3. Clinical phenotypes of SLC25A46-related diseases

#### 3.1. Hereditary motor and sensory neuropathy type VIB (HMSN6B)

In 2015, recessive mutations in SLC25A46 in eight patients from four unrelated families of various ethnic origins were first reported. The proband phenotypes encompassed ADOA-like optic atrophy, CMT-like axonal peripheral neuropathy, and cerebellar atrophy with a variable age of onset and disease course (Table 1) [17]. This new neurodegenerative syndrome is now defined as HMSN6B in OMIM (OMIM: 616505). ADOA and CMT type 2 are hereditary neurodegenerative disorders commonly caused by mutations in the mitochondrial fusion genes OPA1 and MFN2, respectively. However, both diseases lack a genetic diagnosis in up to 60% of patients due to genetic heterogeneity [34, 35].

SLC25A46 provides a new locus in genetic testing for patients with ADOA and CMT-like phenotypes. Indeed, four independent clinical reports published in 2016 and 2017 identified

ID UK family

c.165\_166insC;

p.His56fs\*94;

5 y/8 y Alive (40 y/43 y) +

 +

Normal Normal CSF examination,

n.k.

oxidative enzyme activity,

no ragged red fibers.

p.Gly249Asp

c.746G>A

Abrams et al.

[17]

PL family

c.1005A>T

p.Glu335Asp

 1 y/2 y Alive

(13 mo/11.5 y)

++

 +

 +

 +

↑

Developmental

MG ↑ in urine.

 delay, 3-

Increased

mitochondria.

Abrams et al.

[17]

IT family

c.1018C>T

p.Arg340Cys

 2 y

 Alive (51 y)

 +

 +

+

+

+

↑

CK ↑(225, NR<170 U/L),

Hyperfilamentous.

lactic acid at upper end of

normal range.

Abrams et al.

[17]

US family

c.882\_885dupTTAC;

p.

Prenatal 105 d

+

 +

+

+

n.k.

 n.k.

 Facial and hand

dysmorphism,

aspiration.

 meconium

n.k.

Asn296fs\*297;

p.Pro333Leu

Abrams et al.

c.998C>T

[17]

Moroccan

c.283+3G>T

p.?

Prenatal 7 d

+

 n.k.

 +

+

n.k. ↑

Club foot posture, lactate-

Mitochondrial

fragmentation.

to-pyruvate ratio ↑ and all

individual complexes ↓ in

fibroblasts.

family

Nguyen et al.

[21]

Pakistani

c.413T>G

p.Leu138Arg

 n.k.

 Alive (15 y/20 y) +

 +

+

+

+

 n.k.

 Comprised exotropia,

n.k.

difficulty initiating saccades, spasticity, scoliosis. Old brother with

mild phenotypes.

origin family Charlesworth

et al. [19] Saudi family

c.775C>T

p.Arg259Cys

 28 y Alive

+

 

+

n.k.

 Normal No ragged red fiber or

cytochrome c deficiency,

Occasional

enlarged

mitochondria.

Neurodegenerative Diseases Associated with Mutations in *SLC25A46*

intact sensation and

coordination,

unremarkable

acylcarnitine profile, amino acids, CK and urine

organic acids.

Sulaiman

et al. [26] Tunisian

c.1018C>T

p.Arg340Cys

 1 y/6 y Alive (22 y/19 y) +

 +

n.k.

 +

 n.k.

 Dysarthria, gait instability,

n.k.

http://dx.doi.org/10.5772/intechopen.79992

Babinski sign, abolished

Achilles reflexes, finger-

nose dysmetria, severe

sensorimotor

demyelination.

73

family

Hammer et al.

[23]

SLC25A46

SLC25A46

Age of

Age of death

 Optic

Peripheral

Cerebellar

Hypotonia

Ataxia Lactate Other features

Mitochondrial

dynamics

atrophy

neuropathy

or

or

brainstem

myopathy

atrophy

onset

proteins

mutations


microcephaly (OMIM 607196), neuropathy with bilateral striatal necrosis (OMIM 613710), congenital sideroblastic anemia (OMIM 205950), neonatal epileptic encephalopathy (OMIM 609304), and citrate carrier deficiency (OMIM 190315) [33]. These disorders are characterized by specific metabolic dysfunctions related to the role of the particular carrier that has been affected. Most disease-related SLC25 members have been characterized in terms of substrate identification and associated metabolic pathways, with the exception of two orphan SLC25

SLC25A46 was first mapped to chromosome 5 by genomic sequence analysis in 2006 [27]. Its location was further refined to chromosome 5q22.1 based on sequence alignment with NCBI's standard reference human assembly sequence, that is, the Genome Reference Consortium Human genome build 38. The largest transcript isoform of SLC25A46 contains eight exons, which encode a 418-amino acid protein. Quantitative real-time polymerase chain reaction (PCR) experiments in rodents have demonstrated variable expression of SLC25A46 in all tissues examined, with the highest levels occurring in the hindbrain, spinal cord, and coronal brain sections containing the corpus callosum, fornix, optic chiasm, thalamus, hypothalamus, midbrain, pons,

