See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/334232912 Riboflavin Transporter Deficiency Presentation, Genetics and Treatment Presentation · July 2019 DOI: 10.13140/RG.2.2.29556.09604 CITATIONS READS 0 37 1 author: Keith Massey University of Manitoba 6 PUBLICATIONS 1 CITATION SEE PROFILE Some of the authors of this publication are also working on these related projects: Auditory Neuropathy in Riboflavin Transporter Deficiency View project Epidemiology of Riboflavin Transporter Deficiency View project All content following this page was uploaded by Keith Massey on 04 July 2019. The user has requested enhancement of the downloaded file.
Riboflavin Transporter Deficiency Presentation, Genetics and Treatment Keith Massey Science Director, Cure RTD Foundation Email: Keith.Massey@curertd.org
Riboflavin Transporter Deficiency (RTD) • Early onset, life shortening, neurodegenerative disorder. • Autosomal recessive inheritance. • Previously known as Brown-Vialetto-Van Laere (BVVL) and Fazio-Londe syndromes. • Sensory and/or motor neuropathy and cranial neuropathy. Brown, 1894
Gene Discovery • 2008 2008-2010 2010 The SLC52A1, SLC52A2 and SLC52A3 genes first identified to code for riboflavin (Vitamin B2) transporters (RFVT1, RFVT2 and RFVT3) in humans. • 2010 2010 SLC52A3 gene was found to cause BVVL. Daily high dose riboflavin found to help stabilize or improve clinical symptoms. • 2012 2012 SLC52A2 gene was also found to cause BVVL and patients responded to riboflavin therapy.
RTD Disease Types RTD Type 2 Riboflavin Transporter Deficiency, Type 2 SLC52A2-associated disease RFVT2 riboflavin transporter RTD Type 3 Riboflavin Transporter Deficiency, Type 3 SLC52A3-associated disease RFVT3 riboflavin transporter
Flying under the radar? Daughter First Cousins Once Removed SLC52A2 biallelic mutations SLC52A2 biallelic mutations c.155 C>T and c.935 T>C c.155 C>T and c.916 G>A
Prevalence • Carrier rate: ~1 in 500 people (single SLC52A2 or SLC52A3 known pathogenic mutation) • Estimated Prevalence: ~1 in 1 million births (based on recessive inheritance) • Estimated 50-100 new cases per year Cure RTD Cu D - Patie tients Reported sin ince ce 2010 SLC5 C52A2 (R (RTD Type 2) SLC5 C52A3 (R (RTD TD Type e 3) 3) 125 cases (biallelic mutations) 83 cases es (biallelic mutations) 1 1 case e (heterozygous mutation) 12 12 cas ases es (heterozygous mutation - RF Responsive ) • • 21 deceased, 19 untreated 17 deceased, all untreated • • Mean age of onset: 3.0 3.0 year ears Mean age of onset: 7.6 7.6 year ears SD =3.0, Range: 0-19 19 yea ears SD =8.2, Range: 0-34 34 yea ears • • 55% Female 59% Female Untreated : Mean age of death: 12.6 years (SD=14, Range: 0.9 - 55 years, n =31) Cause of death: Respiratory insufficiency
Riboflavin (Vitamin B2) • Precursor to flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) (active coenzyme) • Riboflavin acquired via intestinal absorption with little storage in the body. • Human genome contains 90 genes encoding for flavin-dependent proteins. • FAD and FMN crucial for cell regulation and energy metabolism.
RVFT Human Tissue Expression Yonezawa 2010
RVFT Function: Intestine and Blood Brain Barrier RVFT: Transmembrane Protein (Apical or Basolateral Facing) Transports Riboflavin (RF) and small amounts of FMN/FAD Active Saturable Transporter
Transporter Kinetics • RF RFVT2 (S (SLC52A2) Km 0.33 +- 0.07 uM (330nM) Vmax 5.2 5.2 +- 0.7 0.7 pmol × mg protein-1 × min-1 • RF RFVT3 (S (SLC52A3) Km 0.98+- 0.11 uM Vmax 63 63.8 .8 +- 9.3 9.3 pmol × mg protein-1 × min-1 Jin et. al 2017
Human Brain Gene Expression Cer Cerebellum GTEX Portal
Clinical Features of RTD Untreated Untreated patients with over 10 years from onset of first symptom. SLC52A2 SLC52A3 (RTD Type 2) (RTD Type 3) N=38 N=18 98% 95% Auditory Neuropathy 80% 82% Pontobulbar Involvement 72% 82% Respiratory Compromise 87% (UL UL) 95% Muscle Weakness 100% 29% Optic Atrophy w/o Nystagmus 100% 29% Sensory Involvement / Gait Ataxia UL UL = Upper limb and axial predominant muscle weakness Other common feature: GI Distress, aggressive scoliosis (type 2), Upper motor neuron and cerebellar signs (type 3), Severe facial weakness (type 3)
Presenting Features SL SLC5 C52A2 SL SLC5 C52A2 SL SLC5 C52A3 SLC5 SL C52A3 0-5 year ears 6-19 19 years 0-5 5 year ears 6-33 years N=77 N=16 N=31 N=28 Auditory Neuropathy 29% 63% 26% 75% Pontobulbar Involvement 9% - 61% 32% Sensory Involvement / Ataxia 62% 32% 6% 4% Optic Atrophy w/o Nystagmus 27% 38% 6% - Muscle Weakness 29% (UL) - 39% 11% Respiratory Compromise 12% 6% 55% 7% Megaloblastic Anemia 5% - - - Symptoms, age at onset, time interval between symptoms, and severity and rate of decline are all highly variable – even between siblings.
