More than an ‘atypical’ phenotype: dual molecular diagnosis of autoimmune lymphoproliferative syndrome and Becker muscular dystrophy

Abstract

Inborn errors of immunity (IEI) are rare and clinically and genetically heterogeneous. Often only a few patients are described for each disorder, making it difficult to rely on conclusive phenotype descriptions. Dual molecular diagnoses or multilocus genomic variations have recently been described in up to 5% of patients undergoing whole exome sequencing.1 Several studies have suggested that up to 10% of primary immunodeficiency diseases (PIDs) may have dual molecular diagnoses.2 Identification of dual molecular diagnoses may enable early protective interventions and potentially targeted or curative therapy, preventing morbidity and mortality. We describe a male patient with the simultaneous presence of two monoallelic variants. As a toddler, a somatic heterozygous variant in FAS led to the diagnosis of autoimmune lymphoproliferative syndrome (ALPS). Subsequently, he developed clinical signs and symptoms that raised suspicion of muscle involvement and finally Becker muscular dystrophy (BMD) was diagnosed following the detection of a hemizygous deletion in the dystrophin (DMD) gene. A 6-month-old boy was referred to our paediatric haematology clinic due to splenomegaly (spleen diameter 9 cm) and lymphadenopathies. Lymphocyte subsets revealed markedly increased CD3+TCRαβ+CD4−CD8− (23·3/30·9% of total lymphocytes/CD3+ lymphocytes). Blood tests showed elevated vitamin B12 (3034 pg/ml, normal values <663 pg/ml) and immunoglobulin G (IgG) levels (25780 mg/l, age-matched normal values 3510–9190 mg/l) (Table I). FAS-mediated apoptosis in T-cell blasts, assessed via CD95 [Fas/apoptosis antigen 1 (APO-1)] ligand (CD95L) stimulation as previously described,3 was defective on two different occasions. Inguinal lymph node biopsy showed paracortical expansion due to polyclonal CD3+TCRαβ+CD4−CD8− cells infiltration (Fig 1A). Genetic analysis of known ALPS-associated genes, performed by targeted next-generation sequencing, showed the presence of the novel c.576_576delA;p.Lys193Argfs*23 FAS variant in 15·48% of the mononuclear cells. Genomic DNA was extracted from buccal epithelial cells and Sanger sequencing did not reveal the presence of the FAS variant (Fig 1B). ALPS due to somatic FAS variant (ALPS-sFAS) was diagnosed. Sirolimus was started, with a decrease in spleen size. At the age of 4 years, while sirolimus was still being administered, the patient was admitted due to hip and shoulder pain, fatigability and generalised weakness in the setting of negative infectious studies and autoimmune evaluation. Lymphoproliferation (splenomegaly and inguinal lymphadenopathies) was still present. Blood tests showed elevated concentrations of creatine kinase (CK, up to 1575 u/l), liver functional tests [aspartate transaminase (AST)/alanine aminotransferase (ALT) 75/35 u/l], erythrocyte sedimentation rate (89 mm/h) and lactate dehydrogenase (607 u/l; Table I]. Neurological examination showed normal muscle tone, strength and tendon reflexes, absence of calf hypertrophy and negative Gowers’ sign. In consideration of subsequent high CK values in repeated samples, a muscle biopsy was taken with the aim of investigating a possible dystrophinopathy, the most frequent cause of CK elevation in male subjects. The histological study revealed the presence of increased connective perimysial tissue with degeneration in a small number of muscle fibres. DMD expression was investigated by means of different antibodies. Immunostaining was reduced for carboxyl- and rod-domain, severely reduced for the amino-terminal domain and absent for D8. Multiplex ligation-dependent probe amplification assay showed a deletion of exon 48 of the DMD gene leading to the diagnosis of BMD. The genetic analysis in the mother showed the same alteration, allowing the diagnosis of DMD gene variant carrier. Therefore, we concluded that there was a combination of two different disorders in the same patient. Somatic FAS variants account for up to 20% of ALPS cases.4 ALPS is characterised by early onset, chronic, non-malignant lymphoproliferation, autoimmune manifestations (mainly cytopenia) and susceptibility to lymphoma. In this context, the presence of ‘atypical’ phenotypic manifestations can represent a diagnostic challenge. Muscle symptoms were initially considered to be part of the autoimmune response in the setting of ALPS. Although the co-existence of muscle weakness and autoimmune diseases is well recognised,5 the association with the presence of FAS variants has not been fully investigated. Only one report has described muscle involvement in one patient with ALPS with a caspase 10 (CASP10) variant.6 Symptoms were deemed ‘atypical’ and no further investigations were performed. We considered the possibility that more than one gene could be contributing to our patient's phenotype. The maternally inherited deletion in the DMD gene explained the muscle phenotype and allowed familiar genetic counselling and targeted follow-up. Dystrophinopathies are a spectrum of progressive X-linked muscle diseases caused by genetic lesions that alter the production or function of the cytoskeletal muscle protein DMD. Their clinical spectrum covers progressive skeletal muscle weakness, cardiomyopathy, muscle cramps and myalgia.7 BMD refers to the milder and variable phenotype of the dystrophinopathies, resulting from variants that preserve the open reading frame of the DMD gene.8 In 2014, Casanova et al.9 reported that 21% of the 232 monogenic PIDs were first reported on the basis of a single case. The 2019 International Union of Immunological Societies Classification of IEI updated the number of immunological diseases to 404, with 430 known genetic defects.10 Compared to the previous classification,11 in just 2 years the list has increased by 50 distinct disorders and 86 different gene defects. Variants in known and novel genes identified additional phenotypes, although the rarity of these disorders hampers the possibility to clearly define a specific phenotypic spectrum. In this context, the presence of derangement from the ‘classic’ phenotype should also prompt consideration of an additional genetic diagnosis, rather than automatically considering the case as ‘atypical’. In our present case, muscle symptoms misleadingly suggested an expansion of the ALPS phenotype, while the dual molecular diagnosis confirmed a combination of two independent diagnoses with the muscle involvement representing a distinct phenotype clearly attributable to BMD (Fig 1C). Two distinct Mendelian diseases may be phenotypically blended and overlapping, as either genotype can cause the phenotype.1, 12 An example of this latter possibility has been described in one family (Fig 1C). Three male subjects had immunodeficiency, leukaemia, lymphoma, anaemia and solid tumours. They all shared a SH2 domain-protein 1A (SH2D1A) variant causing X-linked lymphoproliferative syndrome (XLP) and an additional variant in Fanconi anaemia complementation group B (FANCB), the gene for X-linked Fanconi anaemia.2 Both XLP and Fanconi anaemia are cancer predisposition syndromes. Clinicians have been trained to identify the most evident single explanation for a patient's clinical symptoms. Although challenging cases in rare diseases may ultimately exhibit one disease phenotype, data from large cohorts have revealed that a significant proportion of patients carry two or more Mendelian diseases.1 The concomitant presence of two rare diseases like ALPS and BMD in a single patient is logically assumed to be coincidental, but the possibility of being affected with two inherited genetic conditions should be considered when findings are incoherent with the primary diagnosis. Moreover, our present case highlights that BMD should be suspected and excluded in every male child with a significant increase in CK, with or without associated muscle pain or cramps. We suggest that patients with rare diseases, even if molecularly defined, may benefit from pursuing additional genetic diagnoses. One initial molecular diagnosis does not always preclude the possibility of diagnostic re-evaluation, revealing more than one Mendelian disease in the same individual.

Topics & Concepts