RESEARCH LETTER
Deletion of AFG3L2 Associated With Spinocerebellar Ataxia Type 28 in the Context of Multiple Genomic Anomalies Kenneth A. Myers,1 Jodi Warman Chardon,2 Lijia Huang,2 and Kym M. Boycott2* 1
Division of Neurology, Department of Pediatrics, Alberta Children’s Hospital, University of Calgary, Calgary, Alberta, Canada
2
Department of Genetics, Children’s Hospital of Eastern Ontario, University of Ottawa, Ottawa, Ontario, Canada
Manuscript Received: 22 September 2013; Manuscript Accepted: 20 August 2014
TO THE EDITOR: The spinocerebellar ataxias (SCA) are a genetically and clinically heterogeneous group of disorders characterized by progressive ataxia, gait disturbance, and dysarthria. Of these, spinocerebellar ataxia type 28 (SCA28; OMIM 610246) is one of the less common, comprising an estimated 1.5% of the autosomal dominant progressive SCAs [Cagnoli et al., 2010]. SCA28 was originally characterized in two Italian families exhibiting an autosomal dominantly inherited pattern of juvenile-onset, slowly progressive ataxia with associated ophthalmoplegia, gaze-evoked nystagmus, ptosis, and normal intelligence [Cagnoli et al., 2006; Mariotti et al., 2008]. The causative gene for SCA28 has been identified as ATPase family gene 3-like 2 (AFG3L2), located on the short arm of chromosome 18 (18p11) [Di Bella et al., 2010]. Thus far, the majority of pathogenic variants reported are heterozygous missense mutations primarily in exons 10, 15, and 16 [Cagnoli et al., 2010; Di Bella et al., 2010; Edener et al., 2010; Lo¨bbe et al., 2014]. A homozygous missense mutation in exon 15 of AFG3L2 results in spastic ataxia type 5 (SPAX5; OMIM 614487) [Pierson et al., 2011]. Here, we present the first case of progressive SCA associated with a deletion encompassing the entire AFG3L2 gene, suggesting haploinsufficency as a possible underlying mechanism for SCA28. A 23-year-old man with global developmental delay from infancy developed progressive cerebellar ataxia beginning at 13 years of age. He sat at 1 year and could ambulate with a walker by 8 years of age. Language development was also delayed, with expressive language more affected than receptive language, and he spoke in two to three word sentences in late childhood. He continued to slowly acquire developmental milestones until 13 years of age, when the onset of progressive neurologic deterioration was noted. Over the next years, his balance and gait gradually became increasingly impaired. His gait was wide-based with significant retropulsion early in the course of his deterioration. By the age of 18 years, he could no longer ambulate with a walker and depended on crawling and a wheelchair for mobility. He developed frequent choking episodes secondary to dysphagia. Fine motor coordination declined concurrently, necessitating more help with dressing and
Ó 2014 Wiley Periodicals, Inc.
How to Cite this Article: Myers KA, Warman Chardon J, Huang L, Boycott KM. 2014. Deletion of AFG3L2 associated with spinocerebellar ataxia type 28 in the context of multiple genomic anomalies. Am J Med Genet Part A 164A:3209–3212.
other self-care tasks. Over the same period of time, there was no appreciable decline in cognitive abilities. He had no events suggestive of seizures. When re-assessed at the age of 23 years, he understood simple commands and spoke in short sentences; however, severe dysarthria rendered his speech almost unintelligible. Eye movements were saccadic with end-gaze nystagmus and he was noted to have mild bilateral ptosis. Tone was decreased with normal muscle bulk, power, reflexes, and sensory exam. Dysmetria was present on finger-to-nose testing and he had severe dysdiadochokinesis when attempting rapid alternating movements. His general examination was remarkable for hypertelorism and tapered fingers with 5th finger clinodactyly. There were no signs of abnormal hair distribution. Cytogenetic testing at 1 year of age revealed an abnormal karyotype with an additional X chromosome and a telomeric fusion of one chromosome 18 and the Y chromosome [46,XX, t(Y;18) (p11?;p11?)]. This chromosomal abnormality constituted a diagnosis of Klinefelter syndrome, however, it was unclear whether loss and/or gain of chromosomal materials in 18p and Yp were involved
Correspondence to: Kym Boycott, Department of Genetics, Children’s Hospital of Eastern Ontario, University of Ottawa, 401 Smyth Road, Ottawa, ON, K1H 8L1, Canada. E-mail: [email protected] Article first published online in Wiley Online Library (wileyonlinelibrary.com): 23 September 2014 DOI 10.1002/ajmg.a.36771
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FIG. 1. Sagittal T1-weighted (A) and axial T2-weighted (B) MR images of the brain at 16 years of age demonstrating cerebellar atrophy.