Laboratory Findings Blo Blood • Acylcarnitine profile abnormalities in 50-60% of patients. Selective accumulation of selective short- and medium-chain acylcarnitines. • Riboflavin (+FAD, FMN) levels may be low or normal. • Signs of mild mitochondrial dysfunction in some patients. • >10% - Low hemoglobin and serum iron and/or ferritin levels - Anemia (macrocytic or normocytic) Urin rine Organic ic Acid cid • Elevated ethylmalonic acid in <50% of patients indicative of a defect in riboflavin metabolism
Neurodiagnostic Tests Ele lectromyogram (E (EMG) an and nerve con onduction stu tudies (NC (NCS) • chronic partial denervation • conduction velocities are normal or mildly reduced • indicative of axonal motor / sensory neuropathy Brai ain mag agnetic resonance im imagin ing • Most often unremarkable • ~25% with RTD type 3 have cerebellar and brainstem abnormalities Auditory ry brai ainstem resp sponse (AB (ABR / / BAEP) • absent or abnormal
Audiologic Test Findings in RTD Hearing loss consistent with auditory neuropathy pattern • Absent or abnormal auditory brainstem response (ABR) with cochlear microphonic present • Otoacoustic emissions (OAE) are often present but found to disappear over time in many patients. • Acoustic reflexes often absent • Magnetic resonance imaging (MRI) is generally normal without any evidence of VIII cranial nerve hypoplasia • fMRI with an auditory stimulation protocol showed integrity of the central auditory pathways in patient (Salmina et al 2014)
Audiology Clinical Characteristics Reported in RTD • Progressive hearing loss can occur from early infancy to 25+ years in age • Pure tone thresholds ranging from mild to profound loss • Hearing loss may progress to profound in months or slowly progressive over decades • Loss is bilateral but often asymmetric • Disproportionately poor speech recognition abilities for the degree of hearing loss • Difficulty hearing in noise and hearing fluctuation • Patient with poor speech recognition have reported limited or no benefit from hearing aids • Vestibular deficits are rare
Cochlear Implantation Over 40 RTD patients have received cochlear implants. (Age of implant: 2 to 45 years old) Results are variable but the large majority have received significant benefit and very pleased with CI decision. The few cases with less than optimal outcomes had: • Long-term profound deafness • Extended period of prelingual deafness Cannot discount how much sound awareness can add to quality of life when dealing with multiple disabilities
RTD Type 2: Cochlear Implant Case Example AGE 3mths – 2 years Vision loss, anemia, muscle weakness, sensory gait ataxia, feeding difficulties 2 years Language development normal and first sign of hearing loss became apparent 2.5 years RTD diagnosis and started riboflavin treatment (75mg/kg QID + mitococktail) 3.8 years Received unilateral cochlear implant. ~10-20% accuracy on close word speech set 5.7 years ~95% accuracy on closed word set in CI ear / still ~10-20% in non-implanted ear 7.4 years Received bilateral cochlear implants. 8.0 years ~95% accuracy on open word set using both CIs 2.8 years old old 3.7 years old old 5.7 years old old 4 months on Riboflavin 1.5 years on Riboflavin 3.5 years on Riboflavin
Pathogenic Mutations SLC52A2 (RTD Type 2 ) SLC52A3 (RTD Type 3) • 53 Mutations • 52 Mutations • Mutations distributed throughout all coding exons (2 to 5) • Mostly missense mutations where RFVT retains some function • Combined >15% of mutations across both SLC52A2 and SLC52A3 are: • Nonsense mutations • Single/double nucleotide insertions/deletions causing frameshift • Biallelic “Loss of Function” mutations generally not compatible with life.
Impact of Pathogenic Mutations on RVFT Function • Reduced RFVT cell surface expression Riboflavin Cellular Uptake • RFVT ability to transport riboflavin is impaired • Reduction in mRNA stability (1 mutation) Overall consequence is reduced WT SLC52A2 Mutations riboflavin transport across the cell Foley et al. 2013 membrane .
SLC52A2 Gene Structure ~50% of patients identified have 1 or 2 mutations in: c.9 c.916 G> G>A (Population frequency 14 / ~250,000) c.1 c.1016 T> T>C (Population frequency 18 / ~250,000)
Transmembrane Topology Of RFVT2 Predicted Using TMHMM
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