodactyly of the fifth finger, shallow palmar creases, and right single palmar crease. The skin was fair, and there were no angiokeratomas. Skeletal malformations or hepatosplenomegaly was absent. UO showed excretion of ManGlcNAc and NeuManGlcNAc. A third oligosaccharide was found, corresponding to NeuMan2GlcNAc2 (Figure 3). The brain MRI (Figure 5) showed diffuse and symmetric abnormal increase in white matter T2 signal, predominantly in the frontal lobes and subcortical areas, that extended from the deep periventricular white matter to the subcortical region and corresponded to an abnormal decrease in T1 signal. There were also signs of cerebral volume loss given by the prominence of the sulci and ventricles without substantial change over time. The corpus callosum had normal signal and shape at 6 and 9 years old, with progressive thinning evident at 11 and 13 years old. The cerebellum and brainstem were unaffected. MRI spectroscopy was normal.

Patient D

This patient is a 3-year-old adopted girl, born at 34 3/7 weeks gestational age via C-section due to severe intrauterine growth restriction (Figure 2D). Pregnancy was complicated by maternal use of sertraline and suspected tetrahydrocannabinol exposure. Birth weight was 1,490 g (<first centile), length was 37 cm (<first centile), head circumference was 28 cm (<first centile), and APGAR scores of 1 and 5 at 1 and 5 minutes, respectively. She required continuous positive airway pressure support for respiratory distress and phototherapy for jaundice. A bifid uvula with midline submucous cleft palate and laryngeal cleft were confirmed and repaired. She had a parasacral dimple with an asymmetrical gluteal cleft. A lumbar spine MRI showed a low-lying spinal cord with the tip at L3 and a sinus tract that runs inferiorly to end at the coccyx. Chromosomal microarray revealed no copy number variants but regions of homozygosity of approximately 26% of her total DNA.

At 3 months of age, the patient had recurrent ear infections and diarrhea. At 4 and 6 months of age, BAER and audiometry confirmed bilateral moderate to severe sensorineural hearing loss, and at 8 months, hearing aids were placed. At 9 months, developmental delay was apparent, with lack of speech but the ability to learn sign language. At 1 year, she developed staring spells without EEG abnormalities. Brain MRI was compatible with delayed myelination (Figure 6).

Figure 6. Brain MRI of Patient D.

Brain MRI of patient D at 10 months and 18 months of age. The sequences from left to right for each row of images are sagittal T1, axial T1, axial T2, and axial fluid attenuated inversion recovery (FLAIR).

Exome sequencing identified 2 sets of homozygous variants in the MANBA gene. The first variant, c.434A>C, p.Gln145Pro, was classified as a VUS. Mutation Taster classifies this variant as benign, and PolyPhen2 as possibly damaging (Score 0.611). The Gln145 residue is moderately conserved (absent in mouse), with minor structural impact by Gln145Pro due to hydrogen bond loss within a single domain (eFigure 1C). The second variant, c.1430A>G, p.Tyr477Cys, was also initially classified as a VUS. Mutation Taster and PolyPhen2 classified this variant as damaging. The Tyr477 residue is highly conserved, and the Cys477 change disrupts the contact between the galactose-binding–like domain and the catalytic domain (eFigure 1D). β-mannosidase activity in leukocytes was absent, confirming the diagnosis of β-mannosidosis, and the p.Tyr477Cys variant was reclassified as likely pathogenic. The UO showed 3 abnormal peaks corresponding to the excretion of ManGlcNAc, NeuManGlcNAc, and NeuMan2GlcNAc2 (Figure 3).

At 21 months, the patient developed episodes of recurrent pain and bright red-hot swelling in hands and feet with activity, consistent with erythromelalgia. EMG and nerve conduction tests were normal. At 2 years, a gastrostomy was placed because of poor oral intake and chronic diarrhea. She was diagnosed with autism at 2.5 years. At 3 years, she developed progressive dysphagia requiring exclusive gastrostomy feeding. Family history is relevant for a high degree of consanguinity (the father of the patient is also the maternal grandfather). At 2 years 9 months, her weight was 12.8 kg (38th centile), height was 86.5 cm (10th centile), and BMI was 17.1 (80th centile). Physical examination showed mild coarseness of the face, long palpebral fissures, bilateral epicanthic folds, and flat nose bridge. The uvula was broad, and the palate was intact. The skin and hair were lightly pigmented, with angiokeratomas on the 4 limbs. There was axial hypotonia, with normal strength and reflexes in limbs.