during the telomeric fusion, and therefore the contribution of this t(Y;18) to his developmental delay was inconclusive at that time. Karyotypes for both parents were normal. The family history was significant for Huntington disease (HD; OMIM 143100); the patient’s father was diagnosed at 51 years of age and carried a HD-causing allele of 46 CAG repeats. The patient was tested and found to have an expansion of 39 CAG repeats, which is within the reduced-penetrance range. To investigate the cause of his progressive cerebellar ataxia, brain MRI with spectroscopy was performed at 16 years of age and demonstrated mild cerebellar atrophy (Fig. 1) and a normal cerebrum with neither basal ganglia changes characteristic of Huntington disease nor signs of holoprosencephaly. SCA types 1, 2, 3, 6, 7, 8, 17, and DRPLA, as well as the more common autosomal recessive ataxias (APTX, SETX, POLG1, SIL1, TTPA, and FXN) were excluded by clinical testing (Sanger sequencing and repeat expansion analysis). Screening metabolic studies were normal including lactate, ammonia, plasma amino acids, and urine organic acids. When he was 17 years old, a BAC array (Signature Genomics) that covers 622 loci across the genome, including the holoprosencephaly locus TGIF at the end of 18p, was performed. An extra X chromosome, as well as a distal deletion of 18p (deletion 1), were detected [arr cgh Xp22.33q28 (RP11-1325A17 >RP11-1137B3) 2, 18p11.32p11.31(RP11-683L23 >RP11-835E18) 1] (Fig. 2). The follow-up fluorescence in situ hybridization (FISH) analysis confirmed the presence of a dicentric chromosome derived from the fusion of 18p and Yp (ish der(Y;18)(p11.2;p11.32)(RP11838N2-)[11]/der(Y;18)(p11.2;p11.32)(RP11-838N2þ)[19]). In 19/30 cells analyzed, the TGIF locus was present on the derivative chromosome, and the rearrangement was apparently balanced; while the remainder of the cells showed no TGIF signal on the der(Y;18), indicating the distal 18p deletion is in a mosaic pattern. However, the proximal boundary of this 18p deletion was not known as the nearest proximal BAC clone that was not deleted was 8.4 Mb away. Nevertheless, the Klinefelter syndrome and the mosaic partial monosomy 18p was thought to explain his global developmental delay, while the genetic cause for his
progressive cerebellar ataxia remained unclear. Subsequently, SCA28 was mapped to the end of chromosome 18p (AFG3L2, chr18:12,328,943–12,377,275; hg19). This prompted further investigation of the 18p deletion using a high resolution oligonucleotide chromosomal microarray (Agilent 4 180 K). This microarray analysis detected the presence of a mosaic distal 18p deletion of 6.72 Mb in length including OMIM Morbid Map genes LPIN2 and TGIF (Fig. 2; deletion 1). This result was consistent with the findings from the previous BAC array and FISH analysis. However, a second
FIG. 2. Graphic representation of the cytogenetic findings in the patient reported here. The left t(Y;18) contains only deletion 2 [5.3 Mb, containing 33 RefSeq genes including 3 OMIM Morbid Map genes (NDUFV2, APCDD1, AFG3L2)] and is present in 19/30 cells examined. The right t(Y;18) contains deletion 1 [6.72 Mb, containing 38 RefSeq genes including 2 OMIM Morbid Map genes (LPIN and TGIF)] as well as deletion 2 and is present in 11/30 cells examined.