Given the typical SLC25 molecular structure, the primary sequence of the SLC25A46 protein has been predicted to form six conserved transmembrane alpha helices, TM1–TM6, spanning a region between amino acids 100–418 (Figure 1) [22]. However, the otherwise highly conserved P-X-(D/E)-X-X-(R/K) consensus sequence characteristic of SLC25 proteins is altered in SLC25A46. Moreover, the N-terminus of SLC25A46 is about five times longer than that of other members of the family (100 vs. <20 amino acids). These unusual characteristics suggest that SLC25A46 is unlikely to have a conventional metabolite carrier function. Recently, studies have proposed that unlike most SLC25 members that are located in the inner mitochondrial membrane, SLC25A46 may be anchored to the outer mitochondrial membrane where it may act as a regulator of

In 2015, recessive mutations in SLC25A46 in eight patients from four unrelated families of various ethnic origins were first reported. The proband phenotypes encompassed ADOA-like optic atrophy, CMT-like axonal peripheral neuropathy, and cerebellar atrophy with a variable age of onset and disease course (Table 1) [17]. This new neurodegenerative syndrome is now defined as HMSN6B in OMIM (OMIM: 616505). ADOA and CMT type 2 are hereditary neurodegenerative disorders commonly caused by mutations in the mitochondrial fusion genes OPA1 and MFN2, respectively. However, both diseases lack a genetic diagnosis in up to

SLC25A46 provides a new locus in genetic testing for patients with ADOA and CMT-like phenotypes. Indeed, four independent clinical reports published in 2016 and 2017 identified

and cerebellum, with particularly high levels in mouse embryo cerebellum [27].

mitochondrial dynamics rather than as a substrate transporter.

60% of patients due to genetic heterogeneity [34, 35].

3. Clinical phenotypes of SLC25A46-related diseases

3.1. Hereditary motor and sensory neuropathy type VIB (HMSN6B)

members, namely SLC25A38 and SLC25A46 [33].

72 Recent Advances in Neurodegeneration

Neurodegenerative Diseases Associated with Mutations in *SLC25A46* http://dx.doi.org/10.5772/intechopen.79992 73


ID Italian family

c.42C>G;

p.Tyr14 \*;

Prenatal 1 d/18 d

 n.k.

 n.k.

 +

+

n.k.

 n.k.

 PCH, floppy infant, little

n.k.

respiratory effort and

voluntary movements;

EMG: neurogenic lesion;

loss of spinal motor

neurons, normal CK levels,

serum transferrin IEF, two

siblings were hypotonic and died immediately after

birth.

P. ?

c.462+1G>A

Braunisch

et al. [24]

French

c.425C>T

p.Thr142Ile;

Birth

 15 mo

+

 n.k.

 +

+

n.k. ↑

Leigh syndrome,

Mitochondrial

hyperfusion in

fibroblast.

psychomotor

growth retardation, mild

spastic diplegia; motor

delay; fever, convulsion,

gasping respirations, bilateral intranuclear

ophthalmoplegia,

hyperreflexia,

spasticity.

Neurodegenerative Diseases Associated with Mutations in *SLC25A46*

http://dx.doi.org/10.5772/intechopen.79992

75

 mild

 delay,

(instable

protein)

Canadian

family

Janer et al.

[20]

Note: y, year; mo, month; d, day; n.k., not known; ↑, increase.

Table 1. Clinical phenotypes

 associated with SLC25A46

 mutations.

SLC25A46

SLC25A46

Age of

Age of death

 Optic

Peripheral

Cerebellar

Hypotonia

Ataxia Lactate Other features

Mitochondrial

dynamics

atrophy

neuropathy

or

or

brainstem

myopathy

atrophy

bradycardia at birth, green

amniotic fluid.

onset

proteins

mutations


ID

SLC25A46

SLC25A46

Age of

Age of death

 Optic

Peripheral

Cerebellar

Hypotonia

Ataxia Lactate Other features

Mitochondrial

dynamics

atrophy

neuropathy

or

or

brainstem

myopathy

atrophy

onset

proteins

mutations

Algerian

c.1018C>T

p.Arg340Cys

 2 y

 Alive (31 y)

 +

 +

n.k.

 +

 n.k.

 Subtle white matter

n.k.

74 Recent Advances in Neurodegeneration

changes in cerebellum,

increased tendon reflexes,

no Achilles reflex, positive

Hoffmann sign, no

Babinski sign.

family 1

Hammer et al.

[23]

Algerian

c.479G>C

p.Trp160Ser

 23 y Alive (26 y)

n.k.

 n.k.

 n.k.