Brain MRI (Figure 6) at 10 months showed minimal myelination of the posterior limbs of the internal capsule (Axial T1) without substantial myelin in the lateral thalami or in the central corona radiata. At 19 months, the myelination has slightly progressed, albeit delayed for age.

Patient E

The patient is a 19-year-old man born to healthy parents following a normal pregnancy and delivery (Figure 2E). Birth weight was 2.75 kg at 37 weeks' gestation. Developmental milestones were normal, with no regression. Hearing loss was detected at 2.5 years old, and learning difficulties were detected at 6.5 years old. The symptoms have remained stable throughout his adult life. He is being treated with methylphenidate for behavioral issues, attention deficit and hyperactivity disorder (ADHD), and panic attacks. There is no history of seizures, balance problems, or ataxia. He is also being followed for primary hypogonadism. Brain MRI is normal. The patient was diagnosed with β-mannosidosis at the age of 12 years, and his 2 younger brothers are also affected.

At 19 years of age, his weight was 39.5 kg (fourth centile), height was 158 cm (fourth centile), and BMI was 16 kg/m². The patient had subtle coarse facial features, and angiokeratomas of the thighs, buttocks, groin, palms, and soles. Both the liver and the spleen were palpable at 2 cm below the costal margin.

UO showed a band running in line with the disaccharides control band. The plasma β-mannosidase enzyme activity was 1 nmol/mg/h (normal range 150–1,500), and leukocyte β-mannosidase activity was absent. MANBA sequencing detected a homozygous c.685C>T, p.Gln229Ter variant (Figure 1), classified as pathogenic.

Patient F

The patient is a 31-year-old woman with hearing problems at 4 years of age and requiring hearing aids at 10 years of age (Figure 2F). She was born at 36 weeks; her birth weight was 2,177 g (<first centile). In addition to developmental delay, she was diagnosed with intellectual disability. At 7 years of age, an assessment of her cognitive ability suggested scores in the low range. The patient also had a ventricular septal defect that required surgery at 7 months of age. Additional problems include anxiety and disturbed sleep, some autistic features, and frequent tremors in the upper limbs. There was no history of behavioral problems, aggressiveness, or seizures. She has chronic anemia due to abnormal uterine bleeding. The patient's ancestry is British.

She has no obvious dysmorphic features and no skeletal abnormalities. Mild angiokeratomas were found on her palms. Height was 160 cm, and weight was 67 kg. A loud murmur in her aortic area, and her chest was heard; blood pressure was 141/94. There was no hepatosplenomegaly. Independent walking showed unorthodox wide arm swing, ataxia, and hyperlordotic posture without scoliosis.

Exome sequencing revealed 2 pathogenic variants in MANBA: c.1452_1453del, p.Tyr485CysfsTer27 and c.1753C>T, p.Arg585Ter (Figure 1). The UO showed a disaccharide band present consistent with the diagnosis of β-mannosidosis. The plasma β-mannosidase enzyme activity was 2 nmol/mg/h (normal range 150–1,500) and absent in leukocytes. Brain MRI was normal.

Review of Literature

Table 1 shows a list of the patients with β-mannosidosis reported in the literature. For the analysis, we also included the 6 patients reported in this study, and only the data specifically reported in each scientific article were included in the analysis. Forty-six patients, from 37 different families, have been described in the literature: 17 female patients (41.5%), 24 male patients (58.5%), and 3 patients in which sex was not reported. The mean age of diagnosis was 12.9 years (range 5 months–51 years, median 7 years, interquartile range [IQR] 3.5–19). The mean age at onset of symptoms was 2.4 years (28.7 months, range 0–12 years, median 15.5 months, IQR 4.75–48). The average time between the onset of symptoms and the diagnosis was 10.3 years (range 3 months–49 years, median 6 years, IQR 1.75–13.5). Hearing loss (8 patients, 25%) was the most frequent first symptom reported (eTables 1 and 2). Angiokeratoma corporis diffusum was found as a first symptom in only 2 patients (6.3%), although it is frequently reported in β-mannosidosis (52.6%, eTable 2). The most frequent symptom reported independently of age at onset (eTables 1 and 2) was intellectual disability (91.4%), followed by behavioral abnormalities (82.8%) and hearing loss (74.4%). Compared with α-mannosidosis in which skeletal dysplasia is a main feature,³ it was found in 7.4% of the patients with β-mannosidosis.