MYERS ET AL. nonmosaic deletion of 5.3 Mb at 18p11.23-p11.21 (Fig. 2; deletion 2) was also identified [arr Xp22.33q28(60,701–155,246,644) 2, 18p11.32p11.31(14,275–6,731,478) 1 2, 18p11.23p11.21 (8,253,838–13,557,376) 1; based on hg19]. Interestingly, the AFG3L2 gene, responsible for SCA28, is within deleted region 2, suggesting that heterozygous loss of this gene may contribute to his progressive cerebellar ataxia. The patient reported here presents with a complex set of genomic anomalies, moderate global developmental delay from infancy, and juvenile-onset progressive cerebellar ataxia. The latter deterioration is clinically consistent with the few previously described cases of SCA28 caused by missense mutations in AFG3L2. However, in this case, we believe that the cerebellar ataxia is the result of a heterozygous deletion of AFG3L2. The patient’s early cognitive, language and motor delays, as well as mild dysmorphic features, are likely due to partial 18p monosomy [Turleau, 2008] and Klinefelter syndrome [Lanfranco et al., 2004]. Several other genes in the mosaic and nonmosaic deleted 18p regions are known to be disease-causing but have not been associated with progressive ataxia. The reduced penetrance CAG repeat expansion in the HD gene is unlikely to contribute to the patient’s presentation at this time, as an allele of this size would not typically be symptomatic at this young age [Quarrell et al., 2007]. Juvenile Huntington disease usually involves more than 55 repeats and is initially characterized by behavioral disturbances and learning disorders. Bradykinetic or hypokinetic movement disorders are the most common early motor disturbance [Roos, 2010]. Symptomatic patients usually have brain atrophy, most prominent in the caudate and putamen. The patient’s lack of these clinical and imaging findings argues against a contribution of HD in the overall clinical presentation at this point in time. Taken together, these data suggest that the juvenile-onset SCA in our patient may be secondary to haploinsufficiency of the AFG3L2 gene. AFG3L2 encodes an m-AAA protease subunit, forming the protease either as a homo-oligomer or as a hetero-oligomer with paraplegin as the other protein component [Di Bella et al., 2010]. This m-AAA protease resides in the mitochondrial inner membrane and is responsible for the removal of damaged or misfolded proteins as well as proteolytic activation of essential mitochondrial proteins [Arlt et al., 1996; Martinelli et al., 2009; Almajan et al., 2012]. AFG3L2 expression is particularly high in cerebellar Purkinje cells [Di Bella et al., 2010], accounting for the associated clinical presentation when this protein is dysfunctional. Heterozygous mutations in AFG3L2 result in SCA28, with the majority of clinical reports thus far involving missense changes [Cagnoli et al., 2010; Di Bella et al., 2010; Edener et al., 2010, Lobbe et al., 2014]. Functional studies suggest that the mechanism of the missense mutations may vary depending on the mutation and be either dominant negative or haploinsufficient in their effect [Di Bella et al., 2010]. Haploinsufficient mouse models exhibit progressive late-onset cerebellar degeneration [Maltecca et al., 2009]. In addition, a novel frameshift mutation in AFG3L2 has recently been reported associated with a late-onset SCA in one family [Musova et al., 2013]. The family described by Musova and colleagues [2013] and the patient reported here provides further evidence that haploinsufficiency contributes, at least in part, to the disease mechanism of SCA28.