 +

 n.k.

 Abolished vibration sense,

n.k.

at ankles, nystagmus and

saccadic pursuit, scoliosis.

family 2

Hammer et al.

[23]

Family 1

c.1022T>C

p.Leu341Pro

 Prenatal 14 d/28 d

 n.k.

 n.k.

 +

+

n.k.

 Normal PCH, severe global

developmental

normal respiratory chain

enzymes in muscle and

liver.

 delay,

Increase in mitochondrial

length.

Wan et al. [22]

Family 2

g.chr5:110738771\_

p.?

Prenatal 42 d

+

 +

+

+

n.k. ↑

PCH, occasional myoclonic

n.k.

jerks; EEG: generalized slowing with abnormal

theta rhythm, no epileptic

discharges, sibling with

same phenotype.

11074670del

Wan et al. [22]

Dutch family

c.691C>T

p.Arg231\*;

Prenatal 1 d

+

 n.k.

 +

+

n.k.

 n.k.

 PCH, all three children

n.k.

died within 1 day after

birth, lack of spontaneous

respiration, profound

muscle weakness. Convulsion, spinal motor

neuron degeneration.

g.chr5:110742638\_

p.?

110745029del

Dijk et al. [25]

German

c.736A>T

p. Arg 246 \* Prenatal 1 d/23 d

 n.k.

 n.k.

 +

+

n.k. ↑

PCH, seizures, EEG: low

n.k.

amplitudes with sharp

waves, epileptiform discharges without clinical

equivalents, thrombocytes

↑, lung hypoplasia,

family

Braunisch

et al. [24] Note: y, year; mo, month; d, day; n.k., not known; ↑, increase.

Table 1. Clinical phenotypes associated with SLC25A46 mutations.

homogenous SLC25A46 mutations in an additional nine patients (age range, 7 days to 28 years) from five unrelated families who presented with neurological phenotypes similar to the core features of HMSN6B. Among these nine patients, eight had optic atrophy (the exception was a patient with an age of onset 23 years) and eight had cerebellar atrophy (the exception was a 28-year-old patient without remarkable cerebellar atrophy or axonal neuropathy) (Table 1) [19, 21, 23, 26]. Beyond the key clinical features of optic atrophy, peripheral neuropathy, and cerebellar atrophy, the presently documented population of 17 patients with HMSN6B (or an HMSN6B-like condition) exhibited other clinical symptoms sporadically, including ataxia, hypotonia, myoclonus, dysmetria, nystagmus, speech difficulties, abnormal brain imaging, and elevated lactic acid (Table 1). The clinical manifestations, medical examination findings, and differential diagnoses for these patients were strongly suggestive of a progressive mitochondrial disorder.

onset, clinical features, and disease course. The severity of presentation even varies between siblings. The phenotype spectrum ranges from severe disease at birth with early death to

Neurodegenerative Diseases Associated with Mutations in *SLC25A46*

http://dx.doi.org/10.5772/intechopen.79992

77

4. Mutation spectrum of SLC25A46 and genotype-phenotype correlation

The SLC25A46 gene, located on chromosome 5q22.1, spans approximately 27 kb and is composed of eight exons. The main protein isoform has 418 amino acids and is encoded by a 1257-nucleotide-long open reading frame. Since SLC25A46 mutations associated with neurological disease were first reported in 2015, more than 28 patients with various mutations from 16 unrelated families have been diagnosed genetically, most by whole-exome sequencing, leading to the discovery of a total of 18 pathogenic mutations in the last 2 years (Figure 2). Of these, 50% are missense mutations; 16.7% are nonsense mutations; 11.1% are splice variants; and 22.2% are micro-deletions, insertions, or duplications. The mutation sizes range from a single nucleotide polymorphism to a 2.4-kb deletion. Although some mutations have been found in all exons except exons 2, 6, and 7, 50% of the mutations are located in exon 8, the largest exon, which accounts for half of the SLC25A46 open reading frame (Figure 2). Although there is no suspected mutation hotspot site, the c.1081C>T variant was observed in 3 of 16 independent families (Table 1). The identification and genetic diagnosis of additional

A systemic genotype-phenotype analysis of all available cases indicates that phenotype severity correlates strongly with the magnitude of SLC25A46 protein level reduction caused by each

Figure 2. Schematic diagram of reported pathogenic SLC25A46 variants. Exons 1–8 are represented by blue blocks. Mutations are color coded as follows: red, nonsense and missense mutations that would be expected to destabilize the protein; blue, micro-deletions/insertions/duplications; orange, splice-site mutations; and black, regular missense

manifestation in late childhood with survival beyond 50 years of age.

cases in the future may reveal a SLC25A46 mutation pattern.

4.1. Mutation spectrum of SLC25A46

4.2. Genotype-phenotype correlation

mutations.