The brain findings on CT and MRI were described in 37.5% of the patients (15/40 patients, not including the patients of our cohort), and it was normal in 9 (60%) and abnormal in 6 (40%). Findings on CT included cerebral atrophy,²¹ basal ganglia and white matter calcification,²⁵ and hydrocephalus.²⁷ Brain MRI findings included cortical and subcortical atrophy,²⁷ periventricular and subcortical white matter hyperintensities,¹⁷ hydrocephalus,²⁸ and delayed myelination.¹³

There are 29 reported pathogenic variants in MANBA associated with β-mannosidosis (Table 1, Figure 1): 7 (24.1%) are frameshift (6 deletions, 1 insertion), 8 (27.5%) are missense, 8 (27.5%) are nonsense, 5 (13.7%) are in splice sites, and 1 (3.4%) is a chromosome inverted duplication encompassing 13.1 kb. Most of the patients have private variants (21/34, 61.8%). Only 13 patients (38.2%), all with Roma ancestry (12 from the Czech Republic/Slovakia, 1 from Hungary), share the c.2158-2A>G variant in homozygous status.

Figure 1 shows the location of the variants in the protein (A) and the splicing variants in the gene (B). Seven variants are located in the galactose-binding–like domain and 8 in the catalytic domain. There are 3 positions with recurrent variants: Arg182, Trp192, and Tyr485. The codon Arg182 has 2 missense variants reported (Arg182Trp²⁴ and Arg182Gln,²⁹ patient A). The codon Trp192 belongs to the binding site of MANBA and has 1 nonsense variant (c.575G>A, p.Trp192Ter, patient B) and 1 frameshift variant (c.562_571dup, Trp192Ter).²⁶ The codon Tyr485 is located in the catalytic domain of the protein and has 2 frameshift variants (c.1454_1455del,¹⁹ and c.1452_1453del, patient F) with the same effect on the protein (Tyr485CysfsTer27).

Discussion

β-mannosidosis is classified as a disorder of complex molecule degradation.⁴⁴ Related conditions include α-mannosidosis, fucosidosis, sialidoses, galactosialidoses, mucolipidoses, aspartylglucosaminuria, Schindler, and Kanzaki disease.⁴² The MANBA gene, located on chromosome 4q22-q25, comprises 136,509 bp (NG_012804.2), 17 exons 34, and a transcript of 4,001 bp (NM_005908.4) encoding an 879-amino acid protein²⁰ with a molecular weight of 100 kDa.⁴⁵

Oligosacchariduria, a hallmark of β-mannosidosis,⁴⁶ results from the accumulation of oligosaccharides from the degradation of N-glycans.⁴⁷ Our report expands on the urinary oligosaccharide profile by MALDI-TOF, given that β-mannosidosis was not described in the published study.⁴⁰ ManGlcNAc and NeuManGlcNAc, known markers of the disease,^1,12,48 are present in the urine of patients A, C, and D. In patient B, only NeuManGlcNAc was found, probably reflecting his prior transplant. An additional abnormal peak, corresponding to a Neu-Man2-GlcNac2 structure, was found in patients C and D. In a prior case, a pentasaccharide was reported¹⁴; however, it was not further characterized. The presence of Neu-Man2-GlcNac2 in the urine of the most severely affected patients in our cohort might be explained by incomplete degradation by chitobiase (endo-β-N-acetylglucosaminidase). This enzyme removes one of the 2 reducing N-acetylglucosamine residues.⁴⁷ This is an essential step needed for the subsequent degradation of the N-glycan core by the specific α(1–6)-mannosidase and then the last degradation step by β-mannosidase.⁴⁹ Chitobiase is present in rodents and primates and absent in goats, which explains the more severe phenotype in caprine species.⁵⁰ Since α(1–6)-mannosidase and chitobiase function in tandem for lysosomal N-glycan catabolism, additional studies should be developed to determine whether CTBS (Di-N-Acetyl-Chitobiase) acts as a modifier gene in severe cases.

Establishing a genotype-phenotype correlation is difficult because of the rarity of the disease. Except for the 13 patients from the Roma population, all the patients have private variants and 9 patients (45%) are compound heterozygotes. Three unrelated patients carry variants affecting the Arg182 codon, located in the binding site of the protein. Among these, one compound heterozygote (Arg182Trp/Trp466Ter) exhibited only mild symptoms,²⁴ while another Arg182Gln homozygote (patient A) presented with mild behavioral abnormalities and hearing loss. By contrast, another Arg182Gln homozygote had developmental delay and failure to thrive, although the enzyme activity was not reported.²⁹ The Arg182Trp variant has no detectable enzyme activity in pCS2-transfected cells,²² as well as the Gly392Glu and Ser505Pro variants, which are associated with mild²³ and severe^e1 phenotypes, respectively; therefore, the severity of the phenotype is not explained by the presence or absence of residual enzyme activity. For example, one patient^e2 with residual enzyme activity in leukocytes was a compound heterozygote of a missense (p.Gln94Lys) and a frameshift (p.Met846IlefsTer27) variant (Table 1). This patient had a severe phenotype, including recurrent pulmonary infections, cystic lesions, alveolar hemorrhage, and early demise at 9 years of age due to fungal infection.