3211 Haploinsufficiency of AFG3L2 would be expected in a subset of patients with deletions involving 18p spanning this locus, although progressive ataxia is not a recognized feature of 18p monosomy syndrome and instead the main neurologic abnormalities are hypotonia and myoclonus-dystonia [Nasir et al., 2006; Graziadio et al., 2009; Postma et al., 2009; Kowarik et al., 2011]. However, many of the previously reported patients with monosomy 18p [Turleau, 2008] were only investigated with karyotype or BAC arrays, making genotype/phenotype correlation difficult. In addition, progressive ataxia in monosomy 18p may be under-reported secondary to age of the patients at examination or severe psychomotor delay in older patients such that subtle signs of cerebellar dysfunction may be missed. In summary, the patient’s neurologic presentation is likely ultimately explained by a complex constellation of genomic anomalies including a deletion of 18p (involving the gene for SCA28), Klinefelter syndrome, and a second, mosaic distal 18p deletion. This, to our knowledge, is the first published report of progressive cerebellar ataxia associated with a heterozygous deletion of AFG3L2. Further experimental data is needed to provide additional evidence that AFG3L2 haploinsufficiency is associated with the development of the SCA28 phenotype.
REFERENCES Almajan ER, Richter R, Paeger L, Martinelli P, Barth E, Decker T, Larsson NG, Kloppenburg P, Langer T, Rugarli EI. 2012. AFG3L2 supports mitochondrial protein synthesis and Purkinje cell survival. J Clin Invest 122:4048–4058. Arlt H, Tauer R, Feldmann H, Neupert W, Langer T. 1996. The YTA10-12 complex, an AAA protease with chaperone-like activity in the inner membrane of mitochondria. Cell 85:875–885. Cagnoli C, Mariotti C, Taroni F, Seri M, Brussino A, Michielotto C, Grisoli M, Di Bella D, Migone N, Gellera C, Di Donato S, Brusco A. 2006. SCA28, a novel form of autosomal dominant cerebellar ataxia on chromosome 18p11.22-q11.2. Brain 129:235–242. Cagnoli C, Stevanin G, Brussino A, Barberis M, Mancini C, Margolis RL, Holmes SE, Nobili M, Forlani S, Padovan S, Pappi P, Zaros C, Leber I, Ribai P, Pugliese L, Assalto C, Brice A, Migone N, Durr A, Brusco A. 2010. Missense mutations in the AFG3L2 proteolytic domain account for approximately 1.5% of European autosomal dominant cerebellar ataxias. Hum Mutat 31:1117–1124. Di Bella D, Lazzaro F, Brusco A, Plumari M, Battaglia G, Pastore A, Finardi A, Cagnoli C, Tempia F, Frontali M, Veneziano L, Sacco T, Boda E, Brussino A, Bonn F, Castellotti B, Baratta S, Mariotti C, Gellera C, Fracasso V, Magri S, Langer T, Plevani P, Di Donato S, Muzi-Falconi M, Taroni F. 2010. Mutations in the mitochondrial protease gene AFG3L2 cause dominant hereditary ataxia SCA28. Nat Genet 42:313–321. Edener U, Wollner J, Hehr U, Kohl Z, Schilling S, Kreuz F, Bauer P, Bernard V, Gillessen-Kaesbach G, Zuhlke C. 2010. Early onset and slow progression of SCA28, a rare dominant ataxia in a large four-generation family with a novel AFG3L2 mutation. Eur J Hum Genet 18:965–968. Graziadio C, Rosa RF, Zen PR, Pinto LL, Barea LM, Paskulin GA. 2009. Dystonia, autoimmune disease and cerebral white matter abnormalities in a patient with 18p deletion. Arq Neuropsiquiatr 67:689–691. Kowarik MC, Langer S, Keri C, Hemmer B, Oexle K, Winkelmann J. 2011. Myoclonus-dystonia in 18p deletion syndrome. Mov Disord 26:560–561.
3212 Lanfranco F, Kamischke A, Zitzmann M, Nieschlag E. 2004. Klinefelter’s syndrome. Lancet 364:273–283. Lo¨bbe AM, Kang JS, Hilker R, Hackstein H, Mu¨ller U, Nolte D. 2014. A novel missense mutation in AFG3L2 associated with late onset and slow progression of spinocerebellar ataxia type 28. J Mol Neurosci 52: 493–496. Maltecca F, Magnoni R, Cerri F, Cox GA, Quattrini A, Casari G. 2009. Haploinsufficiency of AFG3L2, the gene responsible for spinocerebellar ataxia type 28, causes mitochondria-mediated Purkinje cell dark degeneration. J Neurosci 29:9244–9254. Mariotti C, Brusco A, Di Bella D, Cagnoli C, Seri M, Gellera C, Di Donato S, Taroni F. 2008. Spinocerebellar ataxia type 28: A novel autosomal dominant cerebellar ataxia characterized by slow progression and ophthalmoparesis. Cerebellum 7:184–188. Martinelli P, La Mattina V, Bernacchia A, Magnoni R, Cerri F, Cox G, Quattrini A, Casari G, Rugarli EI. 2009. Genetic interaction between the m-AAA protease isoenzymes reveals novel roles in cerebellar degeneration. Hum Mol Genet 18:2001–2013. Musova Z, Kaiserova M, Kriegova E, Fillerova R, Vasovcak P, Santava A, Mensikova K, Zumrova A, Krepelova A, Sedlacek Z, Kanovsky P. 2013. A novel frameshift mutation in the AFG3L2 gene in a patient with spinocerebellar ataxia. 2014. Cerebellum 13:331–337.
AMERICAN JOURNAL OF MEDICAL GENETICS PART A Nasir J, Frima N, Pickard B, Malloy MP, Zhan L, Grunewald R. 2006. Unbalanced whole arm translocation resulting in loss of 18p in dystonia. Mov Disord 21:859–863. Pierson TM, Adams D, Bonn F, Martinelli P, Cherukuri PF, Teer JK, Hansen NF, Cruz P, Mullikin JC. for the NISC Comparative Sequencing Program. Blakesley RW, Golas G, Kwan J, Sandler A, Fuentes Fajardo K, Markello T, Tifft C, Blackstone C, Rugarli EI, Langer T, Gahl WA, Toro C, 2011. Whole-exome sequencing identifies homozygous AFG3L2 mutations in a spastic ataxia-neuropathy syndrome linked to mitochondrial m-AAA proteases. PLoS Genet 7:e1002325. Postma AG, Verschuuren-Bemelmans CC, Kok K, van Laar T. 2009. Characteristics of dystonia in the 18p deletion syndrome, including a new case. Clin Neurol Neurosurg 111:880–882. Quarrell OW, Rigby AS, Barron L, Crow Y, Dalton A, Dennis N, Fryer AE, Heydon F, Kinning E, Lashwood A, Losekoot M, Margerison L, McDonnell S, Morrison PJ, Norman A, Peterson M, Raymond FL, Simpson S, Thompson E, Warner J. 2007. Reduced penetrance alleles for Huntington’s disease: A multi-centre direct observational study. J Med Genet 44: e68. Roos RA. 2010. Huntington’s disease: A clinical review. Orphanet J Rare Dis 5:40. Turleau C. 2008. Monosomy 18p. Orphanet J Rare Dis 3:4.
Deletion of AFG3L2 Associated With Spinocerebellar Ataxia Type 28 in the Context of Multiple Genomic Anomalies Kenneth A. Myers,1 Jodi Warman Chardon,2 Lijia Huang,2 and Kym M. Boycott2* 1
Division of Neurology, Department of Pediatrics, Alberta Children’s Hospital, University of Calgary, Calgary, Alberta, Canada
2
Department of Genetics, Children’s Hospital of Eastern Ontario, University of Ottawa, Ottawa, Ontario, Canada
Manuscript Received: 22 September 2013; Manuscript Accepted: 20 August 2014
TO THE EDITOR: The spinocerebellar ataxias (SCA) are a genetically and clinically heterogeneous group of disorders characterized by progressive ataxia, gait disturbance, and dysarthria. Of these, spinocerebellar ataxia type 28 (SCA28; OMIM 610246) is one of the less common, comprising an estimated 1.5% of the autosomal dominant progressive SCAs [Cagnoli et al., 2010]. SCA28 was originally characterized in two Italian families exhibiting an autosomal dominantly inherited pattern of juvenile-onset, slowly progressive ataxia with associated ophthalmoplegia, gaze-evoked nystagmus, ptosis, and normal intelligence [Cagnoli et al., 2006; Mariotti et al., 2008]. The causative gene for SCA28 has been identified as ATPase family gene 3-like 2 (AFG3L2), located on the short arm of chromosome 18 (18p11) [Di Bella et al., 2010]. Thus far, the majority of pathogenic variants reported are heterozygous missense mutations primarily in exons 10, 15, and 16 [Cagnoli et al., 2010; Di Bella et al., 2010; Edener et al., 2010; Lo¨bbe et al., 2014]. A homozygous missense mutation in exon 15 of AFG3L2 results in spastic ataxia type 5 (SPAX5; OMIM 614487) [Pierson et al., 2011]. Here, we present the first case of progressive SCA associated with a deletion encompassing the entire AFG3L2 gene, suggesting haploinsufficency as a possible underlying mechanism for SCA28. A 23-year-old man with global developmental delay from infancy developed progressive cerebellar ataxia beginning at 13 years of age. He sat at 1 year and could ambulate with a walker by 8 years of age. Language development was also delayed, with expressive language more affected than receptive language, and he spoke in two to three word sentences in late childhood. He continued to slowly acquire developmental milestones until 13 years of age, when the onset of progressive neurologic deterioration was noted. Over the next years, his balance and gait gradually became increasingly impaired. His gait was wide-based with significant retropulsion early in the course of his deterioration. By the age of 18 years, he could no longer ambulate with a walker and depended on crawling and a wheelchair for mobility. He developed frequent choking episodes secondary to dysphagia. Fine motor coordination declined concurrently, necessitating more help with dressing and
Ó 2014 Wiley Periodicals, Inc.
How to Cite this Article: Myers KA, Warman Chardon J, Huang L, Boycott KM. 2014. Deletion of AFG3L2 associated with spinocerebellar ataxia type 28 in the context of multiple genomic anomalies. Am J Med Genet Part A 164A:3209–3212.
other self-care tasks. Over the same period of time, there was no appreciable decline in cognitive abilities. He had no events suggestive of seizures. When re-assessed at the age of 23 years, he understood simple commands and spoke in short sentences; however, severe dysarthria rendered his speech almost unintelligible. Eye movements were saccadic with end-gaze nystagmus and he was noted to have mild bilateral ptosis. Tone was decreased with normal muscle bulk, power, reflexes, and sensory exam. Dysmetria was present on finger-to-nose testing and he had severe dysdiadochokinesis when attempting rapid alternating movements. His general examination was remarkable for hypertelorism and tapered fingers with 5th finger clinodactyly. There were no signs of abnormal hair distribution. Cytogenetic testing at 1 year of age revealed an abnormal karyotype with an additional X chromosome and a telomeric fusion of one chromosome 18 and the Y chromosome [46,XX, t(Y;18) (p11?;p11?)]. This chromosomal abnormality constituted a diagnosis of Klinefelter syndrome, however, it was unclear whether loss and/or gain of chromosomal materials in 18p and Yp were involved
Correspondence to: Kym Boycott, Department of Genetics, Children’s Hospital of Eastern Ontario, University of Ottawa, 401 Smyth Road, Ottawa, ON, K1H 8L1, Canada. E-mail: [email protected] Article first published online in Wiley Online Library (wileyonlinelibrary.com): 23 September 2014 DOI 10.1002/ajmg.a.36771
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FIG. 1. Sagittal T1-weighted (A) and axial T2-weighted (B) MR images of the brain at 16 years of age demonstrating cerebellar atrophy.
during the telomeric fusion, and therefore the contribution of this t(Y;18) to his developmental delay was inconclusive at that time. Karyotypes for both parents were normal. The family history was significant for Huntington disease (HD; OMIM 143100); the patient’s father was diagnosed at 51 years of age and carried a HD-causing allele of 46 CAG repeats. The patient was tested and found to have an expansion of 39 CAG repeats, which is within the reduced-penetrance range. To investigate the cause of his progressive cerebellar ataxia, brain MRI with spectroscopy was performed at 16 years of age and demonstrated mild cerebellar atrophy (Fig. 1) and a normal cerebrum with neither basal ganglia changes characteristic of Huntington disease nor signs of holoprosencephaly. SCA types 1, 2, 3, 6, 7, 8, 17, and DRPLA, as well as the more common autosomal recessive ataxias (APTX, SETX, POLG1, SIL1, TTPA, and FXN) were excluded by clinical testing (Sanger sequencing and repeat expansion analysis). Screening metabolic studies were normal including lactate, ammonia, plasma amino acids, and urine organic acids. When he was 17 years old, a BAC array (Signature Genomics) that covers 622 loci across the genome, including the holoprosencephaly locus TGIF at the end of 18p, was performed. An extra X chromosome, as well as a distal deletion of 18p (deletion 1), were detected [arr cgh Xp22.33q28 (RP11-1325A17 >RP11-1137B3) 2, 18p11.32p11.31(RP11-683L23 >RP11-835E18) 1] (Fig. 2). The follow-up fluorescence in situ hybridization (FISH) analysis confirmed the presence of a dicentric chromosome derived from the fusion of 18p and Yp (ish der(Y;18)(p11.2;p11.32)(RP11838N2-)[11]/der(Y;18)(p11.2;p11.32)(RP11-838N2þ)[19]). In 19/30 cells analyzed, the TGIF locus was present on the derivative chromosome, and the rearrangement was apparently balanced; while the remainder of the cells showed no TGIF signal on the der(Y;18), indicating the distal 18p deletion is in a mosaic pattern. However, the proximal boundary of this 18p deletion was not known as the nearest proximal BAC clone that was not deleted was 8.4 Mb away. Nevertheless, the Klinefelter syndrome and the mosaic partial monosomy 18p was thought to explain his global developmental delay, while the genetic cause for his
progressive cerebellar ataxia remained unclear. Subsequently, SCA28 was mapped to the end of chromosome 18p (AFG3L2, chr18:12,328,943–12,377,275; hg19). This prompted further investigation of the 18p deletion using a high resolution oligonucleotide chromosomal microarray (Agilent 4 180 K). This microarray analysis detected the presence of a mosaic distal 18p deletion of 6.72 Mb in length including OMIM Morbid Map genes LPIN2 and TGIF (Fig. 2; deletion 1). This result was consistent with the findings from the previous BAC array and FISH analysis. However, a second
FIG. 2. Graphic representation of the cytogenetic findings in the patient reported here. The left t(Y;18) contains only deletion 2 [5.3 Mb, containing 33 RefSeq genes including 3 OMIM Morbid Map genes (NDUFV2, APCDD1, AFG3L2)] and is present in 19/30 cells examined. The right t(Y;18) contains deletion 1 [6.72 Mb, containing 38 RefSeq genes including 2 OMIM Morbid Map genes (LPIN and TGIF)] as well as deletion 2 and is present in 11/30 cells examined.
MYERS ET AL. nonmosaic deletion of 5.3 Mb at 18p11.23-p11.21 (Fig. 2; deletion 2) was also identified [arr Xp22.33q28(60,701–155,246,644) 2, 18p11.32p11.31(14,275–6,731,478) 1 2, 18p11.23p11.21 (8,253,838–13,557,376) 1; based on hg19]. Interestingly, the AFG3L2 gene, responsible for SCA28, is within deleted region 2, suggesting that heterozygous loss of this gene may contribute to his progressive cerebellar ataxia. The patient reported here presents with a complex set of genomic anomalies, moderate global developmental delay from infancy, and juvenile-onset progressive cerebellar ataxia. The latter deterioration is clinically consistent with the few previously described cases of SCA28 caused by missense mutations in AFG3L2. However, in this case, we believe that the cerebellar ataxia is the result of a heterozygous deletion of AFG3L2. The patient’s early cognitive, language and motor delays, as well as mild dysmorphic features, are likely due to partial 18p monosomy [Turleau, 2008] and Klinefelter syndrome [Lanfranco et al., 2004]. Several other genes in the mosaic and nonmosaic deleted 18p regions are known to be disease-causing but have not been associated with progressive ataxia. The reduced penetrance CAG repeat expansion in the HD gene is unlikely to contribute to the patient’s presentation at this time, as an allele of this size would not typically be symptomatic at this young age [Quarrell et al., 2007]. Juvenile Huntington disease usually involves more than 55 repeats and is initially characterized by behavioral disturbances and learning disorders. Bradykinetic or hypokinetic movement disorders are the most common early motor disturbance [Roos, 2010]. Symptomatic patients usually have brain atrophy, most prominent in the caudate and putamen. The patient’s lack of these clinical and imaging findings argues against a contribution of HD in the overall clinical presentation at this point in time. Taken together, these data suggest that the juvenile-onset SCA in our patient may be secondary to haploinsufficiency of the AFG3L2 gene. AFG3L2 encodes an m-AAA protease subunit, forming the protease either as a homo-oligomer or as a hetero-oligomer with paraplegin as the other protein component [Di Bella et al., 2010]. This m-AAA protease resides in the mitochondrial inner membrane and is responsible for the removal of damaged or misfolded proteins as well as proteolytic activation of essential mitochondrial proteins [Arlt et al., 1996; Martinelli et al., 2009; Almajan et al., 2012]. AFG3L2 expression is particularly high in cerebellar Purkinje cells [Di Bella et al., 2010], accounting for the associated clinical presentation when this protein is dysfunctional. Heterozygous mutations in AFG3L2 result in SCA28, with the majority of clinical reports thus far involving missense changes [Cagnoli et al., 2010; Di Bella et al., 2010; Edener et al., 2010, Lobbe et al., 2014]. Functional studies suggest that the mechanism of the missense mutations may vary depending on the mutation and be either dominant negative or haploinsufficient in their effect [Di Bella et al., 2010]. Haploinsufficient mouse models exhibit progressive late-onset cerebellar degeneration [Maltecca et al., 2009]. In addition, a novel frameshift mutation in AFG3L2 has recently been reported associated with a late-onset SCA in one family [Musova et al., 2013]. The family described by Musova and colleagues [2013] and the patient reported here provides further evidence that haploinsufficiency contributes, at least in part, to the disease mechanism of SCA28.
3211 Haploinsufficiency of AFG3L2 would be expected in a subset of patients with deletions involving 18p spanning this locus, although progressive ataxia is not a recognized feature of 18p monosomy syndrome and instead the main neurologic abnormalities are hypotonia and myoclonus-dystonia [Nasir et al., 2006; Graziadio et al., 2009; Postma et al., 2009; Kowarik et al., 2011]. However, many of the previously reported patients with monosomy 18p [Turleau, 2008] were only investigated with karyotype or BAC arrays, making genotype/phenotype correlation difficult. In addition, progressive ataxia in monosomy 18p may be under-reported secondary to age of the patients at examination or severe psychomotor delay in older patients such that subtle signs of cerebellar dysfunction may be missed. In summary, the patient’s neurologic presentation is likely ultimately explained by a complex constellation of genomic anomalies including a deletion of 18p (involving the gene for SCA28), Klinefelter syndrome, and a second, mosaic distal 18p deletion. This, to our knowledge, is the first published report of progressive cerebellar ataxia associated with a heterozygous deletion of AFG3L2. Further experimental data is needed to provide additional evidence that AFG3L2 haploinsufficiency is associated with the development of the SCA28 phenotype.
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