Molecular Analysis Confirms that FKRP-Related Disorders are Underdiagnosed in Mexican Patients with Neuromuscular Diseases

Abstract

The FKRP-related disorders include the limb-girdle muscular dystrophy type 2I (LGMD2I, MIM#607155) which has a clinical overlap with dystrophinopathies. Except for Brazil, LGMD2I has not been described in other Latin-American countries, despite that 1/64 Mexican-ancestry individuals carry the commonest European p.(Leu276Ile) pathogenic FKRP variant (rs28937900, 1000 Genomes Project Phase 3), suggesting an underdiagnosis. Sequencing of exon 4 of FKRP in 60 unrelated Mexican patients with presumptive diagnoses of dystrophinopathy without DMD gene deletions, or with a neuromuscular disorder of unknown etiology, revealed two patients (2/60, 3.3%): one of them with an unreported pathogenic genotype, p.[Leu276Ile];[Asn463Asp] and a Duchenne-like phenotype; and the other that stands as the third reported case bearing the severe genotype, p.[Asn463Asp];[Asn463Asp] leading to a congenital presentation (MIM#606612). We identified two patients heterozygous for the p.(Arg143Ser) and the novel p.(Arg48Pro) variants, respectively, which were absent in 200 control alleles. Protein modeling suggests that p.(Arg143Ser) is a tolerated change, while p.(Arg48Pro) induces a structural modification, although further molecular confirmation of dystrophinopathy was obtained in the p.(Arg48Pro) patient. Our results justify the inclusion of FKRP-related disorders in the differential diagnosis of Mexican neuromuscular patients. The commonly reported heterozygous FKRP genotypes deserve attention to avoid LGMD2I molecular misdiagnosis.

Predominance of Dystrophinopathy Genotypes in Mexican Male Patients Presenting as Muscular Dystrophy with A Normal Multiplex Polymerase Chain Reaction DMD Gene Result: A Study Including Targeted Next-Generation Sequencing

“…We did not identify the founder LGMD2A or R1-causing CAPN3 pathogenic variant, c.384C > A or p.(Ala116Asp), or the non-founder micro indel CAPN3 mutation described in patients belonging to an endogamic region of Tlaxcala, Mexico [11]. The previously reported FKRP p.[Leu276Ile];[Asn463Asp] heterozygous compound genotype, which to date has been described only in LGMD2I patients of Mexican/Hispanic descent [13,18,19], was herein identified in one non-consanguineous LMGD family (DMD-786), which remarkably was referred with two affected siblings that died during the second and third decade of life; this was attributed to dilated cardiomyopathy, which was corroborated by autopsy in DMD-786. Although cardiomyopathy and other cardiac disturbances are expected in half of LGMD2I or R9 patients [4], dilated cardiomyopathy was not previously described in the five reported Mexican/Hispanic patients bearing the p.[Leu276Ile];[Asn463Asp] FKRP genotype [13,19].…”

“…The previously reported FKRP p.[Leu276Ile];[Asn463Asp] heterozygous compound genotype, which to date has been described only in LGMD2I patients of Mexican/Hispanic descent [13,18,19], was herein identified in one non-consanguineous LMGD family (DMD-786), which remarkably was referred with two affected siblings that died during the second and third decade of life; this was attributed to dilated cardiomyopathy, which was corroborated by autopsy in DMD-786. Although cardiomyopathy and other cardiac disturbances are expected in half of LGMD2I or R9 patients [4], dilated cardiomyopathy was not previously described in the five reported Mexican/Hispanic patients bearing the p.[Leu276Ile];[Asn463Asp] FKRP genotype [13,19]. However, decreased ejection fractions on echocardiogram were noted for two of these patients (at 21 and 22 years of age) [19], suggesting that the cardiological phenotype for patients with this compound heterozygous FKRP genotype has not yet been fully delineated.…”

“…Thus, our study yielded a relatively high diagnostic success rate for NGS (67.5%, n = 27/40) compared to the previous reports, even though we analyzed only a small number of muscular dystrophy-related loci ( n = 11). This may reflect: (a) gender sample bias that favors the identification of dystrophinopathy-related genotypes, especially in male patients at pediatric age (73% of our patients were < 18 years of age); (b) our gene selection criteria, referred to the muscular dystrophy frequencies obtained from muscle immunoanalysis of Mexican patients [dystrophinopathies (52.4%), sarcoglycanopathies (14.1%) and calpaino and caveolinopathies (12.7%)] [12]; and (c) the previous descriptions of LGMD genotypes in Mexican patients [10,11,13,18,19].…”

“…NGS for DMD and the LGMD-related genes was performed by Admera Health (NJ, USA, ). The LGMD genes were selected based on their frequencies identified by immunoanalysis of muscle biopsies from Mexican patients with suspected muscular dystrophies ( CAPN3, DYSF, SGCG, SGCB, SGCA, SGCD and CAV3 ) [12], evidence for a founder effect in our population ( CAPN3 [11] and FKRP [13,18,19]), difficulties performing immunological assessment of the encoded protein in muscle biopsies ( ANO5, CAPN3 and FKRP ) [4,12], inaccuracy in the results of the immunological assessment ( CAPN3 ) [20] or evident overlap of clinical manifestations with DMD/BMD, such as the predominance of childhood onset for proximal or lower muscle weakness, calf pseudohypertrophy, hyperCKemia or cardiomyopathy, as in the cases of TCAP- or FKRP -related disorders and sarcoglycanopathies [1,2,4].…”

“…However, their responsible genotypes were not assessed. Our group recently reported that the FKRP -related disorders, which were not evaluated in the study by Gómez-Díaz et al [12], underlie the genetic etiology of nearly 3% of Mexican patients with a neuromuscular disorder of unknown etiology [13].…”