The c.2158-2A>G variant, observed exclusively in Roma patients, demonstrates intrafamilial variability.^14,18,e3 Two patients had reduced enzyme activity, and 2 had absent activity (Table 1). In addition, 2 affected siblings showed different degrees of severity: the girl showed a coarse face, skeletal dysplasia, short stature, and severe and recurrent infections that caused her demise at 20 years of age, and her older brother had intellectual disability, deafness, mild dysmorphic features, recurrent skin infections, and normal stature, without skeletal dysplasia.¹⁸ The Roma population is a highly endogamic ethnic group, and probably c.2158-2A>G arose as a founder variant in this community. There is a heterozygous frequency of 3.77% in the Czech and Slovak normal-hearing Roma populations, but it is absent in non-Roma Czech and Slovak populations.¹⁴

Facial dysmorphism was observed in 43.3% of patients, but it is usually mild.^e4 Although facial coarsening can be seen in β-mannosidosis, this is not a common feature. Skeletal abnormalities were seen only in 8% and organomegaly in 17.4% of the reported patients.

Neurologic involvement is the most consistent clinical feature in patients with β-mannosidosis, manifested as developmental delay (diagnosed under 5 years of age), intellectual disability (diagnosed after 5 years of age),^e5 behavioral abnormalities, and hearing loss. Intellectual disability has a prevalence of 1%–3% in the general population,^e5 and its etiology is heterogeneous, although up to 40% of cases have a genetic cause.^e6 Mental health comorbidities are common (up to 40%), particularly ADHD (39%) and autism (18%).^e7 The proportion of patients with β-mannosidosis and behavioral abnormalities in this study is 84.6% (eTable 3), suggesting that the behavioral phenotype is intrinsic to β-mannosidosis rather than secondary to intellectual disability. In 3 patients (6.8%), development was initially normal but slowed after the first year of age.^1,25,e8 Patients B, C, and D from our cohort have shown signs of regression of milestones, feature not previously reported in affected patients. Two patients with β-mannosidosis have been reported with peripheral neuropathy.^25,e9,e10

The caprine model of β-mannosidosis shows cranial nerve involvement limited to the eighth cranial nerve, which showed severe reduction of myelin,^e11 and the optic nerve, which showed a decreased number of oligodendrocytes and a generalized myelin deficit that persists in 16-week-old animals, suggesting abnormal stem cell proliferation or early death.^e12 These animal findings correlate with the high incidence of hearing and eye abnormalities in humans.

Hearing loss was the initial symptom in 25% of patients (eTable 2); 95% of them have intellectual disability. Conversely, 78.6% of the patients with intellectual disability also had hearing loss. One patient exhibited hearing loss and angiokeratoma corporis diffusum without intellectual disability.²⁴ In this scenario, we cannot rule out the possibility of underdiagnosis, especially in those cases of hearing loss with milder symptoms. β-mannosidosis should be considered as a differential diagnosis in both syndromic and apparently nonsyndromic hearing loss.

Multiple ophthalmologic manifestations have also been reported in β-mannosidosis (eTable 3). Nystagmus was newly found in 1 patient (patient B). Of interest the MANBA variants c.2013C>A, p.Arg638His, and c.2346T>A, and p.Leu749His have been found in heterozygous state in Chinese patients with autosomal dominant nystagmus.^e13 Although the β-mannosidase enzyme activity was reduced between 40% and 70% of normal controls, these patients did not show any other associated symptoms suggesting of β-mannosidosis.

