Clinical Description
ASAH1-related disorders comprise a spectrum that ranges from Farber disease (FD) to spinal muscular atrophy (SMA) with or without epilepsy. ASAH1-related disorders vary in the age of onset of manifestations, the systems affected, and severity and progression of the disease. While Farber disease has been recognized clinically and diagnosed for decades based on enzyme analysis [Farber 1952, Abul-Haj et al 1962], the recognition of ASAH1-related SMA and associated findings is a recent discovery based on the use of genomic testing; thus, the understanding of the latter ASAH1-related phenotype is still evolving.
Furthermore, although to date SMA-PME and FD have been considered to be two distinct phenotypes with differences in age of onset and primary involvement of different organ systems, a girl with features of both phenotypes illustrates the phenotypic continuum of ASAH1-related disorders that is possible [Teoh et al 2016]. The individual presented at age three years with polyarticular arthritis (without subcutaneous nodules) followed by progressive motor neuron disease without seizures. At age seven years she developed cognitive deficits and a hoarse voice.
Farber Disease (FD)
Farber disease in its classic form is an early-onset, progressive, and fatal disease. With better understanding of the natural history of FD over time, investigators have suggested categorization into several types based on age of onset, severity, and primary manifestations [Levade et al 2009]. Nonetheless, these Farber disease phenotypes can realistically be considered part of a continuum.
Type 1 FD (classic FD) is characterized by the triad of (1) painful, progressive deformity of the joints of the elbows, wrists, hands, knees, and feet; (2) palpable subcutaneous nodules that tend to occur at joints and mechanical pressure points, but can occur elsewhere; and (3) a hoarse cry resulting from granulomas of the larynx and epiglottis. These findings often manifest in the first weeks of life.
Neurologic involvement, reported in a significant proportion of children, can be difficult to assess given the extent of contractures and joint deformity. Many children with type 1 FD have a lower motor neuron disease that manifests as hypotonia and muscle atrophy; EMG studies show chronic denervation [Levade et al 2009]. A minority of children can have infantile spasms [Levade et al 2009].
Other features can include a cherry red spot of the macula [Cogan et al 1966].
Infiltrative pulmonary disease causes respiratory insufficiency that typically results in death before age two years [Ehlert et al 2007, Levade et al 2009].
Type 2 FD ("Intermediate FD") is characterized by age of onset of approximately eight months. Although the classic triad is present, the neurologic involvement is considered less severe than that of type 1 FD [Burck et al 1985, Al Jasmi 2012, Chedrawi et al 2012, Kostik et al 2013]. Seizures become relatively more common over time [Levade et al 2009].
Life expectancy is to mid-childhood.
Type 3 FD (mild FD) is characterized by joint swelling, pain, and contractures with onset after age one year. Many of these children will be mistakenly diagnosed with a juvenile idiopathic arthritis [Schuchman 2014]. Neurologic findings are observed in approximately half of affected children [Levade et al 2009]. Approximately 30% (8/25) of individuals with either type 2 FD or type 3 FD show marked cognitive deficits (IQ<80).
Life expectancy is into the teen years [Samuelsson & Zetterström 1971, Pavone et al 1980, Fiumara et al 1993].
Type 4 FD (neonatal-visceral FD) is characterized by neonatal severe hepatosplenomegaly without the classic triad [Cartigny et al 1985, Qualman et al 1987, Kattner et al 1997].
Death occurs within the first days to weeks of life.
Type 5 FD (neurologic FD) is characterized by six to 12 months of normal development followed by refractory seizures, progressive paraparesis, and speech regression [Eviatar et al 1986, Jameson et al 1987].
Subcutaneous nodules can be present, but are usually mild. Lung infiltration and hepatosplenomegaly do not occur.
Diagnosis of FD prior to the availability of ASAH1 molecular genetic testing. The diagnosis of FD was formerly based on histologic and biochemical findings in individuals with a typical clinical presentation [Levade et al 2009]. Biopsy of the subcutaneous nodules can show characteristic curvilinear granulomatous infiltrations or "Farber bodies" under light microscopy in macrophages, histiocytes, foam cells, and fibroblasts [Schmoeckel 1980, Burck et al 1985].