A new feature of β-mannosidosis is erythromelalgia in patient D, characterized by recurrent episodes of erythema, burning pain, and increased temperature in distal extremities. Primary erythromelalgia due to pathogenic variants in the SCN9A gene,^e14 and secondary causes,^e15,e16 including autoimmune disorders, were ruled out in patient D. Of interest 2 siblings with α-mannosidosis and SLE have been reported, suggesting that the alteration of glycoprotein structures secondary to the oligosaccharide accumulation might trigger the recognition of these structures by the immune system as nonself.^e17 The common finding in erythromelalgia is a central autonomic dysfunction secondary to abnormal sympathetic fibers, which causes impairment in the vasoconstrictor reflexes, along with small and large fiber neuropathy.^e15,e18

Hypomyelination, characterized by a deficit in myelin deposition,^e19 appears as abnormal isointense or decreased T1 signal corresponding to an increase in the T2 signal of the white matter. The brain MRI features of hypomyelination^e20 are represented in 3 of our patients: delayed myelination below 2 years of age (patient D, Figure 6), persistent isointense white matter compared with the gray matter signal in T1 with mild hyperintensity of white matter in T2 (patient B, Figure 4), and hypointense white matter in T1 with hyperintensities in T2 (patient C, Figure 5). Similar patterns occur in fucosidosis and sialic acid disorders, probably reflecting a secondary failure of myelination, unlike primary hypomyelinating conditions such as Pelizaeus-Merzbacher disease.^e21

Despite its classification as a genetic leukoencephalopathy,^e22 β-mannosidosis lacks a well-defined pathophysiologic basis for its MRI features in humans, and there is no literature available on the histopathologic findings. Animal models offer insights: in goats, demyelination is the most prominent neuroradiologic sign of brain injury.⁷ Vacuolation of cells to various extents, reduction of oligodendrocytes, and myelin paucity are prominent.^e11 An absence of myelin sheaths was demonstrated in the corpus callosum, and in the areas with reduced myelin, the number of oligodendrocytes was also significantly decreased.^e23 Proteolipid protein (PLP) expression was markedly reduced, with myelin content at 2%–7% of the normal value in newborn and 4-week goats, demonstrating a severe deficit in myelination.^e24 The myelin-associated glycoprotein (MAG) was also diminished,^e25 although its glycosylation pattern was not affected. The myelin basic protein was not as severely affected as the other myelin proteins evaluated (MAG, PLP, and CNP [2',3'-cyclic-nucleotide 3'-phosphodiesterase]), suggesting interference from the accumulated oligosaccharides to the transport system when myelin is synthesized.

Studies on the pathophysiology of myelination in β-mannosidosis are limited because of the paucity of neuropathologic studies in humans. On the other hand, there is high variable expressivity, even in the same family; most of the pathogenic variants are private; and there is no correlation between residual enzyme activity and severity of the disease.²² Observed MRI abnormalities likely represent a static myelination deficit secondary to oligodendrocyte loss and Schwann cell vacuolation. Potential mechanisms include abnormal signaling on the myelin synthesis or impaired oligodendrocyte maturation, both mechanisms driven by the accumulated oligosaccharides in neuronal tissues. Further research is needed to elucidate the mechanism of impaired myelination in β-mannosidosis.

In conclusion, β-mannosidosis presents primarily with a neurologic phenotype and variable systemic involvement, with a wide range of severity that cannot be explained by a genotype-phenotype correlation. Milder cases may be underdiagnosed. We consider that MANBA should be included in all hearing loss and leukoencephalopathy genetic panels and should be considered as a differential diagnosis in patients with behavioral problems and intellectual disability. Oligosaccharide degradation has an important role in the myelination of the CNS, and modifier genes likely contribute to the pathophysiology of β-mannosidosis; therefore, studies are necessary to delineate the role of glycan metabolism in the myelin formation and maintenance.

Glossary

ADHD

attention deficit and hyperactivity disorder

IQR

interquartile range

MAG

myelin-associated glycoprotein

PLP

proteolipid protein

VUS

variant of uncertain significance

Author Contributions

A.M. Martin Rios: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; study concept or design; analysis or interpretation of data. L.H. Gibbs: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; analysis or interpretation of data. K.M. Stepien: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. K. Hall: drafting/revision of the manuscript for content, including medical writing for content. P.L. Hall: drafting/revision of the manuscript for content, including medical writing for content; analysis or interpretation of data. G. Bentz Pino: drafting/revision of the manuscript for content, including medical writing for content; analysis or interpretation of data. R.Y.-J. Wang: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. N.R. Pillai: drafting/revision of the manuscript for content, including medical writing for content. T. Lund: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. P.J. Orchard: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. V.E. Kimonis: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; study concept or design.

Study Funding

The authors report receiving funding for this study from the parent support organization for beta-mannosidosis: The Lost Enzyme Project (TLEP); A.M. Martin Rios received funding for her Lysosomal Storage Diseases fellowship from Sanofi-Genzyme which permitted her to devote effort towards this project.

Disclosure

The authors report no relevant disclosures. Go to Neurology.org/NG for full disclosures.