Spinal Muscular Atrophy with Progressive Myoclonic Epilepsy (SMA-PME)
SMA-PME is characterized by early-childhood onset of progressive proximal weakness followed by progressive myoclonic and atonic seizures, tremulousness/tremor, and sensorineural hearing loss [Zhou et al 2012, Gan et al 2015, Topaloglu & Melki 2016].
Lower motor neuron disease, the first manifestation in the majority of affected individuals (16/17), is typically evident as weakness between ages three and seven years (median 5 years; range 17 months [Rubboli et al 2015] to 15 years [Dyment et al 2014]). Initially weakness is proximal, and progresses from initial clumsiness/frequent falls to a waddling gait and need for assistive devices for walking.
The lower motor neuron disease also involves the muscles of respiration; thus, recurrent aspiration pneumonias are common (6/16).
Epilepsy. Although seizures often begin in late childhood, after the onset of weakness, exceptions occur [Filosto et al 2016, Topaloglu & Melki 2016].
Myoclonic seizures, which begin as jerking of the upper limbs, are more proximal than distal. Action myoclonus, myoclonic status, and eyelid myoclonus have also been reported [Rubboli et al 2015, Oguz Akarsu et al 2016]. Progressive increase in frequency of the myoclonus contributes significantly to the decreasing motor function [Dyment et al 2014, Rubboli et al 2015].
Atonic seizures of the head and/or torso are also an early presenting seizure type.
Absence seizures are observed in more than half of affected individuals [Gan et al 2015].
Although reported, generalized tonic-clonic seizures are less common than the other types.
Seizures vary in frequency from a few per day initially to a few per minute as the disease evolves. Over time, seizures become refractory to treatment.
Brain MRI is normal.
A generalized tremor, sometimes described as overall tremulousness, has been observed in eight of 16 affected individuals reported to date.
Sensorineural hearing loss (SNHL) that ranges from mild to profound hearing loss at high frequencies has been reported in four of 16 affected individuals [Dyment et al 2014, Gan et al 2015]. The hearing loss was not present at birth.
Musculoskeletal. Scoliosis, observed in five of 16 individuals, ranged from mild [Rubboli et al 2015] to more severe [Zhou et al 2012].
Cognition. Early developmental milestones are typically achieved on time. Cognition is described as normal; however, one child with intellectual disability has been reported [Rubboli et al 2015].
A decline in cognitive ability has been described in children in the last weeks of disease.
In one child progressive cognitive decline was the first manifestation [Sathe & Pearson 2013].
Life expectancy is shortened. The time from disease onset to death has ranged from five to 15 years. Although most affected children die in their late teens [Zhou et al 2012], some individuals who have been symptomatic for more than two decades have lived into their twenties [Gan et al 2015, Filosto et al 2016, Kernohan et al 2017]
Progressive Adult-Onset Brachydactyly Due to Osteolysis
A progressive adult-onset brachydactyly due to osteolysis has been reported in a single family with three affected family members who had progressive shortening of the fingers and toes due to severe osteolysis. Both reduced acid ceramidase activity and biallelic ASAH1 pathogenic variants segregated with the phenotype in the family [Bonafé et al 2016].
Genotype-Phenotype Correlations
No obvious genotype-phenotype correlations have been observed in ASAH1-related disorder to date despite a predominance of nonsense and splice-site variants in SMA-PME and a predominance of missense variants in FD.
While recurrent pathogenic variants have been observed in the FD phenotype (e.g., c.703G>C) and the SMA-PME phenotype (e.g., c.125C>T), to date the only ASAH1 pathogenic variants observed in both phenotypes are those that result in skipping of exon 6 [Bär et al 2001, Bashyam et al 2014, Dyment et al 2014].
There is a correlation in FD between age of death, in situ acid ceramidase activity, and the amount of ceramide accumulation [Levade et al 1